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Transform ID: promise_chains_to_async Language: TypeScript

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What it does

Rewrites fn(...).then(a => ...).then(b => ...) chains as async functions with await and named const bindings for each intermediate value. Both flat (a().then(b).then(c)) and nested (a().then(x => b(x).then(y => y))) shapes are handled.

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Detector pattern

The detector at src/analyze/detectors/typescript/promise-chains.ts walks ts-morph CallExpression nodes whose expression is a PropertyAccessExpression named then, climbing the chain to identify the originating call.

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Preconditions

  1. The function body returns the chain (or the chain is the only statement). Mid-function chains used for side effects only are skipped.
  2. No .catch(...) in the chain. The transform doesn’t synthesize try/catch blocks — pre-existing error handling would change semantics.
  3. No Promise.all / Promise.race / Promise.allSettled / Promise.any inside the chain — combinator semantics aren’t trivially rewritable as sequential awaits.
  4. The enclosing function returns a Promise (so adding async is type-preserving).

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Before / after

async function fetchUser(id) {
  return Promise.resolve({ id: id, name: 'user-' + id });
}

async function fetchPosts(userId) {
  return Promise.resolve([
    { id: 1, userId: userId, title: 'first' },
    { id: 2, userId: userId, title: 'second' },
  ]);
}

export function loadProfile(id) {
  return fetchUser(id).then((u) =>
    fetchPosts(u.id).then((posts) => ({ user: u, posts: posts })),
  );
}

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Edge cases handled

  • Flat chains (a().then(b).then(c)).
  • Nested chains (a().then(x => b(x).then(y => y))).
  • Preserves the enclosing function’s exports / signature shape.

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Edge cases NOT handled (skip via precondition)

  • Chain contains .catch(...) (preconditions: no-catch).
  • Chain contains Promise.all / Promise.race / Promise.allSettled / Promise.any (precondition: no-promise-combinator).
  • Chain handler returns void / does side effects only.
  • Multiple parallel chains on the same value.