Question

According to this answer I want to create my own subclass of Array

QArray = function() {
    Array.apply(this, arguments);
};
QArray.prototype = new Array();
QArray.prototype.constructor = QArray;

which works as expected, method invocation works, but the constructor does not chain to Array.

// test constructor chaining
q = new QArray('1', '2', '3');
assert.equals('1', q[0]);      // => undefined
assert.equals(3, q.length);    // => 0
q = new QArray(10);
assert.equals(10, q.length);   // => 0

This tests pass if I replace QArray with plain Array. Somehow Array seems to be a special case. (I run this in Rhino 1.6 which is Javascript 1.5.) How can I fix my custom subclass of Array?

Answer 1

Since your method already inherits the array methods, you can use array methods which modify the keys, such as Array.prototype.push:

QArray = function() {
    if (arguments.length == 1) this.length = arguments[0]; // Array constructor
    else this.push.apply(this, arguments);
};
QArray.prototype = new Array();
QArray.prototype.constructor = QArray;

q = new QArray('1', '2', '3');   // => [1,2,3]
assert.equals('1', q[0]);        // => true
assert.equals(3, q.length);      // => true
q = new QArray(10);              // => [,,,,,,,,,]
assert.equals(10, q.length);     // => true
q instanceof QArray;             // => true
q instanceof Array;              // => true

I’ve once written a custom array implementation, which behaves like a true array (including length setter/getter). If you need inspiration, have a look at this answer.