Repeat String - Javascript

Question

What is the best or most concise method for returning a string repeated an arbitrary amount of times   The following is my best shot so far   function repeat s  n       var a           while a length  lt  n           a push s             return a join

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Firstly  the OP s questions seems to be about conciseness - which I understand to mean  simple and easy to read   while most answers seem to be about efficiency - which is obviously not the same thing and also I think that unless you implement some very specific large data manipulating algorithms  shouldn t worry you when you come to implement basic data manipulation Javascript functions  Conciseness is much more important   Secondly  as Andr   Laszlo noted  String repeat is part of ECMAScript 6 and already available in several popular implementations - so the most concise implementation of String repeat is not to implement it  -   Lastly  if you need to support hosts that don t offer the ECMAScript 6 implementation  MDN s polyfill mentioned by Andr   Laszlo is anything but concise    So  without further ado - here is my concise polyfill   String prototype repeat   String prototype repeat    function n       return n lt  1   this   this concat this repeat n-1        Yes  this is a recursion  I like recursions - they are simple and if done correctly are easy to understand  Regarding efficiency  if the language supports it they can be very efficient if written correctly   From my tests  this method is  60  faster than the Array join approach  Although it obviously comes nowhere close disfated s implementation  it is much simpler than both    My test setup is node v0 10  using  Strict mode   I think it enables some sort of TCO   calling repeat 1000  on a 10 character string a million times

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function repeat s  n    var r     for  var a 0 a lt n a    r  s  return r

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Tests of the various methods   var repeatMethods         control  function  n s               all of these lines are common to all methods            if  n  0  return             if  n  1    isNaN n   return s          return                divideAndConquer    function  n  s            if  n  0  return             if  n  1    isNaN n   return s          with Math    return arguments callee floor n 2   s  arguments callee ceil n 2   s                linearRecurse  function  n s            if  n  0  return             if  n  1    isNaN n   return s          return s arguments callee --n  s              newArray  function  n  s            if  n  0  return             if  n  1    isNaN n   return s          return  new Array isNaN n    1     n   join s              fillAndJoin  function  n  s            if  n  0  return             if  n  1    isNaN n   return s          var ret               for  var i 0  i lt n  i                ret push s           return ret join                 concat  function  n s            if  n  0  return             if  n  1    isNaN n   return s          var ret               for  var i 0  i lt n  i                ret  s          return ret             artistoex  function  n s            var result               while  n gt 0                if  n amp 1  result  s              n gt  gt  1  s  s                     return result           function testNum len  dev        with Math    return round len 1 dev  random  -0 5        function testString len  dev        return  new Array testNum len  dev    join         var testTime   1000      tests             biggie    str    len  25  dev  12    rep   len  200  dev  50              smalls    str    len  5  dev  5   rep    len  5  dev  5            var testCount   0  var winnar   null  var inflight   0  for  var methodName in repeatMethods        var method   repeatMethods methodName       for  var testName in tests            testCount            var test   tests testName           var testId   methodName     testName          var result                 id  testId              testParams  test                   result count 0            function  result                inflight                setTimeout function                      result start    new Date                    while   new Date   - result start   lt  testTime                        method testNum test rep len  test rep dev   testString test str len  test str dev                        result count                                      result end    new Date                    result rate   1000 result count  result end-result start                  console log result                   if  winnar     null    winnar rate  lt  result rate  winnar   result                  inflight--                  if  inflight  0                        console log  The winner                          console log winnar                                     100 testTime  testCount             result

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Expanding P Bailey s solution   String prototype repeat   function num        return new Array isNaN num   1     num  join this           This way you should be safe from unexpected argument types   var foo    bar   alert foo repeat 3                    Will work   barbarbar  alert foo repeat  3                   Same as above alert foo repeat true                 Same as foo repeat 1   alert foo repeat 0                    This and all the following return an empty alert foo repeat false                string while not causing an exception alert foo repeat null    alert foo repeat undefined    alert foo repeat                      Object alert foo repeat function             Function   EDIT  Credits to jerone for his elegant   num idea

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Just another repeat function   function repeat s  n      var str         for  var i   0  i  lt  n  i          str    s        return str

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Note to new readers  This answer is old and and not terribly practical - it s just  clever  because it uses Array stuff to get   String things done  When I wrote  less process  I definitely meant    less code  because  as others have noted in subsequent answers  it   performs like a pig  So don t use it if speed matters to you    I d put this function onto the String object directly   Instead of creating an array  filling it  and joining it with an empty char  just create an array of the proper length  and join it with your desired string   Same result  less process   String prototype repeat   function  num         return new Array  num   1   join  this       alert   string to repeat n  repeat  4

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If you think all those prototype definitions  array creations  and join operations are overkill  just use a single line code where you need it  String S repeating N times   for  var i   0  result       i  lt  N  i    result    S

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Use Array N 1  join  string to repeat

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There are many ways in the ES-Next ways 1  ES2015 ES6 has been realized this repeat   method   x000D   x000D          str  String    count  Number     const str    hello repeat  n   count   3   let resultString   str repeat count    console log  resultString    n  resultString        resultString    hello repeat  hello repeat  hello repeat         toString       gt   abc   repeat  String prototype repeat    repeat 2       abcabc   repeat   is a generic method      Examples   abc  repeat 0             abc  repeat 1          abc   abc  repeat 2          abcabc   abc  repeat 3 5        abcabcabc   count will be converted to integer      abc  repeat 1 0       RangeError     abc  repeat -1        RangeError x000D   x000D   x000D   2  ES2017 ES8 new add String prototype padStart    x000D   x000D  const str    abc    const times   3   const newStr   str padStart str length   times  str toUpperCase      console log  newStr     newStr       newStr     ABC ABC abc   x000D   x000D   x000D   3  ES2017 ES8 new add String prototype padEnd    x000D   x000D  const str    abc    const times   3   const newStr   str padEnd str length   times  str toUpperCase      console log  newStr     newStr       newStr     abc ABC ABC   x000D   x000D   x000D   refs http   www ecma-international org ecma-262 6 0  sec-string prototype repeat https   developer mozilla org en-US docs Web JavaScript Reference Global Objects String repeat https   developer mozilla org en-US docs Web JavaScript Reference Global Objects String padStart https   developer mozilla org en-US docs Web JavaScript Reference Global Objects String padEnd

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I came here randomly and never had a reason to repeat a char in javascript before   I was impressed by artistoex s way of doing it and disfated s results  I noticed that the last string concat was unnecessary  as Dennis also pointed out   I noticed a few more things when playing with the sampling disfated put together   The results varied a fair amount often favoring the last run and similar algorithms would often jockey for position  One of the things I changed was instead of using the JSLitmus generated count as the seed for the calls  as count was generated different for the various methods  I put in an index  This made the thing much more reliable  I then looked at ensuring that varying sized strings were passed to the functions  This prevented some of the variations I saw  where some algorithms did better at the single chars or smaller strings  However the top 3 methods all did well regardless of the string size    Forked test set  http   jsfiddle net schmide fCqp3 134      repeated string var string    0123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789      count paremeter is changed on every test iteration  limit it s maximum value here var maxCount   200   var n   0    each tests  function  name        var fn   tests name       JSLitmus test   n          name  function  count            var index   0          while  count--                fn call string slice 0  index   string length   index   maxCount               index                          if  fn call   gt    10  length     10     body   prepend   lt h1 gt Error in      name      lt  h1 gt          JSLitmus runAll      I then included Dennis  fix and decided to see if I could find a way to eek out a bit more   Since javascript can t really optimize things  the best way to improve performance is to manually avoid things  If I took the first 4 trivial results out of the loop  I could avoid 2-4 string stores and write the final store directly to the result      final  growing pattern   prototypejs check  count  lt  1   final avoid   function  count        if   count  return         if  count    1  return this valueOf        var pattern   this valueOf        if  count    2  return pattern   pattern      if  count    3  return pattern   pattern   pattern      var result      if  count  amp  1  result   pattern      else result           count  gt  gt   1      do           pattern    pattern          if  count  amp  1  result    pattern          count  gt  gt   1        while  count  gt  1       return result   pattern   pattern      This resulted in a 1-2  improvement on average over Dennis  fix  However  different runs and different browsers would show a fair enough variance that this extra code probably isn t worth the effort over the 2 previous algorithms   A chart  Edit  I did this mostly under chrome  Firefox and IE will often favor Dennis by a couple

