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

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, "!!")); // He!!o World

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');    alert(str);}function setCharAt(str,index,chr) {    if(index > str.length-1) return str;    return str.substring(0,index) + chr + str.substring(index+1);}

<button onclick="rep();">click</button>