> ## Documentation Index
> Fetch the complete documentation index at: https://docs.refactron.dev/llms.txt
> Use this file to discover all available pages before exploring further.

# var_to_const_let

> Replace var declarations with const (or let if reassigned) per binding.

**Transform ID:** `var_to_const_let`
**Language:** TypeScript

## What it does

For every `var` declaration in the file, decide per binding:

* **`const`** if the binding is never reassigned after initialization.
* **`let`** if the binding is reassigned at least once.

Each binding is judged independently — multiple `var` statements in one file may map to a mix of `const` and `let`.

## Detector pattern

The detector at `src/analyze/detectors/typescript/var-declarations.ts` collects all `VariableStatement` nodes with the `var` keyword via ts-morph and then walks the surrounding scope to count reassignments per binding name.

## Preconditions

1. No binding in the file is **referenced before its declaration** (relying on `var` hoisting). If hoisting is used anywhere, the entire file is skipped — the transform refuses to break working hoisted code.
2. No `var` is declared inside a `with` statement (legacy scope rules differ from `let`/`const`).
3. The file contains at least one `var` statement.

## Before / after

<CodeGroup>
  ```ts before.ts theme={null}
  // Legacy var-style declarations.
  export function sumTo(n) {
    var counter = 0;
    for (var i = 1; i <= n; i++) {
      counter = counter + i;
    }
    return counter;
  }

  export function makeGreeting(name) {
    var greeting = 'hi';
    return greeting + ', ' + name;
  }
  ```

  ```ts after.ts theme={null}
  // Legacy var-style declarations.
  export function sumTo(n) {
    let counter = 0;
    for (let i = 1; i <= n; i++) {
      counter = counter + i;
    }
    return counter;
  }

  export function makeGreeting(name) {
    const greeting = 'hi';
    return greeting + ', ' + name;
  }
  ```
</CodeGroup>

## Edge cases handled

* Mixed const/let outcomes inside a single file based on each binding's mutability.
* Multiple independent `var`s in one file (`var a = 1; var b = 2; b = b + 1;` → `const a = 1; let b = 2; b = b + 1;`).
* `for (var i = 0; ...)` becomes `for (let i = 0; ...)` when `i` is mutated by the loop step.

## Edge cases NOT handled (skip via precondition)

* File relies on hoisting (e.g. `console.log(x); var x = 1;`).
* `var` declared inside a `with` block.
* `.cjs` files (handled by [`commonjs_to_esm`](/transforms/commonjs-to-esm) when relevant; `var` inside CommonJS modules has different historical patterns).