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I ve tested the performance of all the proposed approaches   Here is the fastest variant I ve got   String prototype repeat   function count        if  count  lt  1  return         var result       pattern   this valueOf        while  count  gt  1            if  count  amp  1  result    pattern          count  gt  gt   1  pattern    pattern            return result   pattern       Or as stand-alone function   function repeat pattern  count        if  count  lt  1  return         var result           while  count  gt  1            if  count  amp  1  result    pattern          count  gt  gt   1  pattern    pattern            return result   pattern      It is based on artistoex algorithm  It is really fast  And the bigger the count  the faster it goes compared with the traditional new Array count   1  join string  approach     I ve only changed 2 things    replaced pattern   this with pattern   this valueOf    clears one obvious type conversion   added if  count  lt  1  check from prototypejs to the top of function to exclude unnecessary actions in that case  applied optimisation from Dennis answer  5-7  speed up    UPD  Created a little performance-testing playground here for those who interested   variable count   0    100       constant count   1024       Use it and make it even faster if you can

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For all browsers This is about as concise as it gets   function repeat s  n    return new Array n 1  join s      If you also care about performance  this is a much better approach   function repeat s  n    var a    i 0 for  i lt n  a i    s return a join         If you want to compare the performance of both options  see this Fiddle and this Fiddle for benchmark tests  During my own tests  the second option was about 2 times faster in Firefox and about 4 times faster in Chrome  For moderns browsers only   In modern browsers  you can now also do this   function repeat s n    return s repeat n      This option is not only shorter than both other options  but it s even faster than the second option  Unfortunately  it doesn t work in any version of Internet explorer  The numbers in the table specify the first browser version that fully supports the method

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Expanding P Bailey s solution   String prototype repeat   function num        return new Array isNaN num   1     num  join this           This way you should be safe from unexpected argument types   var foo    bar   alert foo repeat 3                    Will work   barbarbar  alert foo repeat  3                   Same as above alert foo repeat true                 Same as foo repeat 1   alert foo repeat 0                    This and all the following return an empty alert foo repeat false                string while not causing an exception alert foo repeat null    alert foo repeat undefined    alert foo repeat                      Object alert foo repeat function             Function   EDIT  Credits to jerone for his elegant   num idea

