Ast Grep Practice

ast-grep Practice

Complement general-purpose lint tools (ESLint, oxlint, Biome, clippy, etc.) with ast-grep for patterns they cannot express. Always prefer reproducible static rules over natural-language prompts.

Installation

bashCopy code
# npm (project-local recommended)
npm install -D @ast-grep/cli
npx ast-grep --help

# or cargo
cargo install ast-grep --locked

# or brew
brew install ast-grep

Package manager selection: If the project's package.json has packageManager set, follow it (pnpm / yarn / etc.). Otherwise install locally with npm. Use the same tool in CI (mixing them splits lockfile and binary resolution paths). Keep global installs on dev machines only; CI and in-repo scripts must always use local references.

Quick Start

Minimum configuration to verify operation:

bashCopy code
mkdir -p rules rule-tests
cat > sgconfig.yml << 'EOF'
ruleDirs:
  - rules
testConfigs:
  - testDir: rule-tests
EOF

cat > rules/no-console-log.yml << 'EOF'
id: no-console-log
language: TypeScript
severity: warning
rule:
  pattern: console.log($$$ARGS)
message: Do not leave console.log behind.
fix: ''
EOF

cat > rule-tests/no-console-log-test.yml << 'EOF'
id: no-console-log
valid:
  - logger.info('ok')
invalid:
  - console.log('debug')
  - "console.log('a', 'b')"
EOF

ast-grep test --skip-snapshot-tests  # run tests
ast-grep scan src/                    # scan project

Principles

• First check whether an existing linter can cover the rule
• Use ast-grep when you need "structural patterns"
• Develop rules with TDD: test-first → rule implementation → CI integration
• Write fix when you can. Prefer rules with auto-fix over detection-only rules

Choosing between ast-grep and existing linters

Signs that ast-grep is the right choice:

• The rule cannot be expressed by configuring existing rules
• It depends on parent/child/sibling AST relationships
• Automatic rewriting (migration) is required

Project setup

sgconfig.yml

yamlCopy code
ruleDirs:            # required: directories holding rule files
  - rules
testConfigs:         # optional: test configuration
  - testDir: rule-tests
utilDirs:            # optional: shared utility rules
  - rule-utils
languageGlobs:       # optional: mapping for non-standard extensions (unnecessary for TS/JS/Python etc.)
  html: ['*.vue', '*.svelte', '*.astro']

Directory layout

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project/
  sgconfig.yml
  rules/
    no-direct-env-access.yml
    prefer-result-type.yml
  rule-tests/
    no-direct-env-access-test.yml
    prefer-result-type-test.yml
    __snapshots__/
  rule-utils/
    is-async-function.yml

ast-grep scan runs every rule under ruleDirs starting from the directory that contains sgconfig.yml.

Rule file structure

yamlCopy code
id: no-direct-env-access
language: TypeScript
severity: warning
rule:
  pattern: process.env.$KEY
  not:
    inside:
      kind: function_declaration
      has:
        pattern: getEnv
      stopBy: end
message: Do not reference process.env directly. Go through getEnv().
note: Ensures type-safe access to environment variables.
fix: getEnv('$KEY')
files:
  - "src/**"
ignores:
  - "src/config.ts"

Fields

Suppression comments

typescriptCopy code
// ast-grep-ignore
someCode()

// ast-grep-ignore: no-direct-env-access
process.env.NODE_ENV

Metavariable pitfalls

Pattern-matching caveats:

• $OBJ.$PROP matches dot access only. It does not match obj['key'] (bracket access)
• $VAR matches exactly one AST node
• $$$VARS matches zero or more AST nodes (variadic arguments, multiple statements, etc.)
• $_ is a wildcard (does not capture). The same name can match different contents
• A metavariable must occupy a whole node: obj.on$EVENT and "hello $NAME" do not work

fix (auto-fix)

Deciding whether to attach a fix

fix is convenient, but since it is applied automatically it can change semantics. Do not attach one (detection-only) when:

• The rewrite changes type safety (e.g. as any → as unknown changes type-inference results)
• Side effects or evaluation order may change (short-circuit behavior, timing of exceptions)
• Context dependence means the correct replacement is not unique (e.g. API migrations that swap argument order; review is mandatory)
• The deletion entangles with other expressions in the same statement

When in doubt, skip fix and document the manual migration steps in note. Only attach fix when you are confident that "replacing everywhere is safe".

