JS pre parsing

Catalog

One 、 What is pre parsing

Two 、 Pre parse

2.1、 Variable pre parsing

2.2、 Function pre parsing

3、 ... and 、 Statement notes

Four 、 Variable promotion reason

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One 、 What is pre parsing

Pre resolution refers to that under the current scope ,JavaScript Before code execution , The browser will default to include var and function The sound

Explicit variables are declared or defined in memory in advance .

Example ：

    <script>
        a = 2;
        var a;
        console.log(a);//2
    </script>

Two 、 Pre parse

2.1、 Variable pre parsing

Variable pre parsing is also called variable promotion , It means that the declaration of variables will be promoted to the top of the current scope , But the assignment of variables will not improve .

Example 1：

    <script>
        console.log(age); // undefined 
        var age = 18;
    </script>

The above code will output undefined Instead of reporting a mistake , Why? ？

This is caused by variable pre parsing .

After pre analysis ：

  Example 2：

  After pre analysis ：

2.2、 Function pre parsing

Function pre parsing is also called function promotion , It means that the function declaration will be promoted to the top of the current scope , But it doesn't call the function .

Example 1：

  Example 1 Function pre parsing process ：

  Example 2：

  Example 2 Function pre parsing process ：

  Be careful ： If you use a function expression to declare a function , Will not be promoted .

3、 ... and 、 Statement notes

（1） Variable declaration and function declaration , Variable declaration promotion takes precedence over function declaration promotion . however , A function declaration overrides an undefined variable with the same name .

Example ：

Pre parsing process ：

 

 （2） Variable declaration and function declaration , Variable declaration promotion takes precedence over function declaration promotion . If the variable has a value , Will not be covered .

Example ：

  Pre parsing process ：

 （3） The following function declaration will override the previous one

    <script>
        function fn() {
            console.log('hello');
        }

        function fn() {
            console.log('world');
        }
        fn(); //world
    </script>

summary ： On declaration （ Whether it's a variable or a function ） Try not to take the same identifier （ name ）.

Four 、 Variable promotion reason

In the compilation phase , The engine declares variables and functions , But the variable will not be assigned , This is mainly due to performance considerations . Variable is declared , But it may not be used later , If it is not used but assigned a value , It's just a waste of memory . In short ,var a This code takes place during the compilation phase , and =1 This code will be based on the actual situation , Occurs during the execution phase , This is the same. " Variable Promotion " Why . Another thing to note , The function declares the whole function body ( Because the function declares that there is no assignment operation ), And the priority is higher than the variable with the same name .

Example ：

    <script>
        console.log(fn()); // Output ：1 \
        console.log(n); // Output ：undefined 
        function fn() {
            return 1;
        }
        var n = 2;
    </script>

In browsing, it is pre resolved as follows ：

    <script>
        var a;

        function fn() {
            return 1;
        }
        console.log(fn());
        console.log(n);
        n = 2;
    </script>