String prototype modifying itself

Question

As far as i know it\'s not possible to modify an object from itself this way:

String.prototype.append = function(val){
    this = this + val;
}

Answer 1

Strings are immutable; what you're asking is like saying, "Why can't I do:

Number.prototype.accumulate = function (x) {
    this = this + x;
};

...?"