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This problem is a well-known    classic  optimization issue for JavaScript  caused by the fact that JavaScript strings are  immutable  and addition by concatenation of even a single character to a string requires creation of  including memory allocation for and copying to  an entire new string   Unfortunately  the accepted answer on this page is wrong  where  wrong  means by a performance factor of 3x for simple one-character strings  and 8x-97x for short strings repeated more times  to 300x for repeating sentences  and infinitely wrong when taking the limit of the ratios of complexity of the algorithms as n goes to infinity  Also  there is another answer on this page which is almost right  based on one of the many generations and variations of the correct solution circulating throughout the Internet in the past 13 years   However  this  almost right  solution misses a key point of the correct algorithm causing a 50  performance degradation   JS Performance Results for the accepted answer  the top-performing other answer  based on a degraded version of the original algorithm in this answer   and this answer using my algorithm created 13 years ago    October 2000 I published an algorithm for this exact problem which was widely adapted  modified  then eventually poorly understood and forgotten  To remedy this issue  in August  2008 I published an article http   www webreference com programming javascript jkm3 3 html explaining the algorithm and using it as an example of simple of general-purpose JavaScript optimizations  By now  Web Reference has scrubbed my contact information and even my name from this article  And once again  the algorithm has been widely adapted  modified  then poorly understood and largely forgotten      Original string repetition multiplication JavaScript algorithm by   Joseph Myers  circa Y2K as a text multiplying function within Text js    published August  2008 in this form by Web Reference    http   www webreference com programming javascript jkm3 3 html  The   article used the function as an example of JavaScript optimizations    which is the only for the strange name  stringFill3            Usage  stringFill3  abc   2      abcabc       function stringFill3 x  n        var s           for                if  n  amp  1  s    x          n  gt  gt   1          if  n  x    x          else break            return s      Within two months after publication of that article  this same question was posted to Stack Overflow and flew under my radar until now  when apparently the original algorithm for this problem has once again been forgotten  The best solution available on this Stack Overflow page is a modified version of my solution  possibly separated by several generations  Unfortunately  the modifications ruined the solution s optimality  In fact  by changing the structure of the loop from my original  the modified solution performs a completely unneeded extra step of exponential duplicating  thus joining the largest string used in the proper answer with itself an extra time and then discarding it    Below ensues a discussion of some JavaScript optimizations related to all of the answers to this problem and for the benefit of all   Technique  Avoid references to objects or object properties  To illustrate how this technique works  we use a real-life JavaScript function which creates strings of whatever length is needed  And as we ll see  more optimizations can be added   A function like the one used here is to create padding to align columns of text  for formatting money  or for filling block data up to the boundary  A text generation function also allows variable length input for testing any other function that operates on text  This function is one of the important components of the JavaScript text processing module   As we proceed  we will be covering two more of the most important optimization techniques while developing the original code into an optimized algorithm for creating strings  The final result is an industrial-strength  high-performance function that I ve used everywhere--aligning item prices and totals in JavaScript order forms  data formatting and email   text message formatting and many other uses   Original code for creating strings stringFill1    function stringFill1 x  n         var s            while  s length  lt  n  s    x       return s         Example of output  stringFill1  x   3      xxx        The syntax is here is clear  As you can see  we ve used local function variables already  before going on to more optimizations   Be aware that there s one innocent reference to an object property s length in the code that hurts its performance  Even worse  the use of this object property reduces the simplicity of the program by making the assumption that the reader knows about the properties of JavaScript string objects   The use of this object property destroys the generality of the computer program  The program assumes that x must be a string of length one  This limits the application of the stringFill1   function to anything except repetition of single characters  Even single characters cannot be used if they contain multiple bytes like the HTML entity  amp nbsp    The worst problem caused by this unnecessary use of an object property is that the function creates an infinite loop if tested on an empty input string x  To check generality  apply a program to the smallest possible amount of input  A program which crashes when asked to exceed the amount of available memory has an excuse  A program like this one which crashes when asked to produce nothing is unacceptable  Sometimes pretty code is poisonous code   Simplicity may be an ambiguous goal of computer programming  but generally it s not  When a program lacks any reasonable level of generality  it s not valid to say   The program is good enough as far as it goes   As you can see  using the string length property prevents this program from working in a general setting  and in fact  the incorrect program is ready to cause a browser or system crash   Is there a way to improve the performance of this JavaScript as well as take care of these two serious problems   Of course  Just use integers   Optimized code for creating strings stringFill2    function stringFill2 x  n         var s            while  n--  gt  0  s    x       return s        Timing code to compare stringFill1   and stringFill2    function testFill functionToBeTested  outputSize         var i   0  t0   new Date         do            functionToBeTested  x   outputSize            t   new Date   - t0           i           while  t  lt  2000        return t i 1000      seconds1   testFill stringFill1  100    seconds2   testFill stringFill2  100      The success so far of stringFill2    stringFill1   takes 47 297 microseconds  millionths of a second  to fill a 100-byte string  and stringFill2   takes 27 68 microseconds to do the same thing  That s almost a doubling in performance by avoiding a reference to an object property   Technique  Avoid adding short strings to long strings  Our previous result looked good--very good  in fact  The improved function stringFill2   is much faster due to the use of our first two optimizations  Would you believe it if I told you that it can be improved to be many times faster than it is now   Yes  we can accomplish that goal  Right now we need to explain how we avoid appending short strings to long strings   The short-term behavior appears to be quite good  in comparison to our original function  Computer scientists like to analyze the  asymptotic behavior  of a function or computer program algorithm  which means to study its long-term behavior by testing it with larger inputs  Sometimes without doing further tests  one never becomes aware of ways that a computer program could be improved  To see what will happen  we re going to create a 200-byte string   The problem that shows up with stringFill2    Using our timing function  we find that the time increases to 62 54 microseconds for a 200-byte string  compared to 27 68 for a 100-byte string  It seems like the time should be doubled for doing twice as much work  but instead it s tripled or quadrupled  From programming experience  this result seems strange  because if anything  the function should be slightly faster since work is being done more efficiently  200 bytes per function call rather than 100 bytes per function call   This issue has to do with an insidious property of JavaScript strings  JavaScript strings are  immutable    Immutable means that you cannot change a string once it s created  By adding on one byte at a time  we re not using up one more byte of effort  We re actually recreating the entire string plus one more byte   In effect  to add one more byte to a 100-byte string  it takes 101 bytes worth of work  Let s briefly analyze the computational cost for creating a string of N bytes  The cost of adding the first byte is 1 unit of computational effort  The cost of adding the second byte isn t one unit but 2 units  copying the first byte to a new string object as well as adding the second byte   The third byte requires a cost of 3 units  etc   C N    1   2   3         N   N N 1  2   O N 2   The symbol O N 2  is pronounced Big O of N squared  and it means that the computational cost in the long run is proportional to the square of the string length  To create 100 characters takes 10 000 units of work  and to create 200 characters takes 40 000 units of work   This is why it took more than twice as long to create 200 characters than 100 characters  In fact  it should have taken four times as long  Our programming experience was correct in that the work is being done slightly more efficiently for longer strings  and hence it took only about three times as long  Once the overhead of the function call becomes negligible as to how long of a string we re creating  it will actually take four times as much time to create a string twice as long    Historical note  This analysis doesn t necessarily apply to strings in source code  such as html    abcd n     efgh n           xyz  n   since the JavaScript source code compiler can join the strings together before making them into a JavaScript string object  Just a few years ago  the KJS implementation of JavaScript would freeze or crash when loading long strings of source code joined by plus signs  Since the computational time was O N 2  it wasn t difficult to make Web pages which overloaded the Konqueror Web browser or Safari  which used the KJS JavaScript engine core  I first came across this issue when I was developing a markup language and JavaScript markup language parser  and then I discovered what was causing the problem when I wrote my script for JavaScript Includes    Clearly this rapid degradation of performance is a huge problem  How can we deal with it  given that we cannot change JavaScript s way of handling strings as immutable objects  The solution is to use an algorithm which recreates the string as few times as possible   To clarify  our goal is to avoid adding short strings to long strings  since in order to add the short string  the entire long string also must be duplicated   How the algorithm works to avoid adding short strings to long strings  Here s a good way to reduce the number of times new string objects are created  Concatenate longer lengths of string together so that more than one byte at a time is added to the output   For instance  to make a string of length N   9   x    x    s        s    x     Now s    x      x    x     Now x    xx      x    x     Now x    xxxx      x    x     Now x    xxxxxxxx      s    x     Now s    xxxxxxxxx  as desired      Doing this required creating a string of length 1  creating a string of length 2  creating a string of length 4  creating a string of length 8  and finally  creating a string of length 9  How much cost have we saved   Old cost C 9    1   2   3   4   5   6   7   9   45   New cost C 9    1   2   4   8   9   24   Note that we had to add a string of length 1 to a string of length 0  then a string of length 1 to a string of length 1  then a string of length 2 to a string of length 2  then a string of length 4 to a string of length 4  then a string of length 8 to a string of length 1  in order to obtain a string of length 9  What we re doing can be summarized as avoiding adding short strings to long strings  or in other words  trying to concatenate strings together that are of equal or nearly equal length   For the old computational cost we found a formula N N 1  2  Is there a formula for the new cost  Yes  but it s complicated  The important thing is that it is O N   and so doubling the string length will approximately double the amount of work rather than quadrupling it   The code that implements this new idea is nearly as complicated as the formula for the computational cost  When you read it  remember that  gt  gt   1 means to shift right by 1 byte  So if n   10011 is a binary number  then n  gt  gt   1 results in the value n   1001   The other part of the code you might not recognize is the bitwise and operator  written  amp   The expression n  amp  1 evaluates true if the last binary digit of n is 1  and false if the last binary digit of n is 0   New highly-efficient stringFill3   function  function stringFill3 x  n         var s            for                 if  n  amp  1  s    x           n  gt  gt   1           if  n  x    x           else break              return s        It looks ugly to the untrained eye  but it s performance is nothing less than lovely   Let s see just how well this function performs  After seeing the results  it s likely that you ll never forget the difference between an O N 2  algorithm and an O N  algorithm   stringFill1   takes 88 7 microseconds  millionths of a second  to create a 200-byte string  stringFill2   takes 62 54  and stringFill3   takes only 4 608  What made this algorithm so much better  All of the functions took advantage of using local function variables  but taking advantage of the second and third optimization techniques added a twenty-fold improvement to performance of stringFill3     Deeper analysis  What makes this particular function blow the competition out of the water   As I ve mentioned  the reason that both of these functions  stringFill1   and stringFill2    run so slowly is that JavaScript strings are immutable  Memory cannot be reallocated to allow one more byte at a time to be appended to the string data stored by JavaScript  Every time one more byte is added to the end of the string  the entire string is regenerated from beginning to end   Thus  in order to improve the script s performance  one must precompute longer length strings by concatenating two strings together ahead of time  and then recursively building up the desired string length   For instance  to create a 16-letter byte string  first a two byte string would be precomputed  Then the two byte string would be reused to precompute a four-byte string  Then the four-byte string would be reused to precompute an eight byte string  Finally  two eight-byte strings would be reused to create the desired new string of 16 bytes  Altogether four new strings had to be created  one of length 2  one of length 4  one of length 8 and one of length 16  The total cost is 2   4   8   16   30   In the long run this efficiency can be computed by adding in reverse order and using a geometric series starting with a first term a1   N and having a common ratio of r   1 2  The sum of a geometric series is given by a 1    1-r    2N   This is more efficient than adding one character to create a new string of length 2  creating a new string of length 3  4  5  and so on  until 16  The previous algorithm used that process of adding a single byte at a time  and the total cost of it would be n  n   1    2   16  17    2   8  17    136   Obviously  136 is a much greater number than 30  and so the previous algorithm takes much  much more time to build up a string   To compare the two methods you can see how much faster the recursive algorithm  also called  divide and conquer   is on a string of length 123 457  On my FreeBSD computer this algorithm  implemented in the stringFill3   function  creates the string in 0 001058 seconds  while the original stringFill1   function creates the string in 0 0808 seconds  The new function is 76 times faster   The difference in performance grows as the length of the string becomes larger  In the limit as larger and larger strings are created  the original function behaves roughly like C1  constant  times N 2  and the new function behaves like C2  constant  times N   From our experiment we can determine the value of C1 to be C1   0 0808    123457 2    00000000000530126997  and the value of C2 to be C2   0 001058   123457    00000000856978543136  In 10 seconds  the new function could create a string containing 1 166 890 359 characters  In order to create this same string  the old function would need 7 218 384 seconds of time   This is almost three months compared to ten seconds   I m only answering  several years late  because my original solution to this problem has been floating around the Internet for more than 10 years  and apparently is still poorly-understood by the few who do remember it  I thought that by writing an article about it here I would help   Performance Optimizations for High Speed JavaScript   Page 3  Unfortunately  some of the other solutions presented here are still some of those that would take three months to produce the same amount of output that a proper solution creates in 10 seconds   I want to take the time to reproduce part of the article here as a canonical answer on Stack Overflow   Note that the best-performing algorithm here is clearly based on my algorithm and was probably inherited from someone else s 3rd or 4th generation adaptation  Unfortunately  the modifications resulted in reducing its performance  The variation of my solution presented here perhaps did not understand my confusing for      expression which looks like the main infinite loop of a server written in C  and which was simply designed to allow a carefully-positioned break statement for loop control  the most compact way to avoid exponentially replicating the string one extra unnecessary time

