Promise do not bubble outside

Question

I need to test function testMe using Mocha. But there is trouble when my unit test throw an error. Here is simpified example

function testMe(callback) {
    new Promise((resolve, reject) => {
        setTimeout(() => resolve([1,2,3]), 1000);
    }).then((result) => {
        callback(null, result);
    }).catch((error) => {
        callback(error, null)
    });
}

testMe((err, result) => {
   if(err) throw new Error();
   if(result.length < 5) throw new Error();
});

In this example after throw runs catch block. But I need to run catch block only after reject.

EDIT:

In this case the script never stop. I don't understand why.

function testMe(callback) {
    new Promise((resolve, reject) => {
        setTimeout(() => resolve([1,2,3]), 1000);
    }).then((result) => {
        callback(null, result);
    }, (error) => {
        callback(error, null)
    }).catch(() => {
        console.log('Do not throw an error but still running');
    });
}

testMe((err, result) => {
   if(err) throw new Error();
   if(result.length < 5) throw new Error();
});

But I need to use it. Because inside testMe I'm using horseman API. — Игорь, Apr 10 '17 at 08:49
[Use `.then(…, …)` instead of `.then(…).catch(…)`](http://stackoverflow.com/q/24662289/1048572) to avoid calling the `callback` twice, though that still won't get you an exception; only an unhandled rejection. — Bergi, Apr 10 '17 at 08:49
No, you don't need to pass a callback to `testMe` - you should have `testMe` *return* a promise, and then check that with mocha. — Bergi, Apr 10 '17 at 08:51
If I use .then(..., ...) instead of .then().catch() I receive UnhandledPromiseRejectionWarning: Unhandled promise rejection (rejection id: 1): Error — Игорь, Apr 10 '17 at 08:52
There is a lot of functions that receive callbacks with common interface. But one of them need to use promise inside. — Игорь, Apr 10 '17 at 08:54
The proper course of action would be to promisify all those functions of course :-) But still, you should not (cannot) `throw` in asynchronous test cases, you need to call Mocha's `done` callback with the error. (Or maybe Mocha supports promise return values, I don't know) — Bergi, Apr 10 '17 at 18:46
Bergi, I don't throw myself. Mocha throws inside expect statement. — Игорь, Apr 10 '17 at 18:48
Then `catch` that and call mocha's asynchronous callback, or make mocha aware that you are using promises. — Bergi, Apr 10 '17 at 18:52
Yes, I am using first option you promote. And there is problem. — Игорь, Apr 10 '17 at 18:55
Where is the [asynchronous `done` callback](https://mochajs.org/#asynchronous-code) in your code? Or why don't you [`return` the promise](https://mochajs.org/#working-with-promises)? — Bergi, Apr 10 '17 at 18:57
Done callback should be at the end of testMe callback handler. — Игорь, Apr 10 '17 at 19:01
The unit test you currently have will simply not work properly with asynchronous code. You *should* change it, promises or not. — Bergi, Apr 10 '17 at 19:09
Current generic unit tests works with callbacks correctly, but the problem is using a promise inside one of functions. — Игорь, Apr 10 '17 at 19:12
Well, the proper fix is to make the tests use promises. But if you absolutely want to go back from your promise function to callbacks, you can [use `.done`](http://stackoverflow.com/questions/26667598/will-javascript-es6-promise-support-done-api). Are you using a particular promise library? — Bergi, Apr 10 '17 at 19:20
@Bergi Thanks, it seems like correct solution. Can you post an answer? — Игорь, Apr 11 '17 at 16:19

rsp · Accepted Answer · 2017-04-10T09:12:31.357

2

When you work with promises then return the promises from functions instead of taking callbacks.

For example, instead of:

function testMe(callback) {
    new Promise((resolve, reject) => {
        // ...
    });
}

use:

function testMe(callback) {
    return new Promise((resolve, reject) => {
        // ...
    });
}

that way you will have the promise available to the caller of the function.

If you need to mix both styles, i.e. returning promises and taking callbacks, consider using a reliable library to handle that for you especially if you have trouble coding the translation between those style yourself:

edited Apr 10 '17 at 09:12

answered Apr 10 '17 at 09:02

rsp

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But I have common mocha test that don't wait a promise. – Игорь Apr 10 '17 at 09:03
How to use these libraries in my case? – Игорь Apr 10 '17 at 09:11

score 1 · Answer 2 · answered Apr 10 '17 at 18:59

You can simply return the promise from the test:

function testMe() {
//             ^^ drop the callback
    return new Promise((resolve, reject) => {
//  ^^^^^^ return the promise
        setTimeout(() => resolve([1,2,3]), 1000);
    });
}

var p = testMe().then(result) => {
//              ^^^^^ use the promise
   if(result.length < 5) throw new Error();
});
return p; // to mocha

Promise do not bubble outside

2 Answers2