Subclassing arrays in javascript doesn't work as expected

js_subclass_array.js

function subArray() {
    this.push.apply(this, arguments);
};

subArray.prototype = [];

var sub = new subArray(1,2,3);

alert(sub.length);  // 3

sub.push(4);

alert(sub.length); // 4

sub[10] = 10;

alert(sub.length);  // 4, should be 11