I read some article and it said the following 2 line are doing the same thing.
fn.call(thisValue);
Function.prototype.call.call(fn, thisValue);
For line 1, my understanding is that every function object in Javascript do have a the method call
inherited from the Function.prototype
and what call
method does is to have the this
keyword inside the function definition of fn
to be thisValue
(the first parameter I passed in the call method. fn
is a function so what I am doing in fn.call(thisValue)
is just invoking fn
and set the this
keyword inside the function to be thisValue
.
But For line 2, I don't get it. Can someone help to explain it what the hack the line 2 is doing.
Let's start with this setup:
function fn() { console.log(this); }
var thisvalue = {fn: fn};
Now you surely understand that thisvalue.fn()
is a method call, and sets the logged this
value to the thisvalue
object.
Next, you seem to know that fn.call(thisvalue)
does exactly the same call. Alternatively, we could write (thisvalue.fn).call(thisvalue)
(parentheses just for structure, could be omitted) as thisvalue.fn === fn
:
thisvalue.fn(…); // is equivalent to
(thisvalue.fn).call(thisvalue, …); // or:
(fn).call(thisvalue, …);
OK, but fn.call(…)
is just a method call as well - the call
method of functions is called on the fn
function.
It can be accessed as such because all function objects inherit this .call
property from Function.prototype
- it's not an own property like .fn
on the thisvalue
object. However, fn.call === Function.prototype.call
is the same as thisvalue.fn === fn
.
Now, we can rewrite that method call of .call
as an explicit invocation with .call()
:
fn.call(thisvalue); // is equivalent to
(fn.call).call(fn, thisvalue); // or:
(Function.protoype.call).call(fn, thisvalue);
I hope you spot the pattern and can now explain why the following work as well:
Function.prototype.call.call.call(Function.prototype.call, fn, thisvalue);
var call = Function.prototype.call; call.call(call, fn, thisvalue);
Breaking this down is left as an exercise to the reader :-)
Since I ended up here trying to understand this question , I'm gonna post my answer here as well.
Let's start with this:
function fn() { console.log(this); }
fn.a = function(){console.log(this)} // "this" is fn because of the . when calling
fn.a() // function fn() { console.log(this); }
So let's dig deeper and try to reimplement the call
function:
Function.prototype.fakeCall = function () {
// taking the arguments after the first one
let arr = Array.prototype.slice.call(arguments, 1);
try{
return this.apply(arguments[0], arr);
}catch(e){}
}
function a(ar){ console.log(ar + this.name) };
let obj = {name : "thierry"};
// a.fakeCall( obj, 'hi ')
Function.fakeCall.fakeCall(a, obj, 'hi ');
Thus when we do this: Function.prototype.fakeCall.fakeCall(a, obj, 'hi ')
what happens is, on the first run we have:
arr = [ obj, 'hi ']
this = Function.fakeCall
so we end up with Function.fakeCall.apply(a, [ obj, 'hi ']);
Then on the second run we have
arr = ['hi']
this = a
so we end up with a.apply(obj, ['hi'])
which is the same as a.call(obj, 'hi');
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