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Use Lodash for Javascript utility functionality  like repeating strings   Lodash provides nice performance and ECMAScript compatibility   I highly recommend it for UI development and it works well server side  too   Here s how to repeat the string  yo  2 times using Lodash    gt    repeat  yo   2   yoyo

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Concatenating strings based on an number   function concatStr str  num       var arr             Construct an array    for  var i   0  i  lt  num  i          arr i    str        Join all elements    str   arr join          return str     console log concatStr  abc   3      Hope that helps

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function repeat pattern  count      for  var result               if  count  amp  1          result    pattern            if  count  gt  gt   1          pattern    pattern        else         return result                You can test it at JSFiddle  Benchmarked against the hacky Array join and mine is  roughly speaking  10  Chrome  to 100  Safari  to 200  Firefox  times faster  depending on the browser

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Simple method   String prototype repeat   function num        num   parseInt num       if  num  lt  0  return         return new Array num   1  join this

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function repeat s  n    var r     for  var a 0 a lt n a    r  s  return r

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This one is pretty efficient  String prototype repeat   function times       var result         var pattern this      while  times  gt  0            if  times amp 1              result  pattern          times gt  gt  1          pattern  pattern            return result

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Here s a 5-7  improvement on disfated s answer   Unroll the loop by stopping at count  gt  1 and perform an additional result    pattnern concat after the loop  This will avoid the loops final previously unused pattern    pattern without having to use an expensive if-check  The final result would look like this   String prototype repeat   function count        if  count  lt  1  return         var result       pattern   this valueOf        while  count  gt  1            if  count  amp  1  result    pattern          count  gt  gt   1  pattern    pattern            result    pattern      return result       And here s disfated s fiddle forked for the unrolled version  http   jsfiddle net wsdfg

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Tests of the various methods   var repeatMethods         control  function  n s               all of these lines are common to all methods            if  n  0  return             if  n  1    isNaN n   return s          return                divideAndConquer    function  n  s            if  n  0  return             if  n  1    isNaN n   return s          with Math    return arguments callee floor n 2   s  arguments callee ceil n 2   s                linearRecurse  function  n s            if  n  0  return             if  n  1    isNaN n   return s          return s arguments callee --n  s              newArray  function  n  s            if  n  0  return             if  n  1    isNaN n   return s          return  new Array isNaN n    1     n   join s              fillAndJoin  function  n  s            if  n  0  return             if  n  1    isNaN n   return s          var ret               for  var i 0  i lt n  i                ret push s           return ret join                 concat  function  n s            if  n  0  return             if  n  1    isNaN n   return s          var ret               for  var i 0  i lt n  i                ret  s          return ret             artistoex  function  n s            var result               while  n gt 0                if  n amp 1  result  s              n gt  gt  1  s  s                     return result           function testNum len  dev        with Math    return round len 1 dev  random  -0 5        function testString len  dev        return  new Array testNum len  dev    join         var testTime   1000      tests             biggie    str    len  25  dev  12    rep   len  200  dev  50              smalls    str    len  5  dev  5   rep    len  5  dev  5            var testCount   0  var winnar   null  var inflight   0  for  var methodName in repeatMethods        var method   repeatMethods methodName       for  var testName in tests            testCount            var test   tests testName           var testId   methodName     testName          var result                 id  testId              testParams  test                   result count 0            function  result                inflight                setTimeout function                      result start    new Date                    while   new Date   - result start   lt  testTime                        method testNum test rep len  test rep dev   testString test str len  test str dev                        result count                                      result end    new Date                    result rate   1000 result count  result end-result start                  console log result                   if  winnar     null    winnar rate  lt  result rate  winnar   result                  inflight--                  if  inflight  0                        console log  The winner                          console log winnar                                     100 testTime  testCount             result

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I came here randomly and never had a reason to repeat a char in javascript before   I was impressed by artistoex s way of doing it and disfated s results  I noticed that the last string concat was unnecessary  as Dennis also pointed out   I noticed a few more things when playing with the sampling disfated put together   The results varied a fair amount often favoring the last run and similar algorithms would often jockey for position  One of the things I changed was instead of using the JSLitmus generated count as the seed for the calls  as count was generated different for the various methods  I put in an index  This made the thing much more reliable  I then looked at ensuring that varying sized strings were passed to the functions  This prevented some of the variations I saw  where some algorithms did better at the single chars or smaller strings  However the top 3 methods all did well regardless of the string size    Forked test set  http   jsfiddle net schmide fCqp3 134      repeated string var string    0123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789      count paremeter is changed on every test iteration  limit it s maximum value here var maxCount   200   var n   0    each tests  function  name        var fn   tests name       JSLitmus test   n          name  function  count            var index   0          while  count--                fn call string slice 0  index   string length   index   maxCount               index                          if  fn call   gt    10  length     10     body   prepend   lt h1 gt Error in      name      lt  h1 gt          JSLitmus runAll      I then included Dennis  fix and decided to see if I could find a way to eek out a bit more   Since javascript can t really optimize things  the best way to improve performance is to manually avoid things  If I took the first 4 trivial results out of the loop  I could avoid 2-4 string stores and write the final store directly to the result      final  growing pattern   prototypejs check  count  lt  1   final avoid   function  count        if   count  return         if  count    1  return this valueOf        var pattern   this valueOf        if  count    2  return pattern   pattern      if  count    3  return pattern   pattern   pattern      var result      if  count  amp  1  result   pattern      else result           count  gt  gt   1      do           pattern    pattern          if  count  amp  1  result    pattern          count  gt  gt   1        while  count  gt  1       return result   pattern   pattern      This resulted in a 1-2  improvement on average over Dennis  fix  However  different runs and different browsers would show a fair enough variance that this extra code probably isn t worth the effort over the 2 previous algorithms   A chart  Edit  I did this mostly under chrome  Firefox and IE will often favor Dennis by a couple

