Ast Grep Codemods

ast-grep NAPI Reference for Codemods

This skill provides the ast-grep rule system reference used by packages/codemods/src/schema-migration/. The codemods use @ast-grep/napi (the Node.js binding) to parse and transform TypeScript/JavaScript ASTs.

Parsing

import { parse, Lang, type SgNode } from '@ast-grep/napi';

const ast = parse(Lang.TypeScript, sourceCode);
const root: SgNode = ast.root();

SgNode Core Methods

Search

// Find first match (returns null if not found)
node.find(matcher: string | number | NapiConfig): SgNode | null

// Find all matches
node.findAll(matcher: string | number | NapiConfig): SgNode[]

// Boolean checks
node.matches(pattern: string): boolean
node.inside(pattern: string): boolean
node.has(pattern: string): boolean

Traversal

node.children(): SgNode[]           // Direct children
node.parent(): SgNode | null        // Parent node
node.child(nth: number): SgNode | null
node.field(name: string): SgNode | null  // Named field (e.g., 'name', 'body', 'source')
node.ancestors(): SgNode[]
node.next(): SgNode | null          // Next sibling
node.nextAll(): SgNode[]
node.prev(): SgNode | null          // Previous sibling
node.prevAll(): SgNode[]

Inspection

node.kind(): string       // Tree-sitter node type (e.g., 'field_definition', 'class_body')
node.text(): string       // Full source text
node.isLeaf(): boolean
node.isNamed(): boolean
node.range(): Range       // { start: Pos, end: Pos } (0-indexed)

Meta-variable Extraction

// After finding with a pattern containing $VAR or $$$VARS:
node.getMatch('VAR'): SgNode | null
node.getMultipleMatches('VARS'): SgNode[]

Code Editing

const edit = node.replace('newCode');  // Returns Edit object
const newSource = root.commitEdits([edit1, edit2]);  // Apply batch edits

NapiConfig Rule Object

The find and findAll methods accept a NapiConfig object for complex queries:

node.findAll({
  rule: { /* rule object */ },
  constraints?: { /* meta-variable constraints */ },
})

Rule Types

1. Atomic Rules

Match individual nodes by their properties.

kind - Match by tree-sitter node type

// Find all class declarations
root.findAll({ rule: { kind: 'class_declaration' } })

// Common TypeScript/JavaScript kinds:
// class_declaration, class_body, field_definition, method_definition,
// import_statement, identifier, property_identifier, decorator,
// call_expression, member_expression, string, template_string

Gotcha: Not all kind names are valid in all grammars. TypeScript uses field_definition, some JavaScript grammars use public_field_definition or class_field. Wrap in try/catch when iterating over multiple possible kinds.

pattern - Match by code pattern with meta-variables

// Simple pattern
root.findAll({ rule: { pattern: 'console.log($ARG)' } })

// Pattern with context (for ambiguous syntax like class members)
root.findAll({
  rule: {
    pattern: {
      context: 'class A { $FIELD = $INIT }',
      selector: 'field_definition',
    }
  }
})

Meta-variables:

• $NAME - matches a single AST node
• $$NAME - matches zero or more nodes (non-greedy)
• $$$NAME - matches zero or more nodes (greedy)

regex - Match node text against regex

// Match identifiers starting with underscore
root.findAll({ rule: { kind: 'identifier', regex: '^_' } })

2. Composite Rules

Combine rules with boolean logic.

all - Every rule must match (AND)

root.findAll({
  rule: {
    all: [
      { kind: 'call_expression' },
      { pattern: '$OBJ.$METHOD($$$ARGS)' },
    ]
  }
})

any - At least one rule must match (OR)

root.findAll({
  rule: {
    any: [
      { kind: 'field_definition' },
      { kind: 'public_field_definition' },
      { kind: 'class_field' },
    ]
  }
})

not - Negate a rule

// Find all identifiers that aren't 'constructor'
root.findAll({
  rule: {
    kind: 'identifier',
    not: { regex: '^constructor$' },
  }
})

matches - Reference a utility rule by ID

root.findAll({
  rule: { matches: 'is-ember-decorator' },
  utils: {
    'is-ember-decorator': {
      kind: 'decorator',
      has: { pattern: '@$NAME', inside: { kind: 'class_body' } },
    }
  }
})

3. Relational Rules

Filter nodes by their position relative to other nodes in the AST.

