Should Promise be rejected with Error or string?

Question

I am writing a code that heavily uses ECMAScript 6-like promises in Javascript.

I cannot decide, if it's more "correct" to reject promise with a string or with an error, since I have seen both of these patterns being used.

That is, if it's better to call

return new Promise(response, reject) {
    reject("Sky is falling.");
}

or

return new Promise(response, reject) {
    reject(new Error("Sky is falling."));
}

Both are valid. The important thing is to reject a Promise with something meaningful. — bvaughn, Mar 29 '15 at 22:01
I hope that it's understandable that my little example is just for illustration and not a real code :) — Karel Bílek, Mar 29 '15 at 22:01
Error is better, because then you get a stack trace as to where the error was. — Stefan Steiger, Jul 15 '19 at 06:46
ESLint's opinion is that `Error` is preferred: https://eslint.org/docs/rules/prefer-promise-reject-errors — vegemite4me, Nov 04 '20 at 10:40

score 21 · Accepted Answer · answered Mar 29 '15 at 22:04

A look at Mozilla's documentation is helpful: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise/reject

Promise.reject("Testing static reject").then(function(reason) {
  // not called
}, function(reason) {
  console.log(reason); // "Testing static reject"
});

Promise.reject(new Error("fail")).then(function(error) {
  // not called
}, function(error) {
  console.log(error); // Stacktrace
});

It shows both strings and errors as valid "reasons" for a rejection. The main thing (I think) is that the "reason" should be meaningful.

If a stack trace is helpful then it may be better to provide an error. If a simple string is all that's needed, then it will suffice.

The `Testing static reject` part is missing now on current version of Mozilla's documentation. There is rejection with `Error` object only. — Martin, Aug 17 '17 at 10:26

Should Promise be rejected with Error or string?

1 Answers1