Basics

yamlCopy code
rule:
  pattern: console.log($ARG)
fix: logger.info($ARG)

Metavariables are usable as-is inside the fix template. Unmatched metavariables become empty strings.

Deletion

yamlCopy code
rule:
  pattern: console.log($$$ARGS)
fix: ''

fix: '' deletes the matched node. Note that a blank line may remain. If you want to also remove trailing ; on statements or trailing commas, always combine with expandEnd (see "Range expansion" below). If leaving a blank line is acceptable, expandEnd is not needed. Rule of thumb: in projects where a formatter (Prettier etc.) runs after, blank lines get tidied automatically, so expandEnd is unnecessary; if no formatter runs, expandEnd is recommended.

Fix when bundling multiple patterns with any:

If each branch under any: can use the same fix template, it is fine to consolidate into one rule:

yamlCopy code
rule:
  any:
    - pattern: $ARR.filter($P).length === 0
    - pattern: $ARR.filter($P).length == 0
fix: '!$ARR.some($P)'   # metavariables shared across both branches + identical template

If the fix differs per branch, always split into separate rules (you cannot write a per-branch fix inside any:). Example: === 0 → !some() and !== 0 → some() should be split. Splitting is acceptable even when the intent is the same (aligning the ids as *-empty / *-nonempty keeps things readable).

Multi-line

yamlCopy code
rule:
  pattern: |
    def foo($X):
      $$$S
fix: |-
  def bar($X):
    $$$S

Indentation is preserved relative to the original code's position.

Range expansion (FixConfig)

When you want to include the trailing comma etc. in the deletion:

yamlCopy code
fix:
  template: ''
  expandEnd:
    regex: ','

Quick rewrites from the CLI

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ast-grep run --pattern 'oldFunc($$$ARGS)' --rewrite 'newFunc($$$ARGS)' --lang typescript .
# --update-all applies everywhere without confirmation

constraints

Add extra conditions to metavariables. Only $ARG is supported ($$$ARGS is not). It filters matches after the rule matches.

Choosing between constraints and structural constraints (has/inside/not):

• You want to constrain the contents of a metavariable → constraints (e.g. $METHOD is one of get / set / delete)
• You want to constrain the structure outside or inside the pattern → has / inside / not / precedes / follows (e.g. inside a specific parent, or having a specific child)
• If you can write the concrete literal directly in the pattern, that is the simplest (pattern: new Set($X) guarantees Set is present)

Writing pattern together with has / not directly under rule is evaluated as AND (matches pattern AND has is true). Use this shape to tack on structural constraints that pattern alone cannot express.

yamlCopy code
rule:
  pattern: $OBJ.$METHOD($$$ARGS)
constraints:
  METHOD:
    regex: '^(get|set|delete)$'
  OBJ:
    kind: identifier

Usable fields: kind, regex, pattern

Note: constrained metavariables inside not may not behave as expected.

transform

Textually transform matched metavariables before using them in fix.

replace (regex replacement)

yamlCopy code
transform:
  NEW_NAME:
    replace:
      source: $NAME
      replace: 'get(\w+)'
      by: 'fetch$1'
fix: $NEW_NAME($$$ARGS)

substring

yamlCopy code
transform:
  INNER:
    substring:
      source: $STR
      startChar: 1
      endChar: -1

Negative indices count from the end. Same semantics as Python slicing.

convert (case conversion)

yamlCopy code
transform:
  SNAKE:
    convert:
      source: $NAME
      toCase: snakeCase
      separatedBy: [caseChange]

Supported cases: camelCase, snakeCase, kebabCase, pascalCase, upperCase, lowerCase, capitalize

rewrite (experimental)

Recursively rewrite nodes inside a metavariable using rewriter rules.

yamlCopy code
transform:
  REWRITTEN:
    rewrite:
      source: $$$BODY
      rewriters: [migrate-api-call]
      joinBy: "\n"

utils (utility rules)