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People overcomplicate this to a ridiculous extent or waste performance  Arrays  Recursion  You ve got to be kidding me   function repeat  string  times      var result        while  times--  gt  0  result    string   return result     Edit  I ran some simple tests to compare with the bitwise version posted by artistoex   disfated and a bunch of other people  The latter was only marginally faster  but orders of magnitude more memory-efficient  For 1000000 repeats of the word  blah   the Node process went up to 46 megabytes with the simple concatenation algorithm  above   but only 5 5 megabytes with the logarithmic algorithm  The latter is definitely the way to go  Reposting it for the sake of clarity   function repeat  string  times      var result        while  times  gt  0        if  times  amp  1  result    string     times  gt  gt   1     string    string       return result

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Good news  String prototype repeat is now a part of JavaScript   quot yo quot  repeat 2      returns   quot yoyo quot   The method is supported by all major browsers  except Internet Explorer  For an up to date list  see MDN  String prototype repeat  gt  Browser compatibility  MDN has a polyfill  for browsers without support

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Concatenating strings based on an number   function concatStr str  num       var arr             Construct an array    for  var i   0  i  lt  num  i          arr i    str        Join all elements    str   arr join          return str     console log concatStr  abc   3      Hope that helps

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Recursive solution using divide and conquer   function repeat n  s        if  n  0  return         if  n  1    isNaN n   return s      with Math    return repeat floor n 2   s  repeat ceil n 2   s

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Here s the JSLint safe version  String prototype repeat   function  num      var a         a length   num  lt  lt  0   1    return a join this

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This may be the smallest recursive one -  String prototype repeat   function n s    s   s       if n gt 0       s    this    s   this repeat --n s    return s

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Use Lodash for Javascript utility functionality  like repeating strings   Lodash provides nice performance and ECMAScript compatibility   I highly recommend it for UI development and it works well server side  too   Here s how to repeat the string  yo  2 times using Lodash    gt    repeat  yo   2   yoyo

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This one is pretty efficient  String prototype repeat   function times       var result         var pattern this      while  times  gt  0            if  times amp 1              result  pattern          times gt  gt  1          pattern  pattern            return result

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Note to new readers  This answer is old and and not terribly practical - it s just  clever  because it uses Array stuff to get   String things done  When I wrote  less process  I definitely meant    less code  because  as others have noted in subsequent answers  it   performs like a pig  So don t use it if speed matters to you    I d put this function onto the String object directly   Instead of creating an array  filling it  and joining it with an empty char  just create an array of the proper length  and join it with your desired string   Same result  less process   String prototype repeat   function  num         return new Array  num   1   join  this       alert   string to repeat n  repeat  4

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This may be the smallest recursive one -  String prototype repeat   function n s    s   s       if n gt 0       s    this    s   this repeat --n s    return s

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For all browsers This is about as concise as it gets   function repeat s  n    return new Array n 1  join s      If you also care about performance  this is a much better approach   function repeat s  n    var a    i 0 for  i lt n  a i    s return a join         If you want to compare the performance of both options  see this Fiddle and this Fiddle for benchmark tests  During my own tests  the second option was about 2 times faster in Firefox and about 4 times faster in Chrome  For moderns browsers only   In modern browsers  you can now also do this   function repeat s n    return s repeat n      This option is not only shorter than both other options  but it s even faster than the second option  Unfortunately  it doesn t work in any version of Internet explorer  The numbers in the table specify the first browser version that fully supports the method

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Good news  String prototype repeat is now a part of JavaScript   quot yo quot  repeat 2      returns   quot yoyo quot   The method is supported by all major browsers  except Internet Explorer  For an up to date list  see MDN  String prototype repeat  gt  Browser compatibility  MDN has a polyfill  for browsers without support

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There are many ways in the ES-Next ways 1  ES2015 ES6 has been realized this repeat   method   x000D   x000D          str  String    count  Number     const str    hello repeat  n   count   3   let resultString   str repeat count    console log  resultString    n  resultString        resultString    hello repeat  hello repeat  hello repeat         toString       gt   abc   repeat  String prototype repeat    repeat 2       abcabc   repeat   is a generic method      Examples   abc  repeat 0             abc  repeat 1          abc   abc  repeat 2          abcabc   abc  repeat 3 5        abcabcabc   count will be converted to integer      abc  repeat 1 0       RangeError     abc  repeat -1        RangeError x000D   x000D   x000D   2  ES2017 ES8 new add String prototype padStart    x000D   x000D  const str    abc    const times   3   const newStr   str padStart str length   times  str toUpperCase      console log  newStr     newStr       newStr     ABC ABC abc   x000D   x000D   x000D   3  ES2017 ES8 new add String prototype padEnd    x000D   x000D  const str    abc    const times   3   const newStr   str padEnd str length   times  str toUpperCase      console log  newStr     newStr       newStr     abc ABC ABC   x000D   x000D   x000D   refs http   www ecma-international org ecma-262 6 0  sec-string prototype repeat https   developer mozilla org en-US docs Web JavaScript Reference Global Objects String repeat https   developer mozilla org en-US docs Web JavaScript Reference Global Objects String padStart https   developer mozilla org en-US docs Web JavaScript Reference Global Objects String padEnd

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Simple recursive concatenation I just wanted to give it a bash  and made this  function ditto  s  r  c         return c--   ditto  s  r    s  c     r     ditto   quot foo quot    quot  quot   128     I can t say I gave it much thought  and it probably shows  -  This is arguably better String prototype ditto   function  c         return --c   this   this ditto  c     this       quot foo quot  ditto  128     And it s a lot like an answer already posted - I know this  But why be recursive at all  And how about a little default behaviour too  String prototype ditto   function         var c   Number  arguments  0        2          r   this valueOf        while   --c             r    this            return r      quot foo quot  ditto     Because  although the non recursive method will handle arbitrarily large repeats without hitting call stack limits  it s a lot slower  Why did I bother adding more methods that aren t half as clever as those already posted  Partly for my own amusement  and partly to point out in the simplest way I know that there are many ways to skin a cat  and depending on the situation  it s quite possible that the apparently best method isn t ideal  A relatively fast and sophisticated method may effectively crash and burn under certain circumstances  whilst a slower  simpler method may get the job done - eventually  Some methods may be little more than exploits  and as such prone to being fixed out of existence  and other methods may work beautifully in all conditions  but are so constructed that one simply has no idea how it works   quot So what if I dunno how it works   quot  Seriously  JavaScript suffers from one of its greatest strengths  it s highly tolerant of bad behaviour  and so flexible it ll bend over backwards to return results  when it might have been better for everyone if it d snapped   quot With great power  comes great responsibility quot   -  But more seriously and importantly  although general questions like this do lead to awesomeness in the form of clever answers that if nothing else  expand one s knowledge and horizons  in the end  the task at hand - the practical script that uses the resulting method - may require a little less  or a little more clever than is suggested  These  quot perfect quot  algorithms are fun and all  but  quot one size fits all quot  will rarely if ever be better than tailor made  This sermon was brought to you courtesy of a lack of sleep and a passing interest  Go forth and code

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function repeat s  n    var r     for  var a 0 a lt n a    r  s  return r

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Note to new readers  This answer is old and and not terribly practical - it s just  clever  because it uses Array stuff to get   String things done  When I wrote  less process  I definitely meant    less code  because  as others have noted in subsequent answers  it   performs like a pig  So don t use it if speed matters to you    I d put this function onto the String object directly   Instead of creating an array  filling it  and joining it with an empty char  just create an array of the proper length  and join it with your desired string   Same result  less process   String prototype repeat   function  num         return new Array  num   1   join  this       alert   string to repeat n  repeat  4

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String prototype repeat is now ES6 Standard    abc  repeat 3     abcabcabc

