Is there a JavaScript function that can pad a string to get to a determined length

Question

I am in need of a JavaScript function which can take a value and pad it to a given length  I need spaces  but anything would do   I found this  Code  String prototype pad   function l  s  t       return s     s    quot   quot     l -  this length   gt  0    s   new Array Math ceil l   s length            1  join s   substr 0  t    t   l   t    1   0   Math ceil l   2             this   s substr 0  l - t    this      Example  var s    quot Jonas quot   document write        lt h2 gt S     bold    s   quot  lt  h2 gt  quot        S pad 20   quot    quot   0      bold    s pad 20   quot    quot   0    quot  lt br   gt  quot        S pad 20   quot        quot   1      bold    s pad 20   quot        quot   1    quot  lt br   gt  quot        S pad 20   quot   quot   2      bold    s pad 20   quot   quot   2      But I have no idea what the heck it is doing and it doesn t seem to work for me

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Taking up Samuel s ideas  upward here  And remember an old SQL script  I tried with this   a 1234   0000  slice a toString   length  a    It works in all the cases I could imagine   a      1 result  0001 a     12 result  0012 a    123 result  0123 a   1234 result  1234 a  12345 result 12345 a    12  result  0012

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Never insert data somewhere  especially not at beginning  like str   pad   str    since the data will be reallocated everytime  Append always at end  Don t pad your string in the loop  Leave it alone and build your pad string first  In the end concatenate it with your main string  Don t assign padding string each time  like str    pad    It is much faster to append the padding string to itself and extract first x-chars  the parser can do this efficiently if you extract from first char   This is exponential growth  which means that it wastes some memory temporarily  you should not do this with extremely huge texts      x000D   x000D  if   String prototype lpad    x000D      String prototype lpad   function pad  len    x000D          while  pad length  lt  len    x000D              pad    pad  x000D            x000D          return pad substr 0  len-this length    this  x000D        x000D    x000D   x000D  if   String prototype rpad    x000D      String prototype rpad   function pad  len    x000D          while  pad length  lt  len    x000D              pad    pad  x000D            x000D          return this   pad substr 0  len-this length   x000D        x000D    x000D   x000D   x000D

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use repeat  it would be more simple       var padLeft function str  pad  fw           return fw gt str length   pad repeat fw-str length  str   str          you can use it like   padeLeft  origin-str    0   20

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Array manipulations are really slow compared to simple string concat  Of course  benchmark for your use case   function string  length  pad char  append        string   string toString        length   parseInt length     1      pad char   pad char              while  string length  lt  length            string   append   string pad char   pad char string            return string

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Using the ECMAScript 6 method String repeat and Arrow functions  a pad function is as simple as   var leftPad    s  c  n    gt  c repeat n - s length    s  leftPad  foo    0   5     returns  00foo    jsfiddle  edit  suggestion from the comments   const leftPad    s  c  n    gt  n - s length  gt  0   c repeat n - s length    s   s    this way  it wont throw an error when s lengthis greater than n  edit2  suggestion from the comments   const leftPad    s  c  n    gt   s   s toString    c   c toString    return s length  gt  n   s   c repeat n - s length    s      this way  you can use the function for strings and non-strings alike

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A variant of  Daniel LaFavers  answer   var mask   function  background  foreground      bg    new String background      fg    new String foreground      bgl   bg length    fgl   fg length    bgs   bg substring 0  Math max 0  bgl - fgl      fgs   fg substring Math max 0  fgl - bgl      return bgs   fgs       For example   mask  00000   11           00011  mask  00011   00           00000  mask  2       3            3  mask  0       111          1  mask  fork                 f     mask          dog            dog

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Here is a JavaScript function that adds specified number of paddings with custom symble  the function takes three parameters        padMe --  string or number to left pad     pads  --  number of pads     padSymble --  custom symble  default is  0        function leftPad padMe  pads  padSymble             if  typeof padMe      undefined                  padMe                           if  typeof pads      undefined                  pads   0                      if  typeof padSymble      undefined                  padSymble    0                        var symble                var result                for var i 0  i  lt  pads   i                  symble    padSymble                     var length   symble length - padMe toString   length          result   symble substring 0  length           return result concat padMe toString                 Here are some results     leftPad 1   1    leftPad 1  4   0001    leftPad 1  4   0    0001    leftPad 1  4           1

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Here s a recursive approach to it    function pad width  string  padding       return  width  lt   string length    string   pad width  padding   string  padding      An example     pad 5   hi    0     gt   000hi

