Replace character at particular index

String.prototype.replaceAt.js

/**
 * String replace character at particular index(es)
 * @param  {Int or IntArray} index     [index to start]
 * @param  {String} character [character which you want to replace]
 * @return {String}           [Replaced character]
 */
String.prototype.replaceAt = function(index, character) {
    if (typeof index !== 'number' && index instanceof Array !== true) {
        throw Error('Please pass a number or an array of number as first argument and your index is ' + index);
    }

    if (index.length === 1 || typeof index === 'number') {
        return this.substr(0, index) + character + this.substr(index + character.length);
    } else {
        var result = this.toString();
        index.forEach(function (val) {
            result = result.substr(0, val) + character + result.substr(val + character.length);
        });
        return result;
    }
    return this;
};