How browsers parse javascript

With the continuous development of Internet technology, JavaScript language has become an essential skill for front-end development. JavaScript is a scripting language that can be executed in the browser to achieve interactive effects on web pages, such as form validation, dynamic loading of content, asynchronous requests, etc. So how does the browser parse JavaScript? Let’s explore it in detail below.

1. Parsing process

After a web page is loaded, the browser will first parse the HTML document. For the <script> tag, the browser will determine whether it is is JavaScript code. If it is JavaScript code, the browser will parse and execute it according to the following steps: </script>

(1) Tokenization

First, the browser will tokenize the JavaScript code and generate a token consisting of lexical units Lexical Token Stream. Lexical units refer to the smallest units in language, such as identifiers, keywords, strings, numbers, etc. The process of tokenization converts each character in the code into a token for subsequent use by the parser.

(2) Syntax analysis

Then, the browser will convert the lexical unit stream into an Abstract Syntax Tree (AST), which is an abstract syntax representation. In this process, the browser analyzes each lexical unit according to JavaScript grammar rules and combines them into a grammatical structure. For example, the browser will generate corresponding syntax nodes for variable declarations, function definitions, expressions, etc. The generated abstract syntax tree retains the structural information and semantic information of the code, making subsequent processing more convenient.

(3) Execution

After the AST is generated, the browser can execute this JavaScript code. During the execution process, the browser will traverse each node in the abstract syntax tree in turn and perform corresponding processing according to the node type. For example, for a variable assignment statement, the browser will find the variable to be assigned from the scope and assign the value of the assignment expression to it. For function call expressions, the browser looks for the definition of the function and executes it. When the entire AST is processed, the JavaScript code is successfully executed.

2. Optimization Technology

Although JavaScript parsers have greatly optimized the process of code parsing and execution, for some complex web applications, they may contain a large number of JavaScript code, causing the page to run slowly or even freeze. In order to solve this situation, browser manufacturers and developers continue to explore various optimization technologies. Here is a brief introduction to several common technologies.

(1) Incremental parsing

When the browser loads a web page containing a large amount of JavaScript code, it may need to perform multiple syntax analysis and executions, which consumes a lot of time and resources. To solve this situation, browsers can use incremental parsing technology. Incremental parsing means that during the process of code parsing and execution, the browser can handle multiple tasks at the same time, thereby minimizing execution time. For example, when the browser is processing a long function call, it can execute the first few lines of code, then interrupt the execution, immediately execute other tasks, wait until the other tasks are completed, and then return to continue executing the remaining code.

(2) JSX compilation

JSX is an XML-like syntax extension and a language feature used in the React framework. When writing a large number of UI components using React, a large amount of JSX code is generated, which can lead to poor performance of the web application. To solve this situation, browsers or developers can use a JSX compiler to convert JSX code into ordinary JavaScript code to improve the running speed of the page.

(3) Pre-compilation

When using JavaScript code of ES6 and above, the browser can use tools such as Babel to convert it into ES5 code to improve the compatibility of the code. In addition, some developers will write some commonly used function libraries to facilitate subsequent development. In order to improve the performance of these function libraries, browsers can precompile them to reduce code parsing and execution time.

3. Summary

The JavaScript parsing process is an important part of the browser executing JavaScript code, and has a direct impact on the running speed and performance of the page. Browser manufacturers and developers continue to explore various optimization techniques to improve the execution efficiency of JavaScript code. When writing JavaScript code, we need to pay attention to the readability and maintainability of the code to improve the efficiency of code parsing and execution, thereby optimizing the performance of the page.

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