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A friend asked about using a JavaScript function to pad left  It turned into a little bit of an endeavor between some of us in chat to code golf it  This was the result   function l p t v       v     return v length gt  t v l p t p v        It ensures that the value to be padded is a string  and then if it isn t the length of the total desired length it will pad it once and then recurse  Here is what it looks like with more logical naming and structure  function padLeft pad  totalLength  value       value   value toString         if  value length  gt   totalLength            return value       else          return padLeft pad  totalLength  pad   value             The example we were using was to ensure that numbers were padded with 0 to the left to make a max length of 6  Here is an example set    x000D   x000D  function l p t v  v     return v length gt  t v l p t p v    x000D   x000D  var vals    6451 123 466750   x000D   x000D  var pad   l 0 6 vals 0      pad with 0 s  max length 6 x000D   x000D  var pads   vals map function i   return l 0 6 i      x000D   x000D  document write pads join   lt br   gt      x000D   x000D   x000D

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String prototype padStart   and String prototype padEnd   are currently TC39 candidate proposals  see github com tc39 proposal-string-pad-start-end  only available in Firefox as of April 2016  a polyfill is available

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pad with default values  I noticed that i mostly need the padLeft for time conversion   number padding  so i wrote this function  function padL a b c    string number length 2 char 0  return  new Array b  2  join c  0  a  slice -b      This simple function supports Number or String as input  default pad is 2 chars  default char is 0  so i can simply write  padL 1      01   if i add the second argument  pad width   padL 1 3      001   third parameter  pad char   padL  zzz  10  x       xxxxxxxzzz   EDIT  BananaAcid if you pass a undefined value or a 0 length string you get 0undefined  so   as suggested  function padL a b c    string number length 2 char 0  return  new Array  b  1  1  join c  0   a       slice - b  2       but this can also be achieved in a shorter way   function padL a b c    string number length 2 char 0  return  new Array b  2  join c  0   a  c  0   slice -b      works also with   padL 0  padL NaN  padL     padL undefined  padL false    And if you want to be able to pad in both ways    function pad a b c d    string number length 2 char 0 0 false Left-1 true Right return a  a  c  0  c new Array b  2  join c  0  d  a c  slice 0 b   c a  slice -b      which can be written in a shorter way without using slice   function pad a b c d    return a  a  c  0     b new Array    b  3 -a length  join c  0  d a b b a       Usage   pad   input     int or string  or undefined NaN false empty string           default 0 or PadCharacter     optional   PadLength     int  default 2   PadCharacter     string or int  default  0    PadDirection     bolean  default 0  padLeft  -  true or 1  is padRight          now if you try to pad  averylongword  with 2     thats not my problem     Said that i give you a tip   Most of the time if you pad you do it for the same value N times   Using any type of function inside a loop slows down the loop     So if you just wanna pad left some numbers inside a long list don t use functions to do this simple thing   use something like this   var arrayOfNumbers  1 2 3 4 5 6 7       paddedArray         len arrayOfNumbers length  while len--    paddedArray len    0000  arrayOfNumbers len   slice -4       if you don t know how the max padding size based on the numbers inside the array   var arrayOfNumbers  1 2 3 4 5 6 7 49095       paddedArray         len arrayOfNumbers length      search the highest number var arrayMax Function prototype apply bind Math max null      get that string length padSize  arrayMax arrayOfNumbers      length     create a Padding string padStr new Array padSize  join 0      and after you have all this static values cached start the loop  while len--    paddedArray len   padStr arrayOfNumbers len   slice -padSize    substr -padSize    console log paddedArray       0   00001  1   00002  2   00003  3   00004  4   00005  5   00006  6   00007  7   49095

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Try this -  function leftPad number         return  number  lt  9   0  number number       call like this var month 3  month leftPad month    output - month 04

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Here s my take  I m not so sure about it s performance  but I find it much more readable than other options I saw around here     var replicate   function len  char      return Array len 1  join char              var padr   function text  len  char      if  text length  gt   len  return text    return text   replicate len-text length  char

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Here is a simple answer in basically one line of code   var value   35    the numerical value var x   5    the minimum length of the string  var padded     00000    value  substr -x     Make sure the number of characters in you padding  zeros here  is at least as many as your intended minimum length  So really  to put it into one line  to get a result of  00035  in this case is   var padded     00000    35  substr -5

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Here s a simple function that I use   var pad function num field       var n        num      var w   n length      var l   field length      var pad   w  lt  l   l-w   0      return field substr 0 pad    n       For example   pad     20                   20 pad    321                  321 pad  12345                12345 pad     15  00000         00015 pad    999                  999 pad   cat                   cat

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For something like this  I might create a one-line function at the point where it is needed   var padleft    s c len    gt    while s length  lt  len  s   c   s  return s      Example     gt  console log  padleft   110    0   8      gt  00000110