inside - Node is contained within a matching ancestor

// Find field_definition nodes that are DIRECT children of class_body
root.findAll({
  rule: {
    kind: 'field_definition',
    inside: {
      kind: 'class_body',
      stopBy: 'neighbor',  // Only check immediate parent
    }
  }
})

has - Node contains a matching descendant

// Find class declarations that have a decorator
root.findAll({
  rule: {
    kind: 'class_declaration',
    has: {
      kind: 'decorator',
      stopBy: 'neighbor',  // Only check direct children
    }
  }
})

follows - Node appears after a matching sibling

// Find nodes that follow a decorator
root.findAll({
  rule: {
    kind: 'field_definition',
    follows: { kind: 'decorator' },
  }
})

precedes - Node appears before a matching sibling

root.findAll({
  rule: {
    kind: 'decorator',
    precedes: { kind: 'method_definition' },
  }
})

The stopBy Parameter (Critical)

Controls how far relational rules search. This is the most important parameter for correct queries.

Common pattern: matching only direct class members

// WRONG: findAll with just kind searches ALL descendants recursively
classBody.findAll({ rule: { kind: 'field_definition' } })
// ^ This picks up nested properties inside object literals!

// RIGHT: Use inside rule with stopBy: 'neighbor' to match direct children only
classBody.findAll({
  rule: {
    kind: 'field_definition',
    inside: { kind: 'class_body', stopBy: 'neighbor' },
  }
})

The field Parameter

Restricts matches to a specific named field position in the parent node.

// Match only the KEY in a key-value pair, not values that happen to match
root.findAll({
  rule: {
    kind: 'pair',
    has: {
      field: 'key',         // Only match the 'key' field position
      regex: 'prototype',
    }
  }
})

Common tree-sitter fields: name, body, source, key, value, left, right, arguments, decorator, type_annotation.

Patterns Used in This Codebase

Finding direct class members (not nested)

import { NODE_KIND_CLASS_BODY, NODE_KIND_FIELD_DEFINITION, NODE_KIND_METHOD_DEFINITION } from './code-processing.js';

const DIRECT_CLASS_MEMBER = { inside: { kind: NODE_KIND_CLASS_BODY, stopBy: 'neighbor' } } as const;

// Properties - try multiple kinds since grammar varies
function findPropertyDefinitions(classBody: SgNode): SgNode[] {
  for (const nodeType of ['field_definition', 'public_field_definition', 'class_field']) {
    try {
      const props = classBody.findAll({ rule: { kind: nodeType, ...DIRECT_CLASS_MEMBER } });
      if (props.length > 0) return props;
    } catch {
      // Kind not valid in this grammar
    }
  }
  return [];
}

// Methods
function findMethodDefinitions(classBody: SgNode): SgNode[] {
  return classBody.findAll({ rule: { kind: NODE_KIND_METHOD_DEFINITION, ...DIRECT_CLASS_MEMBER } });
}

Finding import statements and extracting source

const imports = root.findAll({ rule: { kind: 'import_statement' } });
for (const imp of imports) {
  const source = imp.field('source');     // The string literal after 'from'
  const clause = imp.field('import');     // The import clause (specifiers)
  const sourcePath = source?.text();      // e.g., "'@ember-data/model'"
}

Finding decorators preceding a node

// Walk backwards through siblings collecting decorator nodes
function collectPrecedingDecorators(node: SgNode): string[] {
  const decorators: string[] = [];
  const siblings = node.parent()?.children() ?? [];
  const idx = siblings.indexOf(node);
  for (let i = idx - 1; i >= 0; i--) {
    const sib = siblings[i];
    if (!sib) continue;
    if (sib.kind() === 'decorator') decorators.unshift(sib.text());
    else if (sib.text().trim() !== '') break;
  }
  return decorators;
}

Finding a class that extends a specific base

// Find class with heritage clause
const classDecl = root.find({ rule: { kind: 'class_declaration' } });
const heritage = classDecl?.find({ rule: { kind: 'class_heritage' } });
const identifiers = heritage?.findAll({ rule: { kind: 'identifier' } }) ?? [];
const baseClasses = identifiers.map((id) => id.text());

Pattern matching with context for class fields

// Match decorated class fields like: @attr('string') name;
root.findAll({
  rule: {
    pattern: {
      context: 'class A { @$DECORATOR $FIELD = $VALUE }',
      selector: 'field_definition',
    }
  }
})

Debugging Tips

1. Use node.kind() liberally - When a rule isn't matching, log the actual kinds: classBody.children().map(c => c.kind())
2. Try/catch around findAll with rules - Invalid kind names throw at runtime, not compile time
3. Check stopBy behavior - The default 'neighbor' only searches one level. Use 'end' for recursive search.
4. Use the ast-grep playground - https://ast-grep.github.io/playground.html to test rules interactively