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Fiddle  http   jsfiddle net 3Y9v2   function repeat s  n       return   new Array n 1   join s      alert repeat  R   10

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Fiddle  http   jsfiddle net 3Y9v2   function repeat s  n       return   new Array n 1   join s      alert repeat  R   10

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Here s the JSLint safe version  String prototype repeat   function  num      var a         a length   num  lt  lt  0   1    return a join this

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Here s a 5-7  improvement on disfated s answer   Unroll the loop by stopping at count  gt  1 and perform an additional result    pattnern concat after the loop  This will avoid the loops final previously unused pattern    pattern without having to use an expensive if-check  The final result would look like this   String prototype repeat   function count        if  count  lt  1  return         var result       pattern   this valueOf        while  count  gt  1            if  count  amp  1  result    pattern          count  gt  gt   1  pattern    pattern            result    pattern      return result       And here s disfated s fiddle forked for the unrolled version  http   jsfiddle net wsdfg

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Note to new readers  This answer is old and and not terribly practical - it s just  clever  because it uses Array stuff to get   String things done  When I wrote  less process  I definitely meant    less code  because  as others have noted in subsequent answers  it   performs like a pig  So don t use it if speed matters to you    I d put this function onto the String object directly   Instead of creating an array  filling it  and joining it with an empty char  just create an array of the proper length  and join it with your desired string   Same result  less process   String prototype repeat   function  num         return new Array  num   1   join  this       alert   string to repeat n  repeat  4

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With ES8 you could also use padStart or padEnd for this  eg   var str    cat   var num   23  var size   str length   num     padStart size  str     outputs   catcatcatcatcatcatcatcatcatcatcatcatcatcatcatcatcatcatcatcatcatcatcat

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People overcomplicate this to a ridiculous extent or waste performance  Arrays  Recursion  You ve got to be kidding me   function repeat  string  times      var result        while  times--  gt  0  result    string   return result     Edit  I ran some simple tests to compare with the bitwise version posted by artistoex   disfated and a bunch of other people  The latter was only marginally faster  but orders of magnitude more memory-efficient  For 1000000 repeats of the word  blah   the Node process went up to 46 megabytes with the simple concatenation algorithm  above   but only 5 5 megabytes with the logarithmic algorithm  The latter is definitely the way to go  Reposting it for the sake of clarity   function repeat  string  times      var result        while  times  gt  0        if  times  amp  1  result    string     times  gt  gt   1     string    string       return result

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Simple recursive concatenation I just wanted to give it a bash  and made this  function ditto  s  r  c         return c--   ditto  s  r    s  c     r     ditto   quot foo quot    quot  quot   128     I can t say I gave it much thought  and it probably shows  -  This is arguably better String prototype ditto   function  c         return --c   this   this ditto  c     this       quot foo quot  ditto  128     And it s a lot like an answer already posted - I know this  But why be recursive at all  And how about a little default behaviour too  String prototype ditto   function         var c   Number  arguments  0        2          r   this valueOf        while   --c             r    this            return r      quot foo quot  ditto     Because  although the non recursive method will handle arbitrarily large repeats without hitting call stack limits  it s a lot slower  Why did I bother adding more methods that aren t half as clever as those already posted  Partly for my own amusement  and partly to point out in the simplest way I know that there are many ways to skin a cat  and depending on the situation  it s quite possible that the apparently best method isn t ideal  A relatively fast and sophisticated method may effectively crash and burn under certain circumstances  whilst a slower  simpler method may get the job done - eventually  Some methods may be little more than exploits  and as such prone to being fixed out of existence  and other methods may work beautifully in all conditions  but are so constructed that one simply has no idea how it works   quot So what if I dunno how it works   quot  Seriously  JavaScript suffers from one of its greatest strengths  it s highly tolerant of bad behaviour  and so flexible it ll bend over backwards to return results  when it might have been better for everyone if it d snapped   quot With great power  comes great responsibility quot   -  But more seriously and importantly  although general questions like this do lead to awesomeness in the form of clever answers that if nothing else  expand one s knowledge and horizons  in the end  the task at hand - the practical script that uses the resulting method - may require a little less  or a little more clever than is suggested  These  quot perfect quot  algorithms are fun and all  but  quot one size fits all quot  will rarely if ever be better than tailor made  This sermon was brought to you courtesy of a lack of sleep and a passing interest  Go forth and code

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Firstly  the OP s questions seems to be about conciseness - which I understand to mean  simple and easy to read   while most answers seem to be about efficiency - which is obviously not the same thing and also I think that unless you implement some very specific large data manipulating algorithms  shouldn t worry you when you come to implement basic data manipulation Javascript functions  Conciseness is much more important   Secondly  as Andr   Laszlo noted  String repeat is part of ECMAScript 6 and already available in several popular implementations - so the most concise implementation of String repeat is not to implement it  -   Lastly  if you need to support hosts that don t offer the ECMAScript 6 implementation  MDN s polyfill mentioned by Andr   Laszlo is anything but concise    So  without further ado - here is my concise polyfill   String prototype repeat   String prototype repeat    function n       return n lt  1   this   this concat this repeat n-1        Yes  this is a recursion  I like recursions - they are simple and if done correctly are easy to understand  Regarding efficiency  if the language supports it they can be very efficient if written correctly   From my tests  this method is  60  faster than the Array join approach  Although it obviously comes nowhere close disfated s implementation  it is much simpler than both    My test setup is node v0 10  using  Strict mode   I think it enables some sort of TCO   calling repeat 1000  on a 10 character string a million times

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With ES8 you could also use padStart or padEnd for this  eg   var str    cat   var num   23  var size   str length   num     padStart size  str     outputs   catcatcatcatcatcatcatcatcatcatcatcatcatcatcatcatcatcatcatcatcatcatcat

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Recursive solution using divide and conquer   function repeat n  s        if  n  0  return         if  n  1    isNaN n   return s      with Math    return repeat floor n 2   s  repeat ceil n 2   s