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All options included      function padding stringToBePadded  paddingCharacter  totalLength  padLeftElseRight         will pad any string provided in first argument  with padding character provide in 2nd argument and truncate to lenght provided in third argument  padding left if 4th argument true or undefined  right if false          i e  padding  lode   x   10    -- gt    xxxxxxlode         i e  padding  lode   x   10  true   -- gt    xxxxxxlode         i e  padding  lode   x   10  false   -- gt    lodexxxxxx         i e  padding  12   0   5    -- gt    00012                padLeftElseRight   typeof padLeftElseRight      undefined    padLeftElseRight   true            if  stringToBePadded length  gt  totalLength              console log  string too long to be padded            return stringToBePadded            var paddingString   paddingCharacter repeat totalLength    make long string of padding characters     if   padLeftElseRight           return String paddingString stringToBePadded  slice -totalLength        else           return String stringToBePadded paddingString  slice 0 totalLength

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If you just want a very simple hacky one-liner to pad  just make a string of the desired padding character of the desired max padding length and then substring it to the length of what you want to pad     Example  padding the string store in e with spaces to 25 characters long   var e    hello   e   e                               substring e length    Result   hello                       If you want to do the same with a number as input just call  toString   on it before

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String prototype padLeft   function pad            var s   Array apply null  Array pad   map function     return  0      join       this          return s slice -1   Math max this length  pad             usage     123  padLeft 2  returns   123   12  padLeft 2  returns   12   1  padLeft 2  returns   01

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It s 2014  and I suggest a Javascript string-padding function  Ha   Bare-bones  right-pad with spaces  function pad   str  length         var padding     new Array  Math max  length - str length   1  0       join             return str   padding      Fancy  pad with options          param           str                         input string  or any other type  will be converted to string      param  number   length                      desired length to pad the string to     param  Object    opts      param  string    opts padWith               char to use for padding     param  boolean   opts padLeft false         whether to pad on the left     param  boolean   opts collapseEmpty false   whether to return an empty string if the input was empty     returns  string      function pad   str  length  opts         var padding     new Array  Math max  length -   str        length   1  0       join  opts  amp  amp  opts padWith                   collapse   opts  amp  amp  opts collapseEmpty  amp  amp     str        length      return collapse        opts  amp  amp  opts padLeft   padding   str   str   padding      Usage  fancy    pad   123   5       returns  123     pad  123  5       returns  123    - non-string input  pad   123   5    padWith   0   padLeft  true         returns  00123   pad      5       returns          pad      5    collapseEmpty  true         returns     pad   1234567   5       returns  1234567

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my combination of aboves solutions added to my own  always evolving version       in preperation for ES6 String prototype lpad     String prototype lpad   function  length  charOptional         if  length  lt   this length  return this      return   new Array  length  0  1  join String charOptional           this        slice  - length  0            abc  lpad 5            abc  String 5679  lpad 10 0      0000005679  String   lpad 4  -       ----     repeat string

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es7 is just drafts and proposals right now  but if you wanted to track compatibility with the spec  your pad functions need    Multi-character pad support  Don t truncate the input string Pad defaults to space   From my polyfill library  but apply your own due diligence for prototype extensions      Tests  hello  lpad 4       hello   hello  rpad 4       hello   hello  lpad 10            hello   hello  rpad 10       hello        hello  lpad 10   1234        41234hello   hello  rpad 10   1234        hello12341   String prototype lpad     String prototype lpad   function  length  pad         if  length  lt  this length   return this       pad   pad             let str   this       while  str length  lt  length                 str   pad   str             return str substr  -length         String prototype rpad     String prototype rpad   function  length  pad         if  length  lt  this length   return this       pad   pad             let str   this       while  str length  lt  length                 str    pad             return str substr  0  length

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I found this solution here and this is for me much much simpler   var n   123  String  00000    n  slice -5      returns 00123   00000    n  slice -5      returns 00123            n  slice -5      returns    123   with two spaces    And here I made an extension to the string object   String prototype paddingLeft   function  paddingValue       return String paddingValue   this  slice -paddingValue length        An example to use it   function getFormattedTime date      var hours   date getHours      var minutes   date getMinutes       hours   hours toString   paddingLeft  00      minutes   minutes toString   paddingLeft  00       return   0   1   format hours  minutes       String prototype format   function          var args   arguments      return this replace     d    g  function  match  number            return typeof args number      undefined    args number    match               This will return a time in the format  15 30

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A short way    x  gt  new Array int-x length 1   join char  x  String    Example    x  gt  new Array 6-x length 1   join  0   x   1234     return   001234

