AST (Abstract Syntax Tree) is an intermediate representation after code parsing. It converts JavaScript code into a structured tree-like form for easy analysis and operation. 1. AST is a structured representation of the code, each node represents a syntax construct; 2. It is widely used in code optimization, inspection, formatting, conversion and plug-in development; 3. Common tools include Esprima, Acorn and Babel Parser; 4. Code search, modification and generation can be achieved by traversing AST, such as using estraverse or Babel plug-in for operation. Mastering AST can improve your understanding and control of code.
JavaScript's Abstract Syntax Tree (AST) is a tree-like representation of code structure, which can help us understand code logic more deeply. If you have written JavaScript or used tools like ESLint or Babel, you have actually been indirectly exposed to AST. This article will start from the perspective of practical application and talk about what AST is, why it is useful, and how to start hands-on operation.
What is AST?
Simply put, AST is the intermediate representation after code parsing. When you write a piece of JavaScript code, for example:
const a = 5 3;
The JS engine does not execute the original text directly, but converts it into a tree structure first. Each node of this tree represents a syntax construct, such as variable declaration, assignment operation, addition operation, etc.
AST is not only a data structure inside the compiler, it is also the basis of many developer tools. For example, Babel translates ES6 to ES5, and ESLint checks the code style, all of which are analyzed and modified based on AST.
Why do you need to know about AST?
You may think that AST is a "high-level gameplay", but its practical application scenarios are much wider than imagined:
- Code optimization : When packaging tools such as Webpack and Rollup merge tree-shaking and modules, they rely on AST to determine which code can be safely removed.
- Code inspection and formatting : ESLint, Prettier and other tools use AST to find potential errors or non-compliant specifications.
- Code generation and conversion : Babel is a typical example, which can convert modern JS to a better compatibility version.
- Custom plug-in development : If you want to write your own lint rules or code conversion tool, you can't avoid AST.
In other words, mastering AST allows your understanding of code to no longer be on the surface, but to enter the "operable" level.
How to view and operate AST?
To operate AST, the first step is to parse your code into an AST structure. Commonly used tools are:
- Esprima : an old-fashioned parser, suitable for learning.
- Acorn : Lightweight and supports plug-in extensions.
- Babel Parser (formerly Babylon) : Powerful, supports JSX, Flow, TypeScript and other extensions.
To give a simple example, use Acorn to convert a piece of code to AST:
const acorn = require("acorn");
const code = `const a = 5 3;`;
const ast = acorn.parse(code, { ecmaVersion: 2020 });
console.log(ast);The output is a structured object tree. You can iterate over it, look for nodes of a specific type, or even modify some parts before generating new code.
If you want to save trouble, you can use ready-made online tools, such as AST Explorer , which supports multiple parsers and can display the AST structure in real time.
How to start writing AST-related code?
Once you have AST, you can start doing interesting things, such as:
- Traversal AST to find function calls
- Modify the variable name or expression
- Insert new statements or delete redundant codes
Here is a basic method: use estraverse to traverse AST.
const esprima = require("esprima");
const estraverse = require("estraverse");
const code = "function hello() { console.log('Hello'); }";
const ast = esprima.parseScript(code);
estraverse.traverse(ast, {
enter(node, parent) {
if (node.type === "Identifier" && node.name === "console") {
console.log("Discover console call");
}
},
}); This example will look for all nodes with console identifiers in AST. You can modify enter or leave method as needed to implement different processing logic.
In addition, if you plan to write a plug-in, such as a Babel plug-in to automatically replace a function call, you will often use babel-types and babel-template to create and operate nodes.
Basically that's it. AST looks complicated, but once you master the basic operations, you will find that it is actually not difficult to get started, it is just an underlying ability that is easily overlooked.
The above is the detailed content of Understanding JavaScript Abstract Syntax Trees (AST). For more information, please follow other related articles on the PHP Chinese website!
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