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

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

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


Solution 1:

In JavaScript, strings are immutable, which means the best you can do is 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

Solution 2:

There is no replaceAt function in JavaScript. You can use the following code to replace any character in any string at specified position:

function rep() {
    var str = 'Hello World';
    str = setCharAt(str,4,'a');

function setCharAt(str,index,chr) {
    if(index > str.length-1) return str;
    return str.substr(0,index) + chr + str.substr(index+1);
<button onclick="rep();">click</button>

Solution 3:

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.substr(0, index) + 'x' + s.substr(index + 1);

Solution 4:

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<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).


  • 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.

Solution 5:

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:

String.prototype.replaceAt=function(index, char) {
    var a = this.split("");
    a[index] = char;
    return a.join("");

var str = "hello world";
str = str.replaceAt(3, "#");


Solution 6:

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’