How does javascript work?

JavaScript is a scripting language written in text form. It is mainly used to control the behavior of web pages in web browsers and make the pages dynamically interactive. In this article, we will learn more about how JavaScript works by exploring its workflow.

The workflow of JavaScript mainly includes three parts: parsing, compilation and execution. Among them, the parsing phase converts the JavaScript code into an abstract syntax tree (AST); the compilation phase generates executable bytecode based on the AST; and the execution phase performs actual code execution.

1. Parsing

Parsing is the process of converting text-form code into a syntax tree. During the parsing process, the JavaScript code is first divided into tokens by the tokenizer. At this stage, all spaces, tabs, and newline characters are ignored and do not participate in generating tokens. Token is the smallest unit of syntax analysis. Each Token represents a semantic unit of the code, such as variable names, numbers, function keywords, etc.

During the parsing process, the JavaScript engine will build an abstract syntax tree based on the grammatical rules of the code for further compilation and execution of the code. An abstract syntax tree is a tree structure used to describe program semantics, consisting of a series of nodes and edges. Each node in the tree represents a grammatical structure (such as variables, functions, operators, etc.) in a statement or expression.

2. Compilation

The compilation phase mainly converts the abstract syntax tree generated in the parsing phase into bytecode. Bytecode is a binary code in an intermediate state. Unlike machine instructions, the execution of bytecode requires the support of an interpreter or a virtual machine.

During the compilation process, the JavaScript engine will first perform optimization, including static analysis and runtime optimization. Static analysis reduces redundant calculations and data transmission at runtime by analyzing code structure, reference relationships, data flow transmission and other information. Runtime optimization is to dynamically adjust the execution strategy according to the running status of the code, such as adding caching and inlining to loops.

After the optimization is completed, the compiler converts the abstract syntax tree into bytecode and stores it in memory, reducing the interpretation process at each run. The execution efficiency of bytecode is higher than that of the interpreter, but there is still a certain performance loss compared to the machine code generated by compilation.

3. Execution

The execution phase is the process of converting the bytecode generated during the compilation phase into machine code and executing it. The execution process of JavaScript is single-threaded, which means that all codes are executed sequentially in the same thread. Therefore, all events and operations are processed sequentially in a queue.

During the execution process, since JavaScript code can access APIs provided by browsers such as DOM and BOM, the JavaScript engine will maintain an interactive interface with the browser. When an event is triggered, the JavaScript engine will retrieve the event from the event queue and execute the corresponding callback function. The code in the callback function will re-enter the process of parsing, compilation, and execution until all codes are executed.

During the execution process, the JavaScript engine will also maintain a memory area for storing variables, objects and other data. When a variable is declared, the JavaScript engine allocates a memory to store the variable's value. When a variable is no longer used, the JavaScript engine will automatically reclaim the variable's memory to prevent memory leaks.

Summary

JavaScript is a high-level scripting language with strict grammatical rules and complex semantic structures. Although the execution efficiency of JavaScript has decreased compared with compiled languages, it has the advantages of flexibility and interactivity and is an indispensable part of Web development.

This article introduces the running mechanism of JavaScript in detail from three aspects: parsing, compilation and execution. Understanding the workflow of JavaScript is of great significance to the performance optimization and debugging of JavaScript programs.

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