Create a string of variable length filled with a repeated character

Question

So  my question has been asked by someone else in it s Java form here  Java - Create a new String instance with specified length and filled with specific character  Best solution         but I m looking for its JavaScript equivalent   Basically  I m wanting to dynamically fill text fields with     characters  based on the  maxlength  attribute of each field   So  if an input has has maxlength  3   then the field would be filled with         Ideally there would be something like the Java StringUtils repeat      10    but  so far  the best option that I can think of is to loop through and append the     characters  one at a time  until the max length is reached   I can t shake the feeling that there is a more efficient way to do it than that   Any ideas   FYI - I can t simply set a default value in the input  because the     characters need to clear on focus  and  if the user didn t enter a value  will need to be  refilled  on blur  It s the  refill  step that I m concerned with

User · Answer

ES2015 the easiest way is to do something like

'X'.repeat(data.length)

X being any string, data.length being the desired length.

see: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/repeat

User · Answer

Give this a try  P   x000D   x000D  s       repeat 10  x000D   x000D  document body innerHTML   s x000D   x000D   x000D

User · Answer

Based on answers from Hogan and Zero Trick Pony  I think this should be both fast and flexible enough to handle well most use cases    var hash                                                                           function build string length          if  length    0              return            else if  hash length  lt   length              return hash substring 0  length          else             var result   hash           const half length   length   2           while  result length  lt   half length                  result    result                       return result   result substring 0  length - result length

User · Answer

A great ES6 option would be to padStart an empty string  Like this   var str      padStart 10          Note  this won t work in IE  without a polyfill

User · Answer

I would create a constant string and then call substring on it   Something like   var hashStore                                                var Fiveup   hashStore substring 0 5    var Tenup   hashStore substring 0 10     A bit faster too   http   jsperf com const-vs-join

User · Answer

Unfortunately although the Array join approach mentioned here is terse  it is about 10X slower than a string-concatenation-based implementation  It performs especially badly on large strings  See below for full performance details   On Firefox  Chrome  Node js MacOS  Node js Ubuntu  and Safari  the fastest implementation I tested was   function repeatChar count  ch        if  count    0            return               var count2   count   2      var result   ch          double the input until it is long enough      while  result length  lt   count2            result    result               use substring to hit the precise length target without        using extra memory     return result   result substring 0  count - result length        This is verbose  so if you want a terse implementation you could go with the naive approach  it still performs betweeb 2X to 10X better than the Array join approach  and is also faster than the doubling implementation for small inputs  Code      naive approach  simply add the letters one by one function repeatChar count  ch        var txt           for  var i   0  i  lt  count  i              txt    ch            return txt      Further information    Run speed test in your own browser Full source code of speed test Speed test results

User · Answer

The best way to do this  that I ve seen  is   var str   new Array len   1  join  character      That creates an array with the given length  and then joins it with the given string to repeat   The  join   function honors the array length regardless of whether the elements have values assigned  and undefined values are rendered as empty strings   You have to add 1 to the desired length because the separator string goes between the array elements

User · Answer

You can use the first line of the function as a one-liner if you like   function repeat str  len        while  str length  lt  len  str    str substr 0  len-str length       return str

User · Answer

Version that works in all browsers This function does what you want  and performs a lot faster than the option suggested in the accepted answer   var repeat   function str  count        var array           for var i   0  i  lt   count           array i      str      return array join         You use it like this   var repeatedCharacter   repeat  quot a quot   10    To compare the performance of this function with that of the option proposed in the accepted answer  see this Fiddle and this Fiddle for benchmarks  Version for moderns browsers only In modern browsers  you can now also do this   var repeatedCharacter    quot a quot  repeat 10      This option is even faster  However  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

User · Answer

For Evergreen browsers  this will build a staircase based on an incoming character and the number of stairs to build  function StairCase character  input        let i   0      while  i  lt  input            const spaces       repeat input -  i 1            const hashes   character repeat i   1           console log spaces   hashes           i               Implement   Refresh the console console clear    StairCase     6        You can also add a polyfill for Repeat for older browsers      if   String prototype repeat          String prototype repeat   function count             use strict           if  this    null              throw new TypeError  can  t convert     this     to object                      var str        this          count    count          if  count    count              count   0                    if  count  lt  0              throw new RangeError  repeat count must be non-negative                      if  count    Infinity              throw new RangeError  repeat count must be less than infinity                      count   Math floor count           if  str length    0    count    0              return                          Ensuring count is a 31-bit integer allows us to heavily optimize the            main part  But anyway  most current  August 2014  browsers can t handle            strings 1  lt  lt  28 chars or longer  so          if  str length   count  gt   1  lt  lt  28              throw new RangeError  repeat count must not overflow maximum string size                      var rpt               for                  if   count  amp  1     1                rpt    str                        count  gt  gt  gt   1            if  count    0                break                        str    str                       Could we try             return Array count   1  join this           return rpt

[javascript] Create a string of variable length, filled with a repeated character

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