Rule proposal: report dangling `.then` calls in async functions

Please describe what the rule should do:

What category of rule is this? (place an “X” next to just one item)

[ ] Enforces code style [x] Warns about a potential error [ ] Suggests an alternate way of doing something [ ] Other (please specify:)

Provide 2-3 code examples that this rule will warn about:

// invalid:
async function invalid() {
  foo().then(bar);

  foo().then(result => {
    bar(result);
  });

  foo().then(result => {
    bar(result);
  }).catch(doSomethingElse);
}

// valid:
async function valid() {
  await foo().then(bar);

  await foo().then(result => {
    bar(result);
  });

  return foo().then(result => {
    bar(result);
  }).catch(doSomethingElse); // (unsure about this case -- maybe `catch` could be opt-out)
}

Why should this rule be included in ESLint (instead of a plugin)?

This rule would flag a common error that developers make when they start working with promises and async functions. The invalid examples are typically bugs, because:

• If the foo() promise rejects, the error cannot be caught anywhere and might silently be ignored depending on the environment.
• The promise returned by foo().then(...) might still be pending when the surrounding function returns.
• In most cases, developers intend for errors to be handled somewhere and not silently ignored.
• If the developer actually did intend to swallow errors, they should make that intention explicit by using .catch or a comment, so that the code doesn’t look like it has a bug.

[mysticatea]

Hmm… I’m not sure if this rule is valuable because I think it’s rare cases that people use a mix of .then and await. I probably support if the rule disallows all .then in async functions, but…

About C#, Visual Studio reports function calls that their return type is Task/Task<T> (E.g. async methods) if the call is not in await expressions. The feature has caught async function calls which are without await. It’s very useful personally. On the other hand, ESLint does not know the return type of functions. Yes, .then() probably returns a Promise. But I’m not sure about the value of reporting the restricted case.

[not-an-aardvark]

Closing because there doesn’t seem to be much interest in this.