How does this replaceAt function work?

Question

Could you please explain how this piece of code works?

String.prototype.replaceAt = function(index, character) {
    return this.substr(0, index) + characte         


        

Answer 1

Suppose you execute "thisisatest".replaceAt(3, "h").

Then...

1. this.substr(0, index) returns "thi" : ie the first 3 characters of "thisisatest"
2. character returns "h"
3. this.substr(index+character.length) returns "isatest" : ie all characters of "thisisatest", starting at position 4

So, when you combine this, you get "thihisatest"