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This problem is a well-known    classic  optimization issue for JavaScript  caused by the fact that JavaScript strings are  immutable  and addition by concatenation of even a single character to a string requires creation of  including memory allocation for and copying to  an entire new string   Unfortunately  the accepted answer on this page is wrong  where  wrong  means by a performance factor of 3x for simple one-character strings  and 8x-97x for short strings repeated more times  to 300x for repeating sentences  and infinitely wrong when taking the limit of the ratios of complexity of the algorithms as n goes to infinity  Also  there is another answer on this page which is almost right  based on one of the many generations and variations of the correct solution circulating throughout the Internet in the past 13 years   However  this  almost right  solution misses a key point of the correct algorithm causing a 50  performance degradation   JS Performance Results for the accepted answer  the top-performing other answer  based on a degraded version of the original algorithm in this answer   and this answer using my algorithm created 13 years ago    October 2000 I published an algorithm for this exact problem which was widely adapted  modified  then eventually poorly understood and forgotten  To remedy this issue  in August  2008 I published an article http   www webreference com programming javascript jkm3 3 html explaining the algorithm and using it as an example of simple of general-purpose JavaScript optimizations  By now  Web Reference has scrubbed my contact information and even my name from this article  And once again  the algorithm has been widely adapted  modified  then poorly understood and largely forgotten      Original string repetition multiplication JavaScript algorithm by   Joseph Myers  circa Y2K as a text multiplying function within Text js    published August  2008 in this form by Web Reference    http   www webreference com programming javascript jkm3 3 html  The   article used the function as an example of JavaScript optimizations    which is the only for the strange name  stringFill3            Usage  stringFill3  abc   2      abcabc       function stringFill3 x  n        var s           for                if  n  amp  1  s    x          n  gt  gt   1          if  n  x    x          else break            return s      Within two months after publication of that article  this same question was posted to Stack Overflow and flew under my radar until now  when apparently the original algorithm for this problem has once again been forgotten  The best solution available on this Stack Overflow page is a modified version of my solution  possibly separated by several generations  Unfortunately  the modifications ruined the solution s optimality  In fact  by changing the structure of the loop from my original  the modified solution performs a completely unneeded extra step of exponential duplicating  thus joining the largest string used in the proper answer with itself an extra time and then discarding it    Below ensues a discussion of some JavaScript optimizations related to all of the answers to this problem and for the benefit of all   Technique  Avoid references to objects or object properties  To illustrate how this technique works  we use a real-life JavaScript function which creates strings of whatever length is needed  And as we ll see  more optimizations can be added   A function like the one used here is to create padding to align columns of text  for formatting money  or for filling block data up to the boundary  A text generation function also allows variable length input for testing any other function that operates on text  This function is one of the important components of the JavaScript text processing module   As we proceed  we will be covering two more of the most important optimization techniques while developing the original code into an optimized algorithm for creating strings  The final result is an industrial-strength  high-performance function that I ve used everywhere--aligning item prices and totals in JavaScript order forms  data formatting and email   text message formatting and many other uses   Original code for creating strings stringFill1    function stringFill1 x  n         var s            while  s length  lt  n  s    x       return s         Example of output  stringFill1  x   3      xxx        The syntax is here is clear  As you can see  we ve used local function variables already  before going on to more optimizations   Be aware that there s one innocent reference to an object property s length in the code that hurts its performance  Even worse  the use of this object property reduces the simplicity of the program by making the assumption that the reader knows about the properties of JavaScript string objects   The use of this object property destroys the generality of the computer program  The program assumes that x must be a string of length one  This limits the application of the stringFill1   function to anything except repetition of single characters  Even single characters cannot be used if they contain multiple bytes like the HTML entity  amp nbsp    The worst problem caused by this unnecessary use of an object property is that the function creates an infinite loop if tested on an empty input string x  To check generality  apply a program to the smallest possible amount of input  A program which crashes when asked to exceed the amount of available memory has an excuse  A program like this one which crashes when asked to produce nothing is unacceptable  Sometimes pretty code is poisonous code   Simplicity may be an ambiguous goal of computer programming  but generally it s not  When a program lacks any reasonable level of generality  it s not valid to say   The program is good enough as far as it goes   As you can see  using the string length property prevents this program from working in a general setting  and in fact  the incorrect program is ready to cause a browser or system crash   Is there a way to improve the performance of this JavaScript as well as take care of these two serious problems   Of course  Just use integers   Optimized code for creating strings stringFill2    function stringFill2 x  n         var s            while  n--  gt  0  s    x       return s        Timing code to compare stringFill1   and stringFill2    function testFill functionToBeTested  outputSize         var i   0  t0   new Date         do            functionToBeTested  x   outputSize            t   new Date   - t0           i           while  t  lt  2000        return t i 1000      seconds1   testFill stringFill1  100    seconds2   testFill stringFill2  100      The success so far of stringFill2    stringFill1   takes 47 297 microseconds  millionths of a second  to fill a 100-byte string  and stringFill2   takes 27 68 microseconds to do the same thing  That s almost a doubling in performance by avoiding a reference to an object property   Technique  Avoid adding short strings to long strings  Our previous result looked good--very good  in fact  The improved function stringFill2   is much faster due to the use of our first two optimizations  Would you believe it if I told you that it can be improved to be many times faster than it is now   Yes  we can accomplish that goal  Right now we need to explain how we avoid appending short strings to long strings   The short-term behavior appears to be quite good  in comparison to our original function  Computer scientists like to analyze the  asymptotic behavior  of a function or computer program algorithm  which means to study its long-term behavior by testing it with larger inputs  Sometimes without doing further tests  one never becomes aware of ways that a computer program could be improved  To see what will happen  we re going to create a 200-byte string   The problem that shows up with stringFill2    Using our timing function  we find that the time increases to 62 54 microseconds for a 200-byte string  compared to 27 68 for a 100-byte string  It seems like the time should be doubled for doing twice as much work  but instead it s tripled or quadrupled  From programming experience  this result seems strange  because if anything  the function should be slightly faster since work is being done more efficiently  200 bytes per function call rather than 100 bytes per function call   This issue has to do with an insidious property of JavaScript strings  JavaScript strings are  immutable    Immutable means that you cannot change a string once it s created  By adding on one byte at a time  we re not using up one more byte of effort  We re actually recreating the entire string plus one more byte   In effect  to add one more byte to a 100-byte string  it takes 101 bytes worth of work  Let s briefly analyze the computational cost for creating a string of N bytes  The cost of adding the first byte is 1 unit of computational effort  The cost of adding the second byte isn t one unit but 2 units  copying the first byte to a new string object as well as adding the second byte   The third byte requires a cost of 3 units  etc   C N    1   2   3         N   N N 1  2   O N 2   The symbol O N 2  is pronounced Big O of N squared  and it means that the computational cost in the long run is proportional to the square of the string length  To create 100 characters takes 10 000 units of work  and to create 200 characters takes 40 000 units of work   This is why it took more than twice as long to create 200 characters than 100 characters  In fact  it should have taken four times as long  Our programming experience was correct in that the work is being done slightly more efficiently for longer strings  and hence it took only about three times as long  Once the overhead of the function call becomes negligible as to how long of a string we re creating  it will actually take four times as much time to create a string twice as long    Historical note  This analysis doesn t necessarily apply to strings in source code  such as html    abcd n     efgh n           xyz  n   since the JavaScript source code compiler can join the strings together before making them into a JavaScript string object  Just a few years ago  the KJS implementation of JavaScript would freeze or crash when loading long strings of source code joined by plus signs  Since the computational time was O N 2  it wasn t difficult to make Web pages which overloaded the Konqueror Web browser or Safari  which used the KJS JavaScript engine core  I first came across this issue when I was developing a markup language and JavaScript markup language parser  and then I discovered what was causing the problem when I wrote my script for JavaScript Includes    Clearly this rapid degradation of performance is a huge problem  How can we deal with it  given that we cannot change JavaScript s way of handling strings as immutable objects  The solution is to use an algorithm which recreates the string as few times as possible   To clarify  our goal is to avoid adding short strings to long strings  since in order to add the short string  the entire long string also must be duplicated   How the algorithm works to avoid adding short strings to long strings  Here s a good way to reduce the number of times new string objects are created  Concatenate longer lengths of string together so that more than one byte at a time is added to the output   For instance  to make a string of length N   9   x    x    s        s    x     Now s    x      x    x     Now x    xx      x    x     Now x    xxxx      x    x     Now x    xxxxxxxx      s    x     Now s    xxxxxxxxx  as desired      Doing this required creating a string of length 1  creating a string of length 2  creating a string of length 4  creating a string of length 8  and finally  creating a string of length 9  How much cost have we saved   Old cost C 9    1   2   3   4   5   6   7   9   45   New cost C 9    1   2   4   8   9   24   Note that we had to add a string of length 1 to a string of length 0  then a string of length 1 to a string of length 1  then a string of length 2 to a string of length 2  then a string of length 4 to a string of length 4  then a string of length 8 to a string of length 1  in order to obtain a string of length 9  What we re doing can be summarized as avoiding adding short strings to long strings  or in other words  trying to concatenate strings together that are of equal or nearly equal length   For the old computational cost we found a formula N N 1  2  Is there a formula for the new cost  Yes  but it s complicated  The important thing is that it is O N   and so doubling the string length will approximately double the amount of work rather than quadrupling it   The code that implements this new idea is nearly as complicated as the formula for the computational cost  When you read it  remember that  gt  gt   1 means to shift right by 1 byte  So if n   10011 is a binary number  then n  gt  gt   1 results in the value n   1001   The other part of the code you might not recognize is the bitwise and operator  written  amp   The expression n  amp  1 evaluates true if the last binary digit of n is 1  and false if the last binary digit of n is 0   New highly-efficient stringFill3   function  function stringFill3 x  n         var s            for                 if  n  amp  1  s    x           n  gt  gt   1           if  n  x    x           else break              return s        It looks ugly to the untrained eye  but it s performance is nothing less than lovely   Let s see just how well this function performs  After seeing the results  it s likely that you ll never forget the difference between an O N 2  algorithm and an O N  algorithm   stringFill1   takes 88 7 microseconds  millionths of a second  to create a 200-byte string  stringFill2   takes 62 54  and stringFill3   takes only 4 608  What made this algorithm so much better  All of the functions took advantage of using local function variables  but taking advantage of the second and third optimization techniques added a twenty-fold improvement to performance of stringFill3     Deeper analysis  What makes this particular function blow the competition out of the water   As I ve mentioned  the reason that both of these functions  stringFill1   and stringFill2    run so slowly is that JavaScript strings are immutable  Memory cannot be reallocated to allow one more byte at a time to be appended to the string data stored by JavaScript  Every time one more byte is added to the end of the string  the entire string is regenerated from beginning to end   Thus  in order to improve the script s performance  one must precompute longer length strings by concatenating two strings together ahead of time  and then recursively building up the desired string length   For instance  to create a 16-letter byte string  first a two byte string would be precomputed  Then the two byte string would be reused to precompute a four-byte string  Then the four-byte string would be reused to precompute an eight byte string  Finally  two eight-byte strings would be reused to create the desired new string of 16 bytes  Altogether four new strings had to be created  one of length 2  one of length 4  one of length 8 and one of length 16  The total cost is 2   4   8   16   30   In the long run this efficiency can be computed by adding in reverse order and using a geometric series starting with a first term a1   N and having a common ratio of r   1 2  The sum of a geometric series is given by a 1    1-r    2N   This is more efficient than adding one character to create a new string of length 2  creating a new string of length 3  4  5  and so on  until 16  The previous algorithm used that process of adding a single byte at a time  and the total cost of it would be n  n   1    2   16  17    2   8  17    136   Obviously  136 is a much greater number than 30  and so the previous algorithm takes much  much more time to build up a string   To compare the two methods you can see how much faster the recursive algorithm  also called  divide and conquer   is on a string of length 123 457  On my FreeBSD computer this algorithm  implemented in the stringFill3   function  creates the string in 0 001058 seconds  while the original stringFill1   function creates the string in 0 0808 seconds  The new function is 76 times faster   The difference in performance grows as the length of the string becomes larger  In the limit as larger and larger strings are created  the original function behaves roughly like C1  constant  times N 2  and the new function behaves like C2  constant  times N   From our experiment we can determine the value of C1 to be C1   0 0808    123457 2    00000000000530126997  and the value of C2 to be C2   0 001058   123457    00000000856978543136  In 10 seconds  the new function could create a string containing 1 166 890 359 characters  In order to create this same string  the old function would need 7 218 384 seconds of time   This is almost three months compared to ten seconds   I m only answering  several years late  because my original solution to this problem has been floating around the Internet for more than 10 years  and apparently is still poorly-understood by the few who do remember it  I thought that by writing an article about it here I would help   Performance Optimizations for High Speed JavaScript   Page 3  Unfortunately  some of the other solutions presented here are still some of those that would take three months to produce the same amount of output that a proper solution creates in 10 seconds   I want to take the time to reproduce part of the article here as a canonical answer on Stack Overflow   Note that the best-performing algorithm here is clearly based on my algorithm and was probably inherited from someone else s 3rd or 4th generation adaptation  Unfortunately  the modifications resulted in reducing its performance  The variation of my solution presented here perhaps did not understand my confusing for      expression which looks like the main infinite loop of a server written in C  and which was simply designed to allow a carefully-positioned break statement for loop control  the most compact way to avoid exponentially replicating the string one extra unnecessary time