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Like PHP   const STR PAD RIGHT   1  const STR PAD LEFT   0  const STR PAD BOTH   2           see http   php net str pad     param mixed input      param integer length      param string string      param integer type      return string     function str pad input  length  string  type        if  type     undefined     type     STR PAD LEFT  amp  amp  type     STR PAD BOTH             type   STR PAD RIGHT            if  input toString   length  gt   length             return input        else           if  type     STR PAD BOTH                input    string   input   string             else if  type    STR PAD LEFT                input    string   input             else               input    input   string                      return str pad input toString    length  string  type

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Using the ECMAScript 6 method String repeat  a pad function is as simple as   String prototype padLeft   function char  length         return char repeat Math max 0  length - this length     this      String repeat is currently supported in Firefox and Chrome only  for other implementation  one might consider the following simple polyfill   String prototype repeat   String prototype repeat    function n        return n lt  1   this    this   this repeat n-1

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One liner if you want something compact    x000D   x000D  String prototype pad   function len  chr   x000D          return    new Array len   fill chr   join      this  substring this length    x000D    x000D   x000D   x000D

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Based on the best answers of this question I have made a prototype for String called padLeft  exactly like we have in C     String prototype padLeft   function  paddingChar  totalWidth        if  this toString   length  gt   totalWidth          return this toString         var array   new Array totalWidth         for  i   0  i  lt  array length  i            array i    paddingChar       return  array join       this toString    slice -array length       Usage   var str    12345   console log str padLeft  0   10      Result is   0000012345    JsFiddle

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A little late  but thought I might share anyway  I found it useful to add a prototype extension to Object  That way I can pad numbers and strings  left or right  I have a module with similar utilities I include in my scripts      include the module in your script  there is no need to export var jsAddOns   require   lt path to module gt  jsAddOns                   jsAddOns js                      method prototype for any Object to pad it s toString      representation with additional characters to the specified length        param padToLength required int        entire length of padded string  original   padding      param padChar optional char        character to use for padding  default is white space     param padLeft optional boolean        if true padding added to left        if omitted or false  padding added to right        return padded string or        original string if length is  gt   padToLength     Object prototype pad   function padToLength  padChar  padLeft                get the string value     s   this toString           default padToLength to 0        if omitted  original string is returned     padToLength   padToLength    0          default padChar to empty space     padChar   padChar                  ignore padding if string too long     if  s length  gt   padToLength            return s                create the pad of appropriate length     var pad   Array padToLength - s length  join padChar           add pad to right or left side     if  padLeft            return pad    s                else           return s   pad

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Pad a string to pad length fillig it with pad char  By default the function performs a left pad  unless pad right is set to true   If the value of pad length is negative  less than  or equal to the length of the input string  no padding takes place                                                                                                      if  String prototype pad  String prototype pad   function pad char  pad length  pad right        var result   this     if   typeof pad char      string    amp  amp   pad char length     1   amp  amp   pad length  gt  this length               var padding   new Array pad length - this length   1  join pad char     thanks to http   stackoverflow com questions 202605 repeat-string-javascript 2433358 2433358       result    pad right   result   padding   padding   result           return result      And then you can do   alert   3  pad  0   3       shows  003  alert   hi  pad      3       shows   hi  alert   hi  pad      3  true       shows  hi

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A faster method If you are doing this repeatedly  for example to pad values in an array  and performance is a factor  the following approach can give you nearly a 100x advantage in speed  jsPerf  over other solution that are currently discussed on the inter webs  The basic idea is that you are providing the pad function with a fully padded empty string to use as a buffer  The pad function just appends to string to be added to this pre-padded string  one string concat  and then slices or trims the result to the desired length  function pad pad  str  padLeft      if  typeof str      undefined        return pad    if  padLeft        return  pad   str  slice -pad length       else       return  str   pad  substring 0  pad length          For example  to zero pad a number to a length of 10 digits  pad  0000000000  123 true    To pad a string with whitespace  so the entire string is 255 characters  var padding   Array 256  join          make a string of 255 spaces pad padding 123 true    Performance Test See the jsPerf test here  And this is faster than ES6 string repeat by 2x as well  as shown by the revised JsPerf here Please note that jsPerf is no longer online Please note that the jsPerf site that we originally used to benchmark the various methods is no longer online  Unfortunately  this means we can t get to those test results  Sad but true

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With ES8  there are two options for padding   You can check them in the documentation   https   developer mozilla org en-US docs Web JavaScript Reference Global Objects String padEnd  https   developer mozilla org en-US docs Web JavaScript Reference Global Objects String padStart

