Introduction to acorn-walk

Acorn-walk is a powerful module for walking through the Abstract Syntax Tree (AST) generated by the Acorn JavaScript parser. It provides a rich set of APIs that allow developers to traverse and manipulate syntax trees with ease. In this post, we will explore various acorn-walk APIs and present practical code snippets to demonstrate their usage.

Basic Usage

To start using acorn-walk, first install it via npm:

npm install acorn acorn-walk

Now, let’s parse some JavaScript code and walk through its syntax tree:

const acorn = require('acorn');
const walk = require('acorn-walk');

const code = 'const x = 1 + 2;';
const ast = acorn.parse(code, { ecmaVersion: 2020 });

walk.simple(ast, {
  Literal(node) {
    console.log(`Found a literal: ${node.value}`);
  },
  BinaryExpression(node) {
    console.log(`Found a binary expression: ${node.operator}`);
  },
});

Walking Specific Node Types

Acorn-walk comes with methods like simple, ancestor, full, fullAncestor, and findNodeAt. Here’s a quick overview of each:

simple

walk.simple(ast, {
  Identifier(node) {
    console.log(`Identifier: ${node.name}`);
  }
});

ancestor

walk.ancestor(ast, {
  Literal(node, ancestors) {
    console.log(`Ancestor Count: ${ancestors.length}`);
  }
});

full

walk.full(ast, node => {
  console.log(`Node Type: ${node.type}`);
});

fullAncestor

walk.fullAncestor(ast, (node, ancestors) => {
  console.log(`Node Type: ${node.type}, Ancestor Count: ${ancestors.length}`);
});

findNodeAt

const foundNode = walk.findNodeAt(ast, null, null, node => node.type === 'BinaryExpression');
if (foundNode) {
  console.log(`Found a binary expression at ${foundNode.start}-${foundNode.end}`);
}

App Example

Let’s build a simple app that collects all variable declarations from a JavaScript file:

const fs = require('fs');

function collectVariableDeclarations(filePath) {
  const code = fs.readFileSync(filePath, 'utf8');
  const ast = acorn.parse(code, { ecmaVersion: 2020 });
  const declarations = [];

  walk.simple(ast, {
    VariableDeclaration(node) {
      declarations.push(node);
    }
  });

  return declarations;
}

const declarations = collectVariableDeclarations('example.js');
declarations.forEach(decl => {
  console.log(`Found declaration at line ${decl.loc.start.line}`);
});

This app reads a JavaScript file, parses it into an AST, walks through the tree, and collects variable declarations.

In conclusion, acorn-walk offers a variety of APIs to traverse and manipulate ASTs effectively, making it a valuable tool for static analysis, linting, and code transformation.

Hash: 858aeb0a69b7bcc23d440d06bdf29dd6f88d5aecb88ff89270fb89287c63de3a