How do I replace a character at a particular index in JavaScript

Question

I have a string  let s say Hello world and I need to replace the char at index 3  How can I replace a char by specifying a index   var str    hello world     I need something like  str replaceAt 0  h

User · Answer

I know this is old but the solution does not work for negative index so I add a patch to it  hope it helps someone  String prototype replaceAt function index  character        if index gt -1  return this substr 0  index    character   this substr index character length       else return this substr 0  this length index    character   this substr index character length

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There are lot of answers here  and all of them are based on two methods    METHOD1  split the string using two substrings and stuff the character between them METHOD2  convert the string to character array  replace one array member and join it   Personally  I would use these two methods in different cases  Let me explain    FabioPhms  Your method was the one I initially used and I was afraid that it is bad on string with lots of characters  However  question is what s a lot of characters  I tested it on 10  lorem ipsum  paragraphs and it took a few milliseconds  Then I tested it on 10 times larger string - there was really no big difference  Hm    vsync   Cory Mawhorter  Your comments are unambiguous  however  again  what is a large string  I agree that for 32   100kb performance should better and one should use substring-variant for this one operation of character replacement   But what will happen if I have to make quite a few replacements   I needed to perform my own tests to prove what is faster in that case  Let s say we have an algorithm that will manipulate a relatively short string that consists of 1000 characters  We expect that in average each character in that string will be replaced  100 times  So  the code to test something like this is   var str         A LARGE STRING HERE         for var i 0  i lt 100000  i        var n        Math floor Math random     10     var p   Math floor Math random     1000        replace character  n  on position  p      I created a fiddle for this  and it s here  There are two tests  TEST1  substring  and TEST2  array conversion    Results    TEST1  195ms TEST2  6ms   It seems that array conversion beats substring by 2 orders of magnitude  So - what the hell happened here     What actually happens is that all operations in TEST2 are done on array itself  using assignment expression like strarr2 p    n  Assignment is really fast compared to substring on a large string  and its clear that it s going to win   So  it s all about choosing the right tool for the job  Again

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str   str split      str 3     h   str   str join

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x000D   x000D  function dothis       var x   document getElementById  x   value    var index   document getElementById  index   value    var text   document getElementById  text   value    var length   document getElementById  length   value    var arr   x split        arr splice index  length  text     var result   arr join        document getElementById  output   innerHTML   result    console log result     dothis    x000D   lt input id  x  type  text  value  White Dog  placeholder  Enter Text    gt   lt input id  index  type  number  min  0 value  6  style  width 50px  placeholder  index    gt   lt input id  length  type  number  min  0 value  1  style  width 50px  placeholder  length    gt   lt input id  text  type  text  value  F  placeholder  New character    gt   lt br gt   lt button id  submit  onclick  dothis    gt Run lt  button gt   lt p id  output  gt  lt  p gt  x000D   x000D   x000D   This method is good for small length strings but may be slow for larger text  var x    quot White Dog quot   var arr   x split  quot  quot         quot W quot    quot h quot    quot i quot    quot t quot    quot e quot    quot   quot    quot D quot    quot o quot    quot g quot   arr splice 6  1   F           Here 6 is starting index and 1 is no  of array elements to remove and    final argument  F  is the new character to be inserted      var result   arr join  quot  quot        quot White Fog quot

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You could try  var strArr   str split       strArr 0     h    str   strArr join

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I did a function that does something similar to what you ask  it checks if a character in string is in an array of not allowed characters if it is it replaces it with         var validate   function value           var notAllowed              gt     lt                 amp                                 for var i 0  i lt value length  i                 if notAllowed indexOf value charAt i    gt  -1                  value   value replace value charAt i                       value   validate value                                return value

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You can extend the string type to include the inset method    x000D   x000D  String prototype append   function  index value    x000D    return this slice 0 index    value   this slice index   x000D     x000D   x000D  var s    New string   x000D  alert s append 4  complete      x000D   x000D   x000D    Then you can call the function

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this is easily achievable with RegExp   const str    Hello RegEx    const index   11  const replaceWith    p       Hello RegEx   replace      11          1p    str replace new RegExp         index             1   replaceWith          lt   Hello RegExp

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In Javascript strings are immutable so you have to do something like  var x    Hello world  x   x substring 0  i     h    x substring i 1     To replace the character in x at i with  h

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Generalizing Afanasii Kurakin s answer  we have   x000D   x000D  function replaceAt str  index  ch        return str replace    g   c  i    gt  i    index   ch   c      let str    Hello World   str   replaceAt str  1   u    console log str      Hullo World x000D   x000D   x000D   Let s expand and explain both the regular expression and the replacer function   x000D   x000D  function replaceAt str  index  newChar        function replacer origChar  strIndex            if  strIndex     index              return newChar          else             return origChar            return str replace    g  replacer      let str    Hello World   str   replaceAt str  1   u    console log str      Hullo World x000D   x000D   x000D   The regular expression   matches exactly one character  The g makes it match every character in a for loop  The replacer function is called given both the original character and the index of where that character is in the string  We make a simple if statement to determine if we re going to return either origChar or newChar

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Lets say you want to replace Kth index  0-based index  with  Z    You could use Regex to do this   var re   var re   new RegExp           K           1     str replace re    1A

