I have the following function to test:

// ...
const local = new WeakMap();

export default class User {

  // ...

  async password(password) {
    if (!password) return local.get(this).get('hash'); // remove this for security reasons!
    if (password.length < 6) throw new Error('New password must be at least 6 characters long');
    if (!password.match(passwordPattern)) throw new Error(`New password must match ${passwordPattern}`);
    local.get(this).set('hash', await Password.hash(password));
  }

  // ...

}

Now I want to test this function with mocha, chai and chai-as-promised doing this test-case:

import chai from 'chai';
import chaiAsPromised from 'chai-as-promised';

import User from '../../../server/User';

chai.use(chaiAsPromised);
const expect = chai.expect;

describe('.password()', () => {

  const testuser = new User({username: 'Testuser', password: '123abc'});

  // FINDME
  it(`should throw when too short`, () => {
    return expect(testuser.password('1a')).to.eventually.throw();
  });

  // ...

});

Look for the // FINDME ment in the above code to get the context I'm referring to.

So what happens is that the test(s) do not wait for the async password() function to finish. As I found out ECMAScript7 async functions immediatelly return a thenable, so it should be fine since chai-as-promised use exactly this feature. My guess is, that async functions don't throw errors into this said promise rather than throwing it into the enclosing function itself, not being able to capture it through expect().

For the eager folks: I transpile this code on-the-fly with babel using the following mand:

babel-node --experimental node_modules/mocha/bin/_mocha --watch --recursive -R spec --check-leaks

Update for Babel 6:

Using babel-node 6.4.0, the following worked:

npm install --save-dev babel-preset-stage-0

Then run:

./node_modules/.bin/babel-node --presets stage-0 node_modules/mocha/bin/_mocha --    
recursive -R spec --check-leaks

I have the following function to test:

// ...
const local = new WeakMap();

export default class User {

  // ...

  async password(password) {
    if (!password) return local.get(this).get('hash'); // remove this for security reasons!
    if (password.length < 6) throw new Error('New password must be at least 6 characters long');
    if (!password.match(passwordPattern)) throw new Error(`New password must match ${passwordPattern}`);
    local.get(this).set('hash', await Password.hash(password));
  }

  // ...

}

Now I want to test this function with mocha, chai and chai-as-promised doing this test-case:

import chai from 'chai';
import chaiAsPromised from 'chai-as-promised';

import User from '../../../server/User';

chai.use(chaiAsPromised);
const expect = chai.expect;

describe('.password()', () => {

  const testuser = new User({username: 'Testuser', password: '123abc'});