The call and apply methods in JavaScript talking about _javascript skills

The first argument for call and apply is the parent object that is to invoke the function, which is the invocation context, which is used in the function body to obtain a reference to it.
For example, if you want to invoke the function f with the object O method, you can use the call and apply methods as follows:

Copy Code code as follows:

F.call (o);
F.apply (o);

The following code can be used to understand:

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O.M = f; Temporary method to store F as O
O.M (); Call this temporary method
Delete o.m; Remove this temporary method

Let's take an example.

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function Testfun () {
return THIS.A + this.b;
}
var o = {a:1, b:2};
Testfun.call (o); 3
Testfun.apply (o); 3

The above code execution results are 3, can be understood as return O.A + O.B.
Consider a question, what if the first argument of the call and apply method is null or undefined? Look at one of the following examples:

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var a = ten, B = 20;
function Testfun () {
return THIS.A + this.b;
}
Testfun.call ();
Testfun.apply ();

The result of this code execution is 30. This is because the first argument of call and apply, if passed in null or undefined, is replaced by a global object.
What is the difference between the two methods of call and apply?
For the call method, all arguments after the first invocation context argument are the values to pass in to the function to be called. For example, to call function f in the form of an object o, and pass in two parameters, you can use the following code:

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F.call (o, 1, 2);

The Apply method then puts all the arguments after the first argument into an array,

Copy Code code as follows:

F.apply (O, [1, 2]);

Let's take an example.

Copy Code code as follows:

function Testfun (x, y) {
Return THIS.A + this.b + x + y;
}
var o = {a:1, b:2};
Testfun.call (o, 10, 20);
Testfun.apply (o, [10, 20]);

The execution result of the above code is 33, can be understood as return O.A + O.B + 10 + 20