Detailed introduction to how JavaScript works

This article brings you a detailed introduction to the operating principles of JavaScript. It has certain reference value. Friends in need can refer to it. I hope it will be helpful to you.

JavaScript Operation Principle

Know what it is and why it is so. Here we mainly talk about the understanding of JavaScript operation principle.

JAVA Virtual Machine

First of all, let’s start with the JAVA virtual machine.

First of all, let’s talk about why it is necessary to make a virtual machine, because the machines are different. Without a virtual machine, we would need to compile the code for each machine, which is definitely unreasonable. Therefore, in order to solve this problem, Java introduces the concept of virtual machine (VM), allowing the compiled code to run directly on a virtual machine. No matter what the final target platform is, a virtual consistent one is built on it. Once the virtual machine comes out, you can achieve the effect of compiling once and executing it everywhere.

Fundamentally speaking, Java and C# are not Native languages. The compiled result is not a machine instruction, but a certain own instruction format. It cannot be run by itself and requires a special program to interpret and execute it. This program is What we call a "virtual machine".

On the contrary, languages ​​like C, C, Go, and Rust are Native languages. The compiled results are machine instructions, which can be run directly by themselves. There is no concept of "virtual machine".

JavaScript Engine

The work done by the JavaScript engine is very similar to the JAVA virtual machine

What does it do

The JS engine mainly performs lexical, grammatical and other analysis on the JS code, and compiles the code into executable machine code through a compiler for the computer to execute.

Simply put, its only purpose is to read and compile JavaScript code, which means it can analyze, interpret, optimize, and garbage collect JavaScript code.

Execution process

The JavaScript engine will load the source code, decompose it into strings (also called word segmentation), and then convert these strings into things that the compiler can understand. bytecodes and then execute those bytecodes.

Google's V8 engine is written in C. It can also compile and execute JavaScript source code, handle memory allocation and garbage collection. It is designed to consist of two compilers, which can directly compile source code into machine code. The specific execution process can be seen as the following steps:

1. It performs lexical analysis, that is, Source code is broken down into a series of symbols or strings with clear meaning.

2. Then use a syntax analyzer to analyze these symbols and build them into a syntax tree.

3. Then four JIT (Just-In-Time) processes begin to participate, analyzing and executing the bytecode generated by the parser.

Relationship with ECMAScript

To be precise, each JavaScript engine implements a version of ECMAScript, and JavaScript is a branch of it . As ECMAScript continues to evolve, the JavaScript engine continues to improve. The reason there are so many different engines is because each of them is designed to run in a different web browser, headless browser, or runtime environment like Node.js.

The JavaScript engine is a program, and the JavaScript code we write is also a program. How to make the program understand the program? This requires defining rules. For example:

var a = 1 + 1

The var on the left represents a declaration, which declares the variable a
The one on the right represents the addition of 1 and 1
The equal sign in the middle represents that this is a The semicolon at the end of the assignment statement
indicates the end of the statement
These are the rules. With it, there is a standard for measurement. The JavaScript engine can parse the JavaScript code according to this standard. Then the ECMAScript here defines these rules. Among them, the document ECMAScript 62 defines a complete set of standards for the JavaScript language. These include:

var, if, else, break, continue, etc. are JavaScript keywords
abstract, int, long, etc. are JavaScript reserved words
How to count it as a number and how to count it as a string etc.
defines operators ( , -, >, <, etc.)
defines JavaScript syntax
defines standard processing algorithms for expressions, statements, etc., such as when encountering == How to handle
The standard JavaScript engine will be implemented according to this set of documents. Note that the standard is emphasized here, because there are also implementations that do not follow the standard, such as IE's JS engine. This is why JavaScript has compatibility issues. As for why IE's JS engine is not implemented in accordance with standards, it comes to the browser war. I won't go into details here. Google it yourself.

So, to put it simply, ECMAScript defines the standard of the language, and the JavaScript engine implements it according to it. This is the relationship between the two.

RunTime

Runtime can be simply understood as the current running environment. Different environments provide different api calls, such as the window object in the web browser, DOM Related APIs, etc., these interfaces can provide related JS calls

The above is the detailed content of Detailed introduction to how JavaScript works. For more information, please follow other related articles on the PHP Chinese website!