Why does "push method" works with object? How that mechanism works underhood?
function MyArray() { }
MyArray.prototype = [];
var arr = new MyArray();
arr.push(1, 2, 3);
console.log(arr); // [1, 2, 3] in Chrome
sorry for my english. Thanks!
Asked
Active
Viewed 57 times
1
Jim Button
- 151
- 1
- 8
-
arrays are object too – Rajshekar Reddy Dec 19 '16 at 17:50
-
You're assign the prototype to an array, then pushing to that array...? – Andrew Li Dec 19 '16 at 17:51
-
`MyArray` *is* an array, since you explicitly said you want it to be one via the `prototype`. – Heretic Monkey Dec 19 '16 at 17:51
-
@MikeMcCaughan could you please write a real answer for this. – Soviut Dec 19 '16 at 17:52
-
MyArray is not array – Jim Button Dec 19 '16 at 17:52
-
@Soviut I'm planning on voting to close the question, since it's likely a duplicate, so no, I will not write a real answer for this question. Thanks for the suggestion though. – Heretic Monkey Dec 19 '16 at 17:55
-
But of course Nina comes along and answers all of the questions I'm going to mark as duplicate... – Heretic Monkey Dec 19 '16 at 17:58
-
@Jim Button it is (mostly) ! youve defined that (MyArray.prototype=[]) – Jonas Wilms Dec 19 '16 at 18:03
-
Possible duplicate of [How does JavaScript .prototype work?](http://stackoverflow.com/questions/572897/how-does-javascript-prototype-work) – Heretic Monkey Dec 19 '16 at 18:06
-
@Jonasw No `({}).toString.call(arr) //"[object Object]", Array.isArray(arr) //false` – Jim Button Dec 19 '16 at 18:06
-
[] is an Object with a prototype={ push:...} if you do MyArray.prototype=[], push is inside of MyArrays prototype chain... it may not be an array in javascript but it behaves like that trough the prototyping – Jonas Wilms Dec 19 '16 at 18:10
-
for example, "concat method" is not working: jsfiddle.net/f8r6j24p – Jim Button Dec 20 '16 at 16:52
-
why not use an array? – Nina Scholz Dec 20 '16 at 17:05
-
@NinaScholz it is simple. But question is not about that :) – Jim Button Dec 20 '16 at 17:30
1 Answers
2
MyArray returns an object, even in Chrome, and uses the methods of Array, via assigned prototypal inheritance. The result of an instance of MyArray is still an object, not an array.
function MyArray() { }
MyArray.prototype = [];
var arr = new MyArray();
arr.push(1, 2, 3);
console.log(arr); // { 0: 1, 1: 2, 2: 3, length: 3 }
console.log(typeof arr); // object
console.log(Array.isArray(arr)); // false
console.log(arr instanceof Array); // true
console.log(arr instanceof MyArray); // true
Nina Scholz
- 376,160
- 25
- 347
- 392
-
-
-
for example, "concat method" not working: https://jsfiddle.net/f8r6j24p/ – Jim Button Dec 20 '16 at 16:49