how to add character into string in javasce code example

Example: how to add a letter to a string javascript

String.prototype.splice = function(idx, rem, str) {
    return this.slice(0, idx) + str + this.slice(idx + Math.abs(rem));
};

var result = "foo baz".splice(4, 0, "bar ");
document.body.innerHTML = result; // "foo bar baz"