In JavaScript, why typeof Function.prototype is "function", not "object" like other prototyp

admin管理员组

文章数量:1287913

console.log(typeof String.prototype); // object
console.log(typeof Number.prototype); // object
console.log(typeof Object.prototype); // object
console.log(typeof Boolean.prototype); // object

console.log(typeof Function.prototype); // function

Why does typeof Function.prototype return "function", not "object" like other prototype objects?

Thank you!

console.log(typeof String.prototype); // object
console.log(typeof Number.prototype); // object
console.log(typeof Object.prototype); // object
console.log(typeof Boolean.prototype); // object

console.log(typeof Function.prototype); // function

Why does typeof Function.prototype return "function", not "object" like other prototype objects?

Thank you!

• javascript
• prototype

Share Improve this question Follow edited Oct 26, 2017 at 19:50 Andrew Truckle 19.1k1717 gold badges8282 silver badges215215 bronze badges asked Feb 1, 2011 at 5:38 weilouweilou 4,6171111 gold badges4545 silver badges5656 bronze badges 4

• 5 You could have asked the question without all the HTML. A good question though... – Ateş Göral Commented Feb 1, 2011 at 5:41
• 1 @AtesGoral Fixed now! – doubleOrt Commented Oct 26, 2017 at 19:58
• 1 @Taurus Better late (6 years) than sorry! – Ateş Göral Commented Oct 27, 2017 at 23:14
• 1 @AtesGoral indeed. – doubleOrt Commented Oct 27, 2017 at 23:55

Add a comment  | 

4 Answers 4

Sorted by: Reset to default 17

This seems to be defined in ECMAScript 5:

15.3.4 Properties of the Function Prototype Object

The Function prototype object is itself a Function object (its [[Class]] is "Function") that, when invoked, accepts any arguments and returns undefined.

Its mentioned in the ECMAScript2015

http://www.ecma-international.org/ecma-262/6.0/#sec-properties-of-the-function-prototype-object :

Th Function prototype object is specified to be a function object to ensure compatibility with ECMAScript code that was created prior to the ECMAScript 2015 specification.

This function object does not actually do anything that a function object is meant to do. You can call it with any arguments & it returns undefined. It is a dumb wrt. function object. It's a normal prototype object.

And since it's just there for compatibility reasons, it does not even has a prototype property.

For more elaboration, you can refer this answer: enter link description here

Because function is a native object which among other properties has internal [[Construct]] and [[Call]] properties and also explicit prototype property — the reference to a prototype of the future objects. And its class is function.

F.[[Class]] = "Function"
F.[[Call]] = <reference to function> // function itself

Thus [[Call]] besides the [[Class]] property (which equals to "Function") is the main in respect of objects distinguishing. Therefore the objects having internal [[Call]] property are called as functions. The typeof operator for such objects returns "function" value.

see for reference

Since it has all the methods and props that any function is ought to have, this makes it effectively a function ...

Think about it for a moment and let it sink and you'll get the picture by then :)

本文标签：