promise/always-return

A then() callback that returns nothing passes undefined to the next step in the chain. If the callback started more async work, the chain doesn't wait for it.

The rule checks the first argument of then() when it's a function with a { } body. Every path through it must end with return, throw, process.exit() or process.abort(). A bare return; is enough.

Configure

Off by default. To turn it on:

[lint.rules]
"promise/always-return" = "error" # or "warn"

Every option, set to its default:

[lint.rules]
"promise/always-return" = ["error", {
  ignoreLastCallback = false,
  ignoreAssignmentVariable = ["globalThis"],
}]

Examples

Reported:

fetchUser(id)
  .then(user => {
    user.lastSeen = Date.now();
  })
  .then(user => render(user)); // user is undefined

Not reported:

fetchUser(id)
  .then(user => {
    user.lastSeen = Date.now();
    return user;
  })
  .then(user => render(user));

migrate(database).then(() => {
  process.exit(0);
});

Options

Option	Default	What it does
ignoreLastCallback	false	Allow the last then() of a chain to return nothing, if nothing uses the result.
ignoreAssignmentVariable	["globalThis"]	Allow the last then() of a chain to return nothing if it assigns to one of these variables or to a property of one. Your list replaces the default.

The last then() can be followed by catch() or finally(), and the chain can be awaited:

fetchUser(id)
  .then(user => user.profile)
  .then(profile => {
    render(profile);
  })
  .catch(showError);

If the result is stored or returned, other code can still chain on it, so it's reported:

const rendered = fetchUser(id).then(user => {
  render(user);
});

With the default ignoreAssignmentVariable:

fetchUser(id).then(user => {
  globalThis.currentUser = user;
});

fetchUser(id).then(user => {
  window.currentUser = user;
});