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I think its better to avoid recursion because its costly   x000D   x000D  function padLeft str size padwith    x000D   if size  lt   str length    x000D             not padding is required  x000D    return str  x000D     else   x000D             1- take array of size equal to number of padding char   1  suppose if string is 55 and we want 00055 it means we have 3 padding char so array size should be 3   1   1 will explain below  x000D             2- now join this array with provided padding char  padwith  or default one   0    so it will produce  000  x000D             3- now append  000  with orginal string  str   55   will produce 00055 x000D   x000D             why  1 in size of array   x000D             it is a trick  that we are joining an array of empty element with  0   in our case  x000D             if we want to join items with  0  then we should have at least 2 items in the array to get joined  array with single item doesn t need to get joined   x000D              lt item gt 0 lt item gt 0 lt item gt 0 lt item gt  to get 3 zero we need 4  3 1  items in array    x000D    return Array size-str length 1  join padwith   0   str x000D     x000D    x000D   x000D  alert padLeft  59  5      n    x000D       padLeft  659  5      n    x000D       padLeft  5919  5      n    x000D       padLeft  59879  5      n    x000D       padLeft  5437899  5    x000D   x000D   x000D

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1  function var  padLeft   function paddingString  width  replacementChar        return paddingString length  gt   width   paddingString    padLeft replacementChar   paddingString  width  replacementChar              2  String prototype String prototype padLeft   function width  replacementChar            return this length  gt   width   this toString      replacementChar   this  padLeft width  replacementChar              3  slice   00000    paddingString  slice -5

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I like to do this in case you ever need to pad with multiple characters or tags  e g   amp nbsp   for display     padStringLeft   function s  pad  len        if typeof s      undefined             var c s length  while len  gt  c   s pad s c               return s           padStringRight   function s  pad  len        if typeof s      undefined             var c s length  while len  gt  c   s    pad c               return s

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The key trick in both those solutions is to create an array instance with a given size  one more than the desired length   and then to immediately call the join   method to make a string  The join   method is passed the padding string  spaces probably   Since the array is empty  the empty cells will be rendered as empty strings during the process of joining the array into one result string  and only the padding will remain  It s a really nice technique

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yet another take at with combination of a couple solutions         pad string on left     param  number  number of digits to pad  default is 2     param  string  string to use for padding  default is  0        returns  string  padded string     String prototype paddingLeft   function  b c        if  this length  gt   b  2           return this       return  this  c  0     b new Array    b  3 -this length  join c  0  b this            pad string on right     param  number  number of digits to pad  default is 2     param  string  string to use for padding  default is  0        returns  string  padded string     String prototype paddingRight   function  b c      if  this length  gt   b  2           return this       return  this  c  0     b new Array    b  3 -this length  join c  0  this b

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If you don t mind including a utility library  lodash library has   pad    padLeft and   padRight functions

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ECMAScript 2017 adds a padStart method to the String prototype  This method will pad a string with spaces to a given length  This method also takes an optional string that will be used instead of spaces for padding    abc  padStart 10                      abc   abc  padStart 10   foo         foofoofabc   abc  padStart 6  123465        123abc   abc  padStart 8   0            00000abc   abc  padStart 1                abc    A padEnd method was also added that works in the same manner   For browser compatibility  and a useful polyfill  see this link

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this is my version of function   function str pad str  size  char  right        var s   str           while  s length  lt  size            if  right                s   s   char            else               s   char   s                      return s

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padding string has been inplemented in new javascript version   str padStart targetLength    padString    https   developer mozilla org es docs Web JavaScript Referencia Objetos globales String padStart  If you want your own function check this example   const myString    Welcome to my house   String prototype padLeft   function times   0  str              return  Array times  join str    this     console log myString padLeft 12                      Welcome to my house

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http   www webtoolkit info javascript pad html           Javascript string pad    http   www webtoolkit info         var STR PAD LEFT   1  var STR PAD RIGHT   2  var STR PAD BOTH   3   function pad str  len  pad  dir         if  typeof len      undefined     var len   0        if  typeof pad      undefined     var pad              if  typeof dir      undefined     var dir   STR PAD RIGHT         if  len   1  gt   str length             switch  dir                case STR PAD LEFT                  str   Array len   1 - str length  join pad    str              break               case STR PAD BOTH                  var padlen   len - str length                  var right   Math ceil  padlen   2                    var left   padlen - right                  str   Array left 1  join pad    str   Array right 1  join pad               break               default                  str   str   Array len   1 - str length  join pad               break                switch             return str       It s a lot more readable

[javascript] Is there a JavaScript function that can pad a string to get to a determined length?

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