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defensive.promise-default-fallbacks

Flags promise .catch() handlers that suppress rejected async work with a cheap fallback.

  • Family: defensive
  • Severity: strong
  • Scope: file
  • Requires: file.ast

How it works

The rule looks for promise-chain catch handlers that turn rejection into:

  • an empty handler body like .catch(() => {})
  • a direct sentinel default like null, undefined, false, 0, "", [], or {}
  • a log-and-default block like console.error(error); return false

This is intentionally distinct from the existing try/catch defensive rules. It targets the promise-chain version of the same failure-suppression habit, which shows up frequently in generated async glue code.

To avoid obvious noise, the rule skips very large bundled/generated files over 5000 logical lines.

Flagged examples

export async function loadConfig() {
  return fetchConfig().catch(() => null);
}

export async function readClipboard() {
  return navigator.clipboard.readText().catch(() => {});
}

export async function loadFlag() {
  return fetchFlag().catch((error) => {
    console.error("flag load failed", error);
    return false;
  });
}

Usually ignored

export async function loadConfig() {
  return fetchConfig().catch((error) => {
    throw error;
  });
}

export async function loadConfigResult() {
  return fetchConfig().catch(() => ({ ok: false, reason: "missing" }));
}

How to fix / do this better

A promise catch should usually preserve failure meaning instead of converting it into a cheap sentinel.

Better options:

  • let the rejection propagate
  • transform the error while preserving context
  • return an explicit result type when the caller truly needs a non-throwing contract
  • narrow the fallback to a boundary where a default is genuinely safe
export async function loadConfig() {
  try {
    return await fetchConfig();
  } catch (error) {
    throw new Error("Failed to load config", { cause: error });
  }
}

If a fallback is intentional, make it domain-shaped and explicit rather than null, false, or an empty object that hides why the operation failed.

Scoring

Each flagged promise catch adds 2 points. Log-and-default handlers add 2.5 points. The file total is capped at 8.

Benchmark signal

Small pinned rule benchmark (manifest):

  • Signal rank: #1 of 9
  • Signal score: 0.81 / 1.00
  • Best separating metric: findings / file (1.00)
  • Hit rate: 6/6 AI repos vs 4/5 mature OSS repos
  • Full results: rule signal report