在Mocha中，返回将被拒绝的Promise与调用done（err）具有不同的效果

Question

The mocha docs state

Alternately, instead of using the done() callback, you may return a Promise. This is useful if the APIs you are testing return promises instead of taking callbacks

But on rejection, these two ways appear to have different results:

var Promise = require("bluebird");

describe('failure', function () {
    describe('why2describes', function () {
        it("fails caught", function (done) {
            new Promise(function (resolve, reject) {
                reject(new Error("boom"))
            }).catch(function (err) {
                done(err)
            })
        });

        it("fails return", function (done) {
            return new Promise(function (resolve, reject) {
                reject(new Error("boom"))
            })
        });
    })
});

The first results in

Error: boom

The second results in

Unhandled rejection Error: boom

and then additionally states Error: timeout of 2000ms exceeded. Ensure the done() callback is being called in this test. Error: timeout of 2000ms exceeded. Ensure the done() callback is being called in this test.

Am I doing something wrong with the 2nd case?

Answer 1

Am I doing something wrong with the 2nd case?

Yes. Two things.

1. When you don't have a catch handler attached to a Promise chain, the error happens within the chain, will be lost in the form of rejected promise. Bluebird makes sure that nothing like that happens by detecting and throwing that error

    Unhandled rejection Error: boom 

2. In async case, calling the done function is the way let the test processor to know that the current test has completed. In the second case, you never call done . So it waits for the default timeout, 2000ms, and then fails the test case with that error.

   But if you like to use the Promises/your API returns a Promise, then you should not use done function at all. Your code should look like this

    it("fails return", function () { // no `done` argument here return new Promise(function (resolve, reject) { // ideally you would be doing all the assertions here }) }); 

   The other important thing to note when dealing with Promises based testing is, you should return the promise object.