How do I replace a character at a particular index

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");

Answer 1

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>-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);

}