Why does my text disappear in javascript? [duplicate]

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>

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);