How to write a mutator method in javascript?

Question

Array.prototype.clear = function(){
    this = new Array();
    return true;
}

That code raise an invalid assignment left-hand side error.

How do I change the object itself within one of his methods?

Answer 1

You can't change the reference of what the this value points to, it is immutable.

If you want to clear the current array, you can simply set its length property to zero:

Array.prototype.clear = function(){
  this.length = 0;
  return true;
};

Edit: Looking at the comment made to the sasuke's answer, you could empty the array as in my first example, and then push the elements of another array, for example:

Array.prototype.test = function () {
  var newArray = ['foo', 'bar']; // new array elements
  this.length = 0; // empty original
  this.push.apply(this, newArray); // push elements of new array
};

Answer 2

How about:

Array.prototype.clear = function(){
    this.length = 0;
    return true;
}