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Codemods are automated code transformation tools that modify your source code by analyzing and manipulating its Abstract Syntax Tree (AST). This page explains the technical foundation of Node.js Userland codemods.

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Abstract Syntax Trees (AST)

An Abstract Syntax Tree is a structured representation of your code that the computer can understand and manipulate. When you run a codemod:
1

Parse

Your source code is parsed into an AST representation
2

Analyze

The codemod searches for specific patterns in the AST
3

Transform

Matching nodes are replaced with updated code
4

Generate

The modified AST is converted back to source code

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Example: From Code to AST

Consider this simple code: 
const { isArray } = require('util');
isArray([]);
The AST represents this as a tree structure with nodes like variable_declarator, object_pattern, call_expression, etc. The codemod can identify these patterns and transform them.

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ast-grep: The Pattern Matching Engine

ast-grep is a powerful tool for searching and manipulating ASTs using pattern matching. It provides:
  • Pattern-based search: Find code using intuitive patterns like util.isArray($ARG)
  • Structural matching: Match based on code structure, not just text
  • Language awareness: Understands JavaScript and TypeScript syntax
ast-grep is the core engine powering Node.js Userland codemods. It handles the heavy lifting of AST parsing and traversal.

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The jssg API

The jssg (JavaScript ast-grep) API is the interface codemods use to interact with ASTs. It’s provided by the Codemod.com platform.

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Core Concepts

Every codemod exports a transform function: 
import type { SgRoot, Edit } from "@codemod.com/jssg-types/main";
import type JS from "@codemod.com/jssg-types/langs/javascript";

export default function transform(root: SgRoot<JS>): string | null {
  const rootNode = root.root();
  const edits: Edit[] = [];
  
  // Find nodes, build edits, commit changes
  
  return rootNode.commitEdits(edits);
}
Key types:
  • SgRoot<JS>: The root object containing the parsed AST
  • SgNode<JS>: Individual AST nodes you can query and manipulate
  • Edit: Represents a single code modification
  • Range: Represents a location in the source code

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Finding Nodes

Use pattern matching to locate code: 
// Find all calls to util.isArray
const nodes = rootNode.findAll({
  rule: { pattern: 'util.isArray($ARG)' }
});

// Find specific node kinds
const imports = rootNode.findAll({
  rule: { kind: 'import_statement' }
});

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Creating Edits

Build transformation edits: 
for (const node of nodes) {
  const arg = node.getMatch('ARG');
  if (arg) {
    // Replace util.isArray(x) with Array.isArray(x)
    edits.push(node.replace(`Array.isArray(${arg.text()})`));
  }
}

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Committing Changes

Apply all edits and return the transformed code: 
if (!edits.length) return null;
return rootNode.commitEdits(edits);
Always return null when no changes are made. This signals to the codemod runner that the file should not be modified.

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Real-World Example

Here’s a simplified version of the util.types.isNativeError to Error.isError migration: 
export default function transform(root: SgRoot<JS>): string | null {
  const rootNode = root.root();
  const edits: Edit[] = [];
  
  // Find all util imports/requires
  const statements = [
    ...getNodeRequireCalls(root, 'util'),
    ...getNodeImportStatements(root, 'util'),
  ];
  
  if (!statements.length) return null;
  
  for (const stmt of statements) {
    // Resolve how types.isNativeError is accessed locally
    const binding = resolveBindingPath(stmt, '$.types.isNativeError');
    if (!binding) continue;
    
    // Find all calls using this binding
    const calls = rootNode.findAll({
      rule: { pattern: binding }
    });
    
    // Replace with Error.isError
    for (const call of calls) {
      edits.push(call.replace('Error.isError'));
    }
  }
  
  if (!edits.length) return null;
  return rootNode.commitEdits(edits);
}
This example demonstrates:
  1. Import detection: Finding where util is imported or required
  2. Binding resolution: Determining how types.isNativeError is accessed (could be util.types.isNativeError, types.isNativeError, or isNativeError depending on the import style)
  3. Pattern matching: Locating all usages of the resolved binding
  4. Replacement: Transforming to the new API

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Codemod Utilities

The @nodejs/codemod-utils package provides battle-tested helpers for common operations:

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Import/Require Detection

import { getModuleDependencies } from '@nodejs/codemod-utils/ast-grep/module-dependencies';

// Finds all import/require statements for 'fs'
const fsImports = getModuleDependencies(root, 'fs');

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Binding Resolution

import { resolveBindingPath } from '@nodejs/codemod-utils/ast-grep/resolve-binding-path';

// Given: const { readFile } = require('fs')
// Returns: 'readFile'
const binding = resolveBindingPath(node, '$.readFile');

// Given: const fs = require('fs')
// Returns: 'fs.readFile'
const binding = resolveBindingPath(node, '$.readFile');

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Cleanup Operations

import { removeBinding } from '@nodejs/codemod-utils/ast-grep/remove-binding';
import { removeLines } from '@nodejs/codemod-utils/ast-grep/remove-lines';

// Remove unused import binding
const result = removeBinding(importNode, 'isArray');
if (result?.edit) edits.push(result.edit);

// Remove entire lines (like unused imports)
const cleanCode = removeLines(sourceCode, linesToRemove);
Always use the utility functions from @nodejs/codemod-utils instead of writing your own. They handle edge cases like destructuring, aliasing, and dynamic imports.

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Workflow Structure

Each codemod follows a standard workflow: 
// 1. Import utilities and types
import { getModuleDependencies } from '@nodejs/codemod-utils/ast-grep/module-dependencies';
import type { SgRoot, Edit } from "@codemod.com/jssg-types/main";
import type JS from "@codemod.com/jssg-types/langs/javascript";

// 2. Define the transform function
export default function transform(root: SgRoot<JS>): string | null {
  const rootNode = root.root();
  const edits: Edit[] = [];
  
  // 3. Find relevant imports/requires
  const imports = getModuleDependencies(root, 'module-name');
  if (!imports.length) return null;
  
  // 4. Locate and transform usage patterns
  for (const importNode of imports) {
    // ... find usages and build edits
  }
  
  // 5. Apply edits and return result
  if (!edits.length) return null;
  return rootNode.commitEdits(edits);
}

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Performance Considerations

  • Early exit: Return null immediately if the file doesn’t contain relevant code
  • Batch edits: Collect all edits before calling commitEdits() once
  • Efficient patterns: Use specific patterns to avoid searching the entire AST
// ❌ Inefficient: Commits after each edit
for (const node of nodes) {
  const newCode = rootNode.commitEdits([node.replace(newText)]);
}

// ✅ Efficient: Batch all edits
const edits: Edit[] = [];
for (const node of nodes) {
  edits.push(node.replace(newText));
}
return rootNode.commitEdits(edits);

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Next Steps

When to Use Codemods

Learn when codemods are the right choice for your migration

Safety Best Practices

Ensure safe, reliable code transformations

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Additional Resources

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