What does the browser do with javascript?

JavaScript is a high-level programming language used to give interactivity and dynamic effects to web pages. However, JavaScript code is not run directly on the computer, but is interpreted and executed by the browser. In this article, we will learn how browsers execute JavaScript code.

1. Parsing

When a browser downloads a web page that contains JavaScript code, it automatically parses the code into a series of computer-readable instructions. This process is called parsing. When parsing, the browser creates an abstract syntax tree (AST) to represent the syntactic structure of the code.

The parser understands the meaning of the code through syntax and context (i.e. variables and functions), while converting the code into executable instructions.

2. Precompiling

The browser will precompile the code before parsing and executing JavaScript code. This process consists of two steps: declaration of variables and functions.

During precompilation, the browser scans the declarations of all variables and functions in the code. For variables, the browser adds them to the execution environment. For functions, the browser will hoist the function declaration to the top of the code so that it can be used in other parts of the code.

3. Execution

Once the JavaScript code has been parsed and precompiled, the browser can begin executing the code. During execution, the browser executes instructions in the order of the code and updates variables and functions in the execution environment when necessary.

During execution, JavaScript code accesses variables and functions through the execution environment. The execution environment consists of three parts: scope chain, variable object and this pointer.

A scope chain is a set of nested scopes. In JavaScript, it refers to the nested relationship of functions, which determines the visibility of variables. The variable object represents the execution environment of the function, including all declared variables and functions. The this pointer represents the object currently executing code, and is often used in JavaScript to point to the DOM element currently being manipulated.

4. Optimization and JIT compilation

The browser will also optimize the execution of the code based on the characteristics of the code. For example, browsers will cache frequently executed code so that it can be executed faster the next time it is executed. You can also use a JIT compiler to convert code into native machine code to improve execution speed and performance.

Summary

In the browser, the execution process of JavaScript code can be roughly divided into four stages: parsing, pre-compilation, execution and optimization/JIT compilation. These processes are interrelated. From code parsing to final execution, the browser needs to be interpreted and executed. Understanding these processes helps us gain a deeper understanding of how JavaScript code is executed by the browser.

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