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desc  repeat string    param  n - times    param  d - delimiter       String prototype repeat   function  n  d        return --n   this    d          this repeat n  d         this      this is how to repeat string several times using delimeter

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To repeat a string in a specified number of times  we can use the built-in repeat   method in JavaScript  Here is an example that repeats the following string for 4 times  const name    quot king quot    const repeat   name repeat 4    console log repeat    Output   quot kingkingkingking quot   or we can create our own verison of repeat   function like this  function repeat str  n      if   str     n        return        let final    quot  quot     while  n        final    s      n--        return final     console log repeat  quot king quot   3     originally posted at https   reactgo com javascript-repeat-string

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String prototype repeat is now ES6 Standard    abc  repeat 3     abcabcabc

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I ve tested the performance of all the proposed approaches   Here is the fastest variant I ve got   String prototype repeat   function count        if  count  lt  1  return         var result       pattern   this valueOf        while  count  gt  1            if  count  amp  1  result    pattern          count  gt  gt   1  pattern    pattern            return result   pattern       Or as stand-alone function   function repeat pattern  count        if  count  lt  1  return         var result           while  count  gt  1            if  count  amp  1  result    pattern          count  gt  gt   1  pattern    pattern            return result   pattern      It is based on artistoex algorithm  It is really fast  And the bigger the count  the faster it goes compared with the traditional new Array count   1  join string  approach     I ve only changed 2 things    replaced pattern   this with pattern   this valueOf    clears one obvious type conversion   added if  count  lt  1  check from prototypejs to the top of function to exclude unnecessary actions in that case  applied optimisation from Dennis answer  5-7  speed up    UPD  Created a little performance-testing playground here for those who interested   variable count   0    100       constant count   1024       Use it and make it even faster if you can

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Just another repeat function   function repeat s  n      var str         for  var i   0  i  lt  n  i          str    s        return str

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To repeat a string in a specified number of times  we can use the built-in repeat   method in JavaScript  Here is an example that repeats the following string for 4 times  const name    quot king quot    const repeat   name repeat 4    console log repeat    Output   quot kingkingkingking quot   or we can create our own verison of repeat   function like this  function repeat str  n      if   str     n        return        let final    quot  quot     while  n        final    s      n--        return final     console log repeat  quot king quot   3     originally posted at https   reactgo com javascript-repeat-string

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function repeat s  n    var r     for  var a 0 a lt n a    r  s  return r

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If you think all those prototype definitions  array creations  and join operations are overkill  just use a single line code where you need it  String S repeating N times   for  var i   0  result       i  lt  N  i    result    S

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Use Array N 1  join  string to repeat

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Simple method   String prototype repeat   function num        num   parseInt num       if  num  lt  0  return         return new Array num   1  join this

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function repeat pattern  count      for  var result               if  count  amp  1          result    pattern            if  count  gt  gt   1          pattern    pattern        else         return result                You can test it at JSFiddle  Benchmarked against the hacky Array join and mine is  roughly speaking  10  Chrome  to 100  Safari  to 200  Firefox  times faster  depending on the browser

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desc  repeat string    param  n - times    param  d - delimiter       String prototype repeat   function  n  d        return --n   this    d          this repeat n  d         this      this is how to repeat string several times using delimeter

[javascript] Repeat String - Javascript

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