> 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/basics/manipulating-ast.md).

# Manipulating ASTs

The `go-fAST` library provides a powerful and flexible mechanism to walk and transform Abstract Syntax Trees (ASTs). This guide explains how you can **safely and correctly modify ASTs** during traversal using custom visitors.

### 💡 Key Concepts

#### Node Replacement Rules

When modifying the AST, it’s **critical** that replacement nodes conform to the expected type. For example:

* You **can** replace an `*ast.Expression` with another `*ast.Expression` wrapping a different inner `Expr`.
* Since you must replace nodes with their equivilent types, as an example what you **cannot** do is replace an `*ast.ForStatement` with an `*ast.Identifier`.

#### Statement vs Expression

All nodes in the AST are either:

* A **statement**, implementing the `ast.Stmt` interface.
* An **expression**, implementing the `ast.Expr` interface.

Always ensure you're replacing nodes **with compatible interface types**.

### 🔁 Example: Replacing Number Literals with Strings

Below is an example demonstrating how to replace every number literal with a string literal containing `"foo"`:

```go
goCopyEdittype ExampleVisitor struct {
    ast.NoopVisitor
}

func (v *ExampleVisitor) VisitExpression(n *ast.Expression) {
    // Step 1: Visit child expressions first
    n.VisitChildrenWith(v)

    // Step 2: Check if this is a number literal
    _, isNumberLiteral := n.Expr.(*ast.NumberLiteral)
    if !isNumberLiteral {
        return // Not a number literal; no replacement needed
    }

    // Step 3: Create a new string literal expression
    replacement := "foo"
    *n = ast.Expression{
        Expr: &ast.StringLiteral{
            Value:   replacement,
        },
    }
}
```

### 🔍 Input JavaScript (Before Visitor)

```js
var x = 42;
var y = x + 8;
console.log(y);
```

### 🔄 Transformed JavaScript (After Visitor)

```js
var x = "foo";
var y = x + "foo";
console.log(y);
```

#### Notes:

* The `VisitChildrenWith(v)` call ensures recursive traversal before mutation (to ensure children of this Expression are visited and replaced)
* The replacement assigns directly to `*n`, ensuring the parent retains the correct `*ast.Expression` type.

### Statement & Expression Types

If a node is a statement, it will implement the `ast.Stmt` interface. If it is an expression, it will implement the `ast.Expr` interface.


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