JS scope closure, pre-interpretation and this keyword comprehensive case analysis _javascript skills

This article analyzes the JS scope closure, the pre interpretation and this keyword. Share to everyone for your reference, specific as follows:

var number = 2;
var obj = {number:5,
  fn1: (function () {
  this.number *= 2;
  number=number*2;
  var number=3;
    return function () {
      this.number *= 2;
      number*=3;
      alert (number);
    }
  )()
};
var fn1 = obj.fn1;
alert (number);
FN1 ();
Obj.fn1 ();
alert (window.number);
alert (Obj.number);

Resolution

1. FN1 itself is followed by (), so var fn1 = obj.fn1; He was executed, but the return function inside was not executed.

2. Alert (number), the output of the global number, is originally 2, after the var fn1 = obj.fn1;

This.number *= 2;
number=number*2;
var number=3;

These three words

Note: Any anonymous method executed directly, his this point to window

So This.number *= 2, so that the global variable becomes 4, which is output 4.

and number=number*2; He was going to see the Number,var number=3 in the scope block; The scope declaration was first, but there was no assignment, so number=number*2; it doesn't really work.

3. Execution to FN1 () is the execution of the return function inside the FN1, which is the three words

This.number *= 2;
number*=3;
alert (number);

This is still an anonymous method, so This.number *= 2; makes the global variable 8,number*=3; To find number in the scope block, the outer definition number is 3, so alert (number) is 9, if alert ( This.number); point to global variable, that is, output 8

4. OBJ.FN1 (), still executes the return function inside the FN1, which is the three words

This.number *= 2;
number*=3;
alert (number);

But then this point to Obj,this.number *= 2, make the number in obj into 10,number*=3, or to find the scope block, because the above becomes 9, so this is 27,alert (number); The output is number of the scope block, that is, 27, if alert (this.number), point to obj number, that is, output 10

5. Alert (window.number); After the previous rounds, the global variable becomes 8 (that is, execution obj.fn1 (); no effect on global variables)

6. Alert (obj.number); only this sentence obj.fn1 ();, changed obj.number, so output 10

(function () {
   var a=10;
  fn ();
  function fn () {
    var a=a+10;
    Console.log (a);
     return A;
  }
  Console.log (a);
  Console.log (FN () +10);
}) ();

A in the FN function is first declared but not assigned, and then the operation is performed, and he does not look for a variable with the same name outside the function because he is already declared inside. An operation that is not numeric and numeric, outputting nan

Console.log (FN () +10); This sentence outputs two values: Console.log (FN ()) and Console.log (FN () +10);

Results:

NaN
10
NaN
NaN

If the title is changed into

(function () {
  var a=10;
  fn ();
  function fn () {
    a=a+10;
    Console.log (a);
    return A;
  }
  Console.log (a);
  Console.log (FN () +10);
}) ();

Results:

20

20

30

40

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