Pre parsing of JS

Catalog

One 、js The engine executes the code in two steps : Pre parsing and code execution

Two 、 Pre parsing is divided into variable pre parsing （ Variable Promotion ） And function pre parsing （ Function enhancement ）

1. Variable Promotion

2. Function enhancement

3、 ... and 、 Case study

One 、js The engine executes the code in two steps : Pre parsing and code execution

1. Pre parsing will js Everything in it var,function Raise the declaration of to the front of the current scope

2. Code execution , Execute from top to bottom in code order

Two 、 Pre parsing is divided into variable pre parsing （ Variable Promotion ） And function pre parsing （ Function enhancement ）

1. Variable Promotion

Is to promote all variable declarations to the front of the current scope , But the assignment operation is not promoted .

2. Function enhancement

To go to the front of the current function declaration is to elevate all the scope , But don't call the function .

3、 ... and 、 Case study

 console.log(num1);
        var num1 = 12;
// amount to 
        // var num1;
        // console.log(num1);
        // num1=12;

So the output undefined

 fn();

        function fn() {
            var a = 20;
            console.log(a);
        }
        // amount to 

        // function fn() {
        //     var a = 20;
        //     console.log(a);
        // }
        // fn();

It can output normally 20

 fn1();
        var fn1 = function() {
            console.log(22);
        }

Function expression The call must be written below the function expression