Reference shared rules defined under utilDirs with matches.

yamlCopy code
# rule-utils/is-async-function.yml
id: is-async-function
language: TypeScript
rule:
  any:
    - kind: function_declaration
      has:
        field: async
        regex: async
    - kind: arrow_function
      has:
        field: async
        regex: async

yamlCopy code
# rules/async-no-try-catch.yml
id: async-no-try-catch
language: TypeScript
rule:
  all:
    - matches: is-async-function
    - has:
        pattern: await $EXPR
        stopBy: end
    - not:
        has:
          kind: try_statement
          stopBy: end
message: async function lacks try-catch.
severity: warning

Testing

There are two kinds of tests. Do not conflate them:

• Classification test (test --skip-snapshot-tests): only verifies that the code listed under valid / invalid is classified correctly. This is the one to run in CI.
• Snapshot test (test / test -U): pins the match positions and fix results on invalid code as snapshots and detects regressions. Generate for the first time with -U, then have humans review afterwards. Run at least once before CI.

Test file format

The id inside the test file must match the id of the rule file. The filename is free (convention: {rule-id}-test.yml).

yamlCopy code
# rule-tests/no-direct-env-access-test.yml
id: no-direct-env-access
valid:
  - getEnv('NODE_ENV')
  - "function setup() { return getEnv('PORT') }"
invalid:
  - process.env.NODE_ENV
  - process.env.PORT

Running tests

bashCopy code
# classification test (is valid/invalid correct?)
ast-grep test --skip-snapshot-tests

# generate / update snapshots
ast-grep test -U

# interactive snapshot review
ast-grep test --interactive

Test result markers:

• . : pass
• N : noisy (false positive — matches valid code)
• M : missing (false negative — fails to match invalid code)

Workflow

1. Write the test file under rule-tests/ (Red)
2. Write the rule under rules/ (Green)
3. Verify with ast-grep test --skip-snapshot-tests
4. Generate snapshots with ast-grep test -U
5. Review the snapshots and commit

CI integration

justfile

justCopy code
ast-grep-test:
  ast-grep test

ast-grep-lint:
  ast-grep scan

check: format-check typecheck ast-grep-lint test

GitHub Actions

Align tools with the dev environment (use pnpm in CI if the project uses pnpm, npm if it uses npm):

yamlCopy code
- uses: actions/setup-node@v7
  with: { node-version: 24, cache: npm }   # for pnpm projects: pnpm/action-setup@v6 + cache: pnpm

- run: npm ci   # for pnpm: pnpm install --frozen-lockfile

- name: ast-grep rule tests
  run: npx ast-grep test --skip-snapshot-tests

- name: ast-grep scan
  run: npx ast-grep scan --error

severity and exit codes:

• ast-grep scan exits non-zero by default if at least one finding has error severity
• Passing --error makes warning / hint also cause non-zero exit (use when CI should fail on warnings too)
• You can specify a severity like --error=error to tighten gradually
• --format json for structured output (for integrating with other tools)

Looking up kind names

Kind names depend on the language's Tree-sitter grammar.

bashCopy code
# AST dump (named nodes only, use these when writing rules)
ast-grep run --pattern 'YOUR_CODE' --lang typescript --debug-query=ast

# CST dump (all nodes, including anonymous tokens)
ast-grep run --pattern 'YOUR_CODE' --lang typescript --debug-query=cst

See references/kind-catalog.md for a per-language catalog of common kinds (covers TypeScript / Rust / Go / Python).

Practical rule examples

TypeScript: forbid as any casts (detection only, no fix)

yamlCopy code
id: no-as-any
language: TypeScript
severity: error
rule:
  pattern: $EXPR as any
message: as any disables the type system. Go through as unknown or a type guard.
note: |
  Why no auto-fix: mechanically replacing `as any` → `as unknown` changes type
  inference results and introduces new compile errors at call sites. Detection-only,
  migrate manually.

When matching a type assertion like as any, $EXPR as any works on the as_expression node. $EXPR matches the whole left-hand side, so it matches both JSON.parse(raw) as any and (value as any).

TypeScript: rewrite a deprecated API

yamlCopy code
id: migrate-old-api
language: TypeScript
severity: error
rule:
  pattern: oldClient.fetch($URL, $OPTS)
fix: newClient.request($URL, $OPTS)
message: oldClient.fetch is deprecated. Migrate to newClient.request.

Forbid a specific import

yamlCopy code
id: no-lodash-import
language: TypeScript
severity: warning
rule:
  pattern: import $_ from 'lodash'
message: Do not import lodash wholesale. Use lodash/xxx.
fix: import $_ from 'lodash/xxx' // TODO: fix the correct path

TypeScript: forbid direct fetch inside React components

yamlCopy code
id: no-fetch-in-component
language: TypeScript
severity: warning
rule:
  pattern: fetch($$$ARGS)
  inside:
    any:
      - kind: function_declaration
        has:
          field: return_type
          pattern: JSX.Element
      - kind: arrow_function
        inside:
          kind: variable_declarator
          regex: '^[A-Z]'
    stopBy: end
message: Do not fetch directly inside a component. Use hooks or a server action.

Rust: forbid unwrap()

yamlCopy code
id: no-unwrap
language: Rust
severity: warning
rule:
  pattern: $EXPR.unwrap()
  not:
    inside:
      kind: function_item
      regex: '#\[test\]'
      stopBy: end
message: Do not use unwrap() outside tests. Use ? or expect().
note: unwrap() panics, so avoid it in production code.

Rust: flag unsafe blocks

yamlCopy code
id: flag-unsafe-block
language: Rust
severity: warning
rule:
  kind: unsafe_block
message: unsafe block. Explain the safety rationale in a comment.

Rust: migrate println! to log macros

yamlCopy code
id: no-println-in-lib
language: Rust
severity: warning
rule:
  pattern: println!($$$ARGS)
  not:
    inside:
      kind: function_item
      regex: 'fn main'
      stopBy: end
message: Do not use println! in library code. Use log::info! etc.
fix: log::info!($$$ARGS)
files:
  - "src/lib.rs"
  - "src/**/mod.rs"
  - "src/**/*.rs"
ignores:
  - "src/main.rs"
  - "src/bin/**"

Go: detect ignored errors

yamlCopy code
id: no-ignored-error
language: Go
severity: error
rule:
  kind: short_var_declaration
  has:
    kind: identifier
    regex: '^_$'
    field: left
  has:
    kind: call_expression
    field: right
    stopBy: end
message: Do not ignore errors with _. Handle them appropriately.

Go: prevent forgetting defer Close()

yamlCopy code
id: defer-close-after-open
language: Go
severity: warning
rule:
  kind: short_var_declaration
  has:
    pattern: os.Open($PATH)
    field: right
    stopBy: end
  not:
    precedes:
      pattern: defer $_.Close()
      stopBy:
        kind: return_statement
message: Add defer Close() immediately after os.Open.

Python: forbid bare except

yamlCopy code
id: no-bare-except
language: Python
severity: warning
rule:
  kind: except_clause
  not:
    has:
      kind: identifier
      stopBy: neighbor
message: Do not use bare except. Specify the exception type.

Python: migrate print() to a logger

yamlCopy code
id: no-print-in-src
language: Python
severity: warning
rule:
  pattern: print($$$ARGS)
  not:
    inside:
      kind: function_definition
      regex: 'def main'
      stopBy: end
message: Use logger instead of print().
fix: logger.info($$$ARGS)
files:
  - "src/**"

References

In-skill details

• references/rule-yaml.md — all rule YAML fields, evaluation order, metavariable binding scope, any: + fix consolidation/splitting, $$$ARGS empty match, etc.
• references/testing.md — classification vs snapshot tests, multi-line code notation, snapshot operation
• references/cli.md — subcommands and flags in general, --error / exit codes / --format json
• references/kind-catalog.md — per-language kind catalog (TypeScript / Rust / Go / Python)

Official

• ast-grep docs: https://ast-grep.github.io/
• Rule reference: https://ast-grep.github.io/reference/yaml.html
• sgconfig: https://ast-grep.github.io/reference/sgconfig.html
• Playground: https://ast-grep.github.io/playground.html
• Rule catalog: https://ast-grep.github.io/catalog/