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CemKalyoncu  Thanks for the great answer   I also adapted it slightly to make it more like the Array splice method  and took  Ates  note into consideration    spliceString function string  index  numToDelete  char          return string substr 0  index    char   string substr index numToDelete         var myString  hello world    spliceString myString myString lastIndexOf  l   2  mhole        hello wormhole

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If you want to replace characters in string  you should create mutable strings  These are essentially character arrays  You could create a factory     function MutableString str        var result   str split          result toString   function           return this join                return result        Then you can access the characters and the whole array converts to string when used as string     var x   MutableString  Hello      x 0     B      yes  we can alter the character   x push          good performance  no new string is created   var y    Hi    x     converted to string   Hi  Bello

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The methods on here are complicated  I would do it this way   var myString    this is my string   myString   myString replace myString charAt number goes here    insert replacement here      This is as simple as it gets

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Here is my solution using the ternary and map operator  More readable  maintainable end easier to understand if you ask me   It is more into es6 and best practices    x000D   x000D  function replaceAt     x000D    const replaceAt   document getElementById  replaceAt   value  x000D   x000D    const str    ThisIsATestStringToReplaceCharAtSomePosition   x000D    const newStr   Array from str  map  character  charIndex    gt  charIndex      replaceAt - 1         character  join      x000D   x000D    console log  New string    newStr     x000D    x000D   lt input type  number  id  replaceAt  min  1  max  44  oninput  replaceAt     gt  x000D   x000D   x000D

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x000D   x000D  var str    hello world   console log str   var arr       str   arr 0     H   str   arr join      console log str   x000D   x000D   x000D

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Here is a version I came up with if you want to style words or individual characters at their index in react javascript   replaceAt  yourArrayOfIndexes  yourString orArrayOfStrings      Working example  https   codesandbox io s ov7zxp9mjq  function replaceAt indexArray      string         const replaceValue   i   gt  string i     lt b gt  string i   lt  b gt       indexArray forEach replaceValue       return string      And here is another alternate method  function replaceAt indexArray      string         const startTag     lt b gt        const endTag     lt  b gt        const tagLetter   i   gt  string splice i  1  startTag   string i    endTag       indexArray forEach tagLetter       return string join          And another     function replaceAt indexArray      string         for  let i   0  i  lt  indexArray length  i              string   Object assign string               indexArray i     lt b gt  string indexArray i    lt  b gt                        return string

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You can use the following function to replace Character or String at a particular position of a String  To replace all the following match cases use String prototype replaceAllMatches   function   String prototype replaceMatch   function matchkey  replaceStr  matchIndex        var retStr   this  repeatedIndex   0      for  var x   0   matchkey    null   amp  amp   retStr indexOf matchkey   gt  -1   x              if  repeatedIndex    0  amp  amp  x    0                repeatedIndex   retStr indexOf matchkey             else      matchIndex  gt  0             repeatedIndex   retStr indexOf matchkey  repeatedIndex   1                     if  x    matchIndex                retStr   retStr substring 0  repeatedIndex    replaceStr   retStr substring repeatedIndex    matchkey length                matchkey   null     To break the loop                      return retStr       Test   var str    yash yas  dfdas       console log  Index Matched replace      str replaceMatch  as        2     console log  Index Matched replace      str replaceMatch  y        1       Output   Index Matched replace    yash yas  dfd     Index Matched replace    yash  as  dfdas

User · Answer

In JavaScript  strings are immutable  which means the best you can do is to create a new string with the changed content and assign the variable to point to it   You ll need to define the replaceAt   function yourself   String prototype replaceAt   function index  replacement        return this substr 0  index    replacement   this substr index   replacement length       And use it like this   var hello    Hello World   alert hello replaceAt 2             Should display He  o World

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One-liner using String replace with callback  no emoji support       0 - index to replace   f  - replacement string  dog  replace    g   c  i    gt  i    0   f   c      fog    Explained     String replace will call the callback on each pattern match   in this case - each character  dog  replace    g  function  character  index       if  index    0    we want to replace the first character      return  f     return character   leaving other characters the same

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This works similar to Array splice   String prototype splice   function  i  j  str        return this substr 0  i    str   this substr j  this length

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Work with vectors is usually most effective to contact String   I suggest the following function   String prototype replaceAt function index  char        var a   this split          a index    char      return a join          Run this snippet    x000D   x000D  String prototype replaceAt function index  char    x000D      var a   this split      x000D      a index    char  x000D      return a join      x000D    x000D   x000D  var str    hello world   x000D  str   str replaceAt 3        x000D   x000D  document write str   x000D   x000D   x000D

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You can t  Take the characters before and after the position and concat into a new string   var s    Hello world   var index   3  s   s substring 0  index     x    s substring index   1

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There is no replaceAt function in JavaScript  You can use the following code to replace any character in any string at specified position   x000D   x000D  function rep         var str    Hello World       str   setCharAt str 4  a        alert str      function setCharAt str index chr        if index  gt  str length-1  return str      return str substring 0 index    chr   str substring index 1     x000D   lt button onclick  rep     gt click lt  button gt  x000D   x000D   x000D

[javascript] How do I replace a character at a particular index in JavaScript?

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