> For the complete documentation index, see [llms.txt](https://gofast.disasm.dev/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://gofast.disasm.dev/getting-started/depth-first-traversal.md).

# Depth-First Traversal

## Depth-First Traversal in AST

**Depth-first traversal** is a method of exploring a tree structure, such as an Abstract Syntax Tree (AST), by diving deep into each branch before backtracking. In `go-fAST`, this is the default traversal strategy used when implementing custom visitors.

This page walks through what depth-first traversal means in practice, using a JavaScript example and mapping it to the order of AST node visitation.

***

### 📘 What It Means

In a depth-first traversal:

* You visit a node
* Then recursively visit its children
* Then move on to the next sibling

This pattern is ideal for AST processing because it naturally follows the syntactic structure of code.

***

### 📄 JavaScript Example

Here’s a simple JavaScript program:

```javascript
var a = 1;
var b = 2;
function foo(d, e) {
   return d + e;
}
console.log(foo(a, b));
```

***

### 🔍  Traversal Order

The **depth-first traversal** would look like this:

1. **Program** (root)
2. **VariableDeclaration** (var a = 1)
   * **VariableDeclarator**
     * **Identifier** `a`
     * **NumberLiteral** `1`
3. **VariableDeclaration** (var b = 2)
   * **VariableDeclarator**
     * **Identifier** `b`
     * **NumberLiteral** `2`
4. **FunctionDeclaration** `foo`
   * **Identifier** `foo`
   * **ParameterList**
     * **Identifier** `d`
     * **Identifier** `e`
   * **BlockStatement**
     * **ReturnStatement**
       * **BinaryExpression** `d + e`
         * **Identifier** `d`
         * **Identifier** `e`
5. **ExpressionStatement** (console.log(...))
   * **CallExpression**
     * **MemberExpression** `console.log`
       * **Identifier** `console`
       * **Identifier** `log`
     * **Arguments**
       * **CallExpression** `foo(a, b)`
         * **Identifier** `foo`
         * **Identifier** `a`
         * **Identifier** `b`

***

### 💡 Visualization Tip

Imagine the tree as a nested structure:

```
Program
├── VarDecl (a)
│   ├── Identifier
│   └── NumberLiteral
├── VarDecl (b)
│   ├── Identifier
│   └── NumberLiteral
├── FunctionDecl (foo)
│   ├── Identifier
│   ├── Params
│   │   ├── Identifier (d)
│   │   └── Identifier (e)
│   └── Block
│       └── Return
│           └── BinaryExpr (d + e)
│               ├── Identifier
│               └── Identifier
└── ExpressionStmt (console.log)
    └── CallExpr
        ├── MemberExpr (console.log)
        │   ├── Identifier
        │   └── Identifier
        └── CallExpr (foo(a, b))
            ├── Identifier
            ├── Identifier
            └── Identifier
```

***

### ✅ Summary

* go-fAST performs **depth-first traversal** by default via `VisitWith()`
* This means each node is visited before its siblings, and all its children are visited first


---

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