What different between Object.prototype.toString and toString?

Question

As we know, we can use the Object.prototype.toString.call(arg) if we need to detect object class.

But In my test, Object.prototype.toString.call(arg) === toString.

So, why don't we use toString.call(arg) to replace Object.prototype.toString.call(arg) which is a long type.

In the Polyfill of Array.isArray, It also use Object.prototype.toString.call(arg).

different between Object.toString and Object.prototype.toString

These are totally different methods. instance.toString gives a string presentation of the instance of an object, and is often overridden. And as you've noticed, calling Object.prototype.toString gives a "Class" of the object. For example, consider the return values of [].toString() and Object.prototype.toString.call([]); ... — Teemu
– Teemu, Commented Jul 12, 2019 at 9:39

CertainPerformance · Accepted Answer · 2019-07-12 09:43:30Z

4

It's not necessary, it just makes the code's intent more clear. When you use the toString standalone function, usually that'll refer to window.toString - window is an object, and objects inherit from Object.prototype, so window.toString === Object.prototype.toString. But relying on this sort of thing implicitly can be confusing and can produce bugs.

There's also no guarantee that there's not some other function you've defined called toString, eg:

(() => {
  const toString = () => 'here is a string';
  
  // many lines of code here
  class Foo {}
  const f = new Foo();
  // Does not result in "[object Object]":
  console.log(toString.call(f));
})();

Explicitly using Object.prototype.toString instead makes the code more understandable and predictable at a glance.

(You're still free to use toString alone instead if you want, it probably won't break anything, but it's probably not a great idea in general.)

edited Jul 12, 2019 at 9:43

answered Jul 12, 2019 at 9:41

CertainPerformance

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5 Comments

Kobe Over a year ago

Why do you use an IIFE here?

Daan Over a year ago

@Kobe To avoid polluting the global namespace.

CertainPerformance Over a year ago

Not that important, it just distinguishes the locally scoped toString from the toString visible on the top level, though even if it was on the top level, It doesn't overwrite window.toString if it's declared with const. It doesn't really matter, just slightly reduces the possibility of confusion from looking at the snippet

Jonas Wilms Over a year ago

And window.toString might not necessarily be Object.prototype.toString (or does the Web Spec say that?) ...

CertainPerformance Over a year ago

@JonasWilms Yeah, could be, that's why I said usually. window.toString is writable, it's not even a property of window, it's just inheriting from Object.prototype

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