Question

Very glad to learn of this way of basically subclassing a JavaScript Array (code copied from link):

function SomeType() {
    this.push(16);
}

SomeType.prototype = [];
SomeType.prototype.constructor = SomeType; // Make sure there are no unexpected results

console.log(new SomeType()); // Displays in console as [16]

But this isn’t quite complete. Is there a way to fake subclass the Array like this and get the [] method?

var a = [];
a[3]  = true;
console.log(a.length); //=> 4

var s = new SomeType();
s[3]  = true;
console.log(s.length); //=> 1

This way you can still treat it as an array when doing a for loop:

for (var i = 0; i < s.length; i++) {
  var item = s[i];
}

Answer 1

Only works with browsers with __proto__ (which is deprecated ), so it’s not cross-browser :

var CustomArray = function ( ) {
  var array = [ 16 ];
  array.__proto__ = this.__proto__;
  return array;
};

CustomArray.prototype = [];
CustomArray.prototype.constructor = CustomArray;

var array = new CustomArray( );
console.log( array instanceof Array );       // true
console.log( array instanceof CustomArray ); // true

array[ 3 ] = 42;
console.log( array.length );                 // 4

I don’t think there is any other way to do it.