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Transform ID: var_to_const_let Language: TypeScript

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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.

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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.

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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.

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Before / after

// 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;
}

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Edge cases handled

  • Mixed const/let outcomes inside a single file based on each binding’s mutability.
  • Multiple independent vars 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.

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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 when relevant; var inside CommonJS modules has different historical patterns).