mocha done() 和 async await 的矛盾问题

Question

I have the following test case:

it("should pass the test", async function (done) {
        await asyncFunction();
        true.should.eq(true);
        done();
    });

Running it asserts:

Error: Resolution method is overspecified. Specify a callback or return a Promise; not both.

And if I remove the done(); statement, it asserts:

Error: Timeout of 2000ms exceeded. For async tests and hooks, ensure "done()" is called; if returning a Promise, ensure it resolves.

How to solve this paradox?

Answer 1

You need to remove the done parameter as well, not just the call to it. Testing frameworks like Mocha look at the function's parameter list (or at least its arity) to know whether you're using done or similar.

Using Mocha 3.5.3, this works for me (had to change true.should.be(true) to assert.ok(true) as the former threw an error):

const assert = require('assert');

function asyncFunction() {
    return new Promise(resolve => {
        setTimeout(resolve, 10);
    });
}

describe('Container', function() {
  describe('Foo', function() {
    it("should pass the test", async function () {
        await asyncFunction();
        assert.ok(true);
    });
  });
});

But if I add done :

describe('Container', function() {
  describe('Foo', function() {
    it("should pass the test", async function (done) {  // <==== Here
        await asyncFunction();
        assert.ok(true);
    });
  });
});

...then I get

Error: Timeout of 2000ms exceeded. For async tests and hooks, ensure "done()" is called; if returning a Promise, ensure it resolves.

Answer 2

Removing done as a param from it worked for me! Instead only use expect/should. Example is as follows:

getResponse(unitData, function callBack(unit, error, data){ try {
    return request.post(unit, function (err, resp) {
        if (!err && resp.statusCode === 200) {
            if (resp.body.error) {
                return callback(obj, JSON.stringify(resp.body.error), null); 
            }
            return callback(obj, null, resp); 
        } else {
            if (err == null) {  
                err = { statusCode: resp.statusCode, error: 'Error occured.' };
            }
            return callback(obj, err, null); 
        }
    });
} catch (err) {
    return callback(obj, err, null);
}}

BEFORE:

it('receives successful response', async (done) => { 
const getSomeData = await getResponse(unitData, function callBack(unit, error, data){ 
    expect(data.statusCode).to.be.equal(200); 
    done(); 
}) })

AFTER (works):

it('receives successful response', async () => { 
const getSomeData = await getResponse(unitData, function callBack(unit, error, data){
     expect(data.statusCode).to.be.equal(200); 
}) })

Answer 3

Sometimes there are cases you need to use async/await + done function in mocha.

For example, in one of my socket.io unit test cases, I have to call db functions with async functions and test socket event handlers which are callback functions:

context("on INIT_CHAT", ()=> {
  it("should create a room", async (done) => {
    const user = await factory.create("User");
    socket.emit("INIT_CHAT", user);
    
    socket.on("JOIN_CHAT", async (roomId) => {
       const room = await ChatRoom.findByPk(roomId);
       expect(room).to.exist;
       // then I need to close a test case here
       done();
    });
  });
});

This will causes the exact same error as in the OP:

Error: Resolution method is overspecified. Specify a callback or return a Promise; not both.

My Workaround:

I just wrapped the entire test code in a promise generator:

context("on INIT_CHAT", ()=> {
  it("should create a room", async () => {
    const asyncWrapper = () => {
      return new Promise(async (resolve) => {
        const user = await factory.create("User");
        socket.emit("INIT_CHAT", user);
        socket.on("JOIN_CHAT", async (roomId) => {
          const room = await ChatRoom.findByPk(roomId);
          expect(room).to.exist;
          resolve(true);
        });
      });
    });
    await asyncWrapper();
  });
});