---
id: wiki-2026-0508-ast-traversal
title: AST Traversal
category: 10_Wiki/Topics
status: verified
canonical_id: self
aliases: [ast-walk, syntax-tree-traversal, visitor-pattern]
duplicate_of: none
source_trust_level: A
confidence_score: 0.9
verification_status: applied
tags: [ast, compilers, codemod, static-analysis]
raw_sources: []
last_reinforced: 2026-05-10
github_commit: pending
tech_stack:
  language: typescript
  framework: ts-morph
---

# AST Traversal

## 매 한 줄
> **"매 code 의 tree 의 walk 한다"**. 매 AST (Abstract Syntax Tree) traversal 의 compiler/linter/codemod/IDE 의 foundation. 매 2026 의 tree-sitter (Atom, 2018; 매 GitHub 의 sole semantic search engine) + ts-morph + Babel + RustPython AST 의 dominant tooling.

## 매 핵심

### 매 traversal strategies
- **Pre-order DFS**: 매 visit parent → children. 매 default (most visitors).
- **Post-order DFS**: 매 visit children → parent. 매 type inference, dead-code elim.
- **BFS**: 매 level-by-level. 매 rare — scope analysis.
- **Visitor pattern**: 매 node-type-keyed callbacks. 매 ESLint, Babel, ts-morph standard.

### 매 mutating vs read-only
- **Read-only**: linter, complexity metrics, security scanner.
- **Mutating**: codemod, formatter, transpiler. 매 immutability + new tree 의 produce 의 best-practice (Babel: `path.replaceWith`).

### 매 응용
1. ESLint rule — 매 pattern detection.
2. Codemod — jscodeshift, ts-morph, ast-grep.
3. Tree-sitter query — 매 IDE syntax highlight, code nav.
4. AST-based diffing — 매 difftastic, semantic diff.

## 💻 패턴

### Babel visitor (JavaScript)
```js
import { parse } from '@babel/parser';
import traverse from '@babel/traverse';
import generate from '@babel/generator';
import * as t from '@babel/types';

const code = `console.log("hello"); foo("world");`;
const ast = parse(code);

traverse(ast, {
  CallExpression(path) {
    const callee = path.node.callee;
    if (t.isMemberExpression(callee) &&
        t.isIdentifier(callee.object, { name: 'console' })) {
      // strip console.* calls
      path.remove();
    }
  }
});

console.log(generate(ast).code); // foo("world");
```

### ts-morph (TypeScript refactor)
```ts
import { Project, SyntaxKind } from 'ts-morph';

const project = new Project({ tsConfigFilePath: 'tsconfig.json' });
for (const sf of project.getSourceFiles()) {
  sf.forEachDescendant(node => {
    if (node.getKind() === SyntaxKind.CallExpression) {
      const ce = node.asKindOrThrow(SyntaxKind.CallExpression);
      if (ce.getExpression().getText() === 'fetch') {
        ce.replaceWithText(`httpClient.${ce.getText().replace('fetch', '')}`);
      }
    }
  });
}
await project.save();
```

### Python — `ast` + NodeTransformer
```python
import ast

class StripPrint(ast.NodeTransformer):
    def visit_Expr(self, node):
        if (isinstance(node.value, ast.Call)
            and isinstance(node.value.func, ast.Name)
            and node.value.func.id == "print"):
            return None  # remove
        return node

src = "print('hi')\nx = 1"
tree = ast.parse(src)
StripPrint().visit(tree)
ast.fix_missing_locations(tree)
print(ast.unparse(tree))  # x = 1
```

### tree-sitter query (multi-language)
```scheme
; queries/python/calls.scm — find all decorated functions
(decorated_definition
  (decorator (call function: (identifier) @decorator-name))
  definition: (function_definition name: (identifier) @func-name))
```

```python
import tree_sitter_python as tspython
from tree_sitter import Language, Parser, Query

PY = Language(tspython.language())
parser = Parser(PY)
tree = parser.parse(b"@app.route('/')\ndef home(): pass")
q = Query(PY, open("queries/python/calls.scm").read())
for node, name in q.captures(tree.root_node):
    print(name, node.text)
```

### ast-grep (rule-based, polyglot)
```yaml
# rule.yml
id: no-console-log
language: js
rule:
  pattern: console.log($$$)
fix: ''
```

```bash
ast-grep scan -r rule.yml --update
```

### Rust — syn (proc-macro / codegen)
```rust
use syn::{visit::Visit, ItemFn, File};

struct FnCounter(usize);
impl<'ast> Visit<'ast> for FnCounter {
    fn visit_item_fn(&mut self, i: &'ast ItemFn) {
        self.0 += 1;
        syn::visit::visit_item_fn(self, i);
    }
}

let src = std::fs::read_to_string("src/lib.rs").unwrap();
let file: File = syn::parse_file(&src).unwrap();
let mut c = FnCounter(0);
c.visit_file(&file);
println!("functions: {}", c.0);
```

### ESLint custom rule
```js
module.exports = {
  meta: { type: 'problem', schema: [] },
  create(ctx) {
    return {
      'CallExpression[callee.name="eval"]'(node) {
        ctx.report({ node, message: 'eval is forbidden' });
      }
    };
  }
};
```

### Pre-order vs post-order (manual walk)
```ts
function walkPre(node: Node, fn: (n: Node) => void) {
  fn(node);
  for (const c of node.children) walkPre(c, fn);
}
function walkPost(node: Node, fn: (n: Node) => void) {
  for (const c of node.children) walkPost(c, fn);
  fn(node);
}
```

## 매 결정 기준
| Goal | Tool |
|---|---|
| JS/TS lint rule | ESLint visitor |
| TS large refactor | ts-morph |
| Python codemod | LibCST (preserves whitespace) > ast |
| Polyglot pattern search | ast-grep, Semgrep |
| IDE / syntax highlight | tree-sitter |
| Rust macro | syn / quote |
| Babel plugin | @babel/traverse |

**기본값**: 매 ts-morph (TS refactor) · ast-grep (polyglot scan) · LibCST (Python codemod).

## 🔗 Graph
- 부모: [[Static-Analysis]]
- 변형: [[Visitor-Pattern]]
- 응용: [[ESLint]] · [[Refactoring_Best_Practices|Refactoring]]
- Adjacent: [[Architectural-Constraint-Enforcement]] · [[Architecture_Refactor]]

## 🤖 LLM 활용
**언제**: 매 codebase 의 LLM-driven structural query — 매 ast-grep + Claude 의 hybrid (LLM 의 generate pattern, ast 의 execute).
**언제 X**: 매 LLM 의 raw text find/replace — 매 AST-aware tool 의 사용.

## ❌ 안티패턴
- **Regex on code**: 매 multiline construct 의 break — 매 AST 의 사용.
- **Mutating during traversal**: 매 visitor 의 reentrancy bug — 매 collect-then-apply.
- **Ignore comments/whitespace**: 매 codemod 의 lose comments — 매 LibCST/Recast 의 사용.
- **Single-pass dependence**: 매 transformation order 의 fragile — 매 idempotent 의 design.

## 🧪 검증 / 중복
- Verified (Babel docs; ts-morph guide; tree-sitter playground; *Crafting Interpreters* — Nystrom).
- 신뢰도 A.

## 🕓 Changelog
| 날짜 | 변경 |
|---|---|
| 2026-05-08 | Phase 1 |
| 2026-05-10 | Manual cleanup — Babel/ts-morph/tree-sitter/ast-grep patterns |