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This article mainly introduces the relevant knowledge of how to achieve precompilation in JavaScript, the content is detailed and easy to understand, the operation is simple and fast, and it has a certain reference value. I believe you will gain something after reading this JavaScript article on how to realize precompilation. Let's take a look at it.

JavaScript running Trilogy

What does the script execution js engine do?

Grammar analysis

Pre-compilation

Interpretive execution

There are two more steps before executing the code

Parsing is simple: the engine checks your code for any low-level syntax errors

Interpretation and execution, as the name implies, is the execution of code.

The simple understanding of precompilation is to open up some space in memory and store some variables and functions.

Understanding the pre-compilation is also helpful for everyone to understand the scope.

When does JS precompilation occur?

The misunderstanding of my thinking also happened here.

When exactly does pre-compilation happen?

I hope you don't let the above process mislead you.

Mistakenly assume that precompilation occurs only before the execution of the code block in the script

There's nothing wrong with that.

Precompilation actually occurs before execution in the script code

But most of it happens before the function is executed.

An example of JS precompilation

Before giving an example, consider these concepts:

Variable declaration var...

Function declaration function...

Var a = 1 / variable declaration function b (y) {/ / function declaration var x = 1; console.log ('so easy');}; var c = function () {/ / is a variable declaration rather than a function declaration! / /...} b (100); var d = 0

Let's see what the engine did to this code.

1. The GO global object (Global Object) (that is, the familiar window object) is created when the page is generated.

two。 The first script file loads

3. After the script is loaded, analyze whether the syntax is legal

4. Start precompilation

Look for the variable declaration, and as the GO attribute, the value is assigned to undefined

Look up the function declaration. As a GO attribute, the value is assigned to the function body.

/ / pseudocode GO/window = {/ / Page loading creates GO while creating document, navigator, screen and other attributes, omitting a: undefined, c: undefined, b: function (y) {var x = 1; console.log ('so easy');}}

Interpret the execution code (until function b is executed)

/ / pseudo code GO/window = {/ / variables are initialized with the execution flow a: 1, c: function () {/ /...}, b: function (y) {var x = 1; console.log ('so easy');}}

Pre-compilation occurs before function b is executed

Create an AO active object (Active Object)

Look for formal parameters and variable declarations, and assign values to undefined

Real parameter values are assigned to formal parameters

Find the function declaration and assign the value to the function body

/ / pseudo code AO = {/ / create AO at the same time, create arguments and other attributes, omit y: 100, x: undefined} here

Explain the code in the execution function

After the execution of the first script file, load the second script file

After the second script file is loaded, syntax analysis is performed.

After the syntax analysis, the precompilation begins.

Repeat the initial precompilation steps.

Everyone should pay attention.

Variable declaration and function declaration occur in the precompilation stage, there is no initialization behavior (assignment), and anonymous functions do not participate in precompilation.

Variable initialization occurs only during the interpretation execution phase

This is the end of the article on "how to realize pre-compilation in JavaScript". Thank you for reading! I believe that everyone has a certain understanding of the knowledge of "how to achieve pre-compilation in JavaScript". If you want to learn more knowledge, you are welcome to follow the industry information channel.

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