Front-end development-JavaScript lexical analysis

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

1. Parameters of the analysis function

2. Variable declarations for analytic functions

3. function declaration expressions for analytic functions

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

Example 1:

function func (age) {    console.log (age);    var age =;    Console.log (age);    Function age () {    }    Console.log (age);} Func (18);

Lexical analysis:

The first step is to analyze the function parameters:

Formal parameter: 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 a function of age, then assign function age () {} to ao.age, i.e. 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 the age is still 25, and the result is 25, running as follows

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:

function func (age) {    var;    Console.log (age);    var age =;    Console.log (age);    Function age () {    }    Console.log (age);} Func (18);

Operation Result:

Example 3:

function func (age) {    var;    Console.log (age);    var age =;    Console.log (age);    Function age () {        console.log (age);    }    Age ();    Console.log (age);} Func (18);

Operation Result:

Example 4:

function func (age) {    var;    Console.log (age);    Function age () {        console.log (age);    }    Age ();    Console.log (age);} Func (18);

Operation Result:

Example 5:

function func (age) {    console.log (age);    var = function age () {        console.log (age);    };    Age ();    Console.log (age);} Func (18);

Operation Result:

　

Summary:
The function takes two steps to run: Lexical analysis and statement execution

Lexical analysis:
1. Formal parameters and arguments, actual participation
2. Analyze the variable declaration, such as var age = 18; extract the age=undefined, but do not execute the assignment statement
3. Parse function declaration, such as function age () {} equals var age = function () {}, but the body does not perform

Statement execution:
1.console.log (age);
2.var age = 25;
3.function () {Console.log (age);} is not executed, age () only executes

Precautions:
1. Function declaration Functions age () {console.log;} whole will be assigned to Ao.age
2.var age = function Age () {console.log;} belongs to lexical analysis second step variable declaration

Front-end development-JavaScript lexical analysis