Subclassing Javascript Arrays. TypeError: Array.prototype.toString is not generic

Juriy Zaytsev (@kangax) just today released a really good article on the subject.

He explores various alternatives like the Dean Edwards iframe borrowing technique, direct object extension, prototype extension and the usage of ECMAScript 5 accessor properties.

At the end there is no perfect implementation, each one has its own benefits and drawbacks.

Definitely a really good read:

  • How ECMAScript 5 still does not allow to subclass an array

ES6

class SubArray extends Array {
    last() {
        return this[this.length - 1];
    }
}
var sub = new SubArray(1, 2, 3);
sub // [1, 2, 3]
sub instanceof SubArray; // true
sub instanceof Array; // true

Original Answer: (Not recommended, may cause performance issues)

Copy-pasting from article mentioned in the accepted answer for more visibility

Using __proto__

function SubArray() {
  var arr = [ ];
  arr.push.apply(arr, arguments);
  arr.__proto__ = SubArray.prototype;
  return arr;
}
SubArray.prototype = new Array;

Now you can add your methods to SubArray

SubArray.prototype.last = function() {
  return this[this.length - 1];
};

Initialize like normal Arrays

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

Behaves like normal Arrays

sub instanceof SubArray; // true
sub instanceof Array; // true

Tags:

Javascript

Inheritance

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