typeof函数不是函数

Question

Colleague showed me the next code, which blown my mind:

 const x = Object.create(function() {}); // variant: const x = Object.create(Array); console.log(x.call) // ƒ call() { [native code] } console.log(typeof x.call) // "function" console.log(x.call instanceof Function) // true x.call() // Uncaught TypeError: x.call is not a function 

I understand that x.call is prototyped from function , it's not own x 's property:

x.hasOwnProperty('call') // false

But why x.call can't actually being executed? Is it something related to call keyword ?

Answer 1

原因x是一个继承自Function.prototype call的对象，但是call是要在函数上调用的，因此，如果您尝试在一个普通对象上执行它，则它将失败。

Answer 2

The core idea behind Object.create boils down to this:

function create(prt){
    var noOP = function(){};
    noOP.prototype = prt;
    return new noOP;
}

So, the returned value is NOT a function, it is an object. To illustrate, I'll first store a function:

var u = function(){return 5}

Now I'll use Object.create on it:

var res = create(u);

Consoling it will give you >noOP {} , so it is a plain object. The problem starts from here:

res.hasOwnProperty("prototype") //false

So the newly created object has "prototype" but this is in fact inherited from u :

res.prototype === u.prototype //true

Similary, "call" is again inherited from u which in turn u inherits from its constructor's (Function) prototype:

res.call === u.call //true
res.call === Function.prototype.call //also true

And here is your problem, if you look at the EcmaScript implementation of call , it expects a this and this should be callable. Isolate call from res :

var x = res.call; //ƒ call() { [native code] }

Now I will "call" the call, we will pass 3 arguments, 1st for what to call, 2nd for setting this inside that callable, 3rd and so forth for arguments for the callable:

x.call(function(a){console.log("hey");console.log(a);console.log(this);},5,5)
//hey
//5
//Number {5}

Now try the same on your created object res either by res.call or x.call :

x.call(res,5,5) //TypeError: x.call is not a function

In the end, it boils down to returned object from Object.create not being callable.