On the procedure of JavaScript lexical analysis

Article Original address: https://www.cnblogs.com/zingp/p/6102561.html

JavaScript code before running a similar compilation process is lexical analysis, lexical analysis is mainly three steps:

• Analysis parameters
• Declaration of the Re-analysis variable
• Analysis function declaration

The steps are as follows:

• The function generates an active object (active objects) at run-moment, called AO
• First step: Analyze the parameters:

1. The function receives the form parameter, which is added to the Ao property, and at this time the value is undefine, i.e. Ao.age=undefine
2. Receives an argument, adds an attribute to the AO, overwrites the previous undefine

• The second step: Analysis Variable declaration: such as Var age, or Var age=18;

1. If the AO has no age attribute in the previous analysis parameter, the AO attribute is added as Undefine, which is ao.age=undefine
2. If the AO above already has the age attribute, no modification is made

• The third step: the Declaration of the analysis function:
• If you have function age () {} to assign the functions to Ao.age, overwrite the values of the previous analysis

See a code to practice practiced hand:

1 function func (age) {2     Console.log (age), 3     var = 4     Console.log (age), 5     function age () {6     } 7     Console.log (age); 8  9}10 func (18);

Lexical analysis:

The first step is to analyze the function parameters:
Formal parameters: Ao.age = undefined
Actual parameter: Ao.age = 18
The second step is to analyze the local variables:
The 3rd line of code has VAR age, but at this point the first step has ao.age = 18, so do not make any changes
i.e. Ao.age = 18
The third step is to analyze the function declaration:
The 5th line of code has the function age, which is paid to Ao.age, which is ao.age = function age () {}

So, when executing the code:

The 2nd line of code runs when the age is the lexical analysis of the ao.age, the result is: "function age () {};

The 3rd line of code: 25 is assigned to age, at this time age=25;

The 4th line of code runs when age has been assigned a value of 25, resulting in 25;

　　5th, 6 lines of code is a function expression, so no action is done;

The 7th line of code runs when age is still 25, and the result is 25. Look at the results of the browser execution, bingo~~

Lexical analysis should pay attention to the var age = function () {}, this statement, participate in the second and third steps;

When executing code, be aware that the function expression does not do anything, and only declares that the variable is not assigned, and age is still equal to ao.age.

Example 2:

1 function func (age) {2     var, 3     Console.log (age), 4     var age = + 5     console.log (age), 6     function A GE () {7     } 8     Console.log (age); 9}11 func (18);

Answer:

Example 3:

1 function func (age) {2     var, 3     Console.log (age), 4     var age = + 5     console.log (age), 6     funct Ion Age () {7         Console.log (age), 8     } 9 Age     (),     console.log (age),}13 func (18);

Answer:

Example 4:

1 function func (age) {2     var, 3     Console.log (age), 4     function age () {5         console.log (age); 6     } 7
   
    age (); 8     Console.log (age); 9}11 func (18);
   

Answer:

Example 5:

1 function func (age) {2     Console.log (age), 3     var = function age () {4         Console.log (age), 5     };6 Age     (), 7     Console.log (age), 8}9 func (18);

On the procedure of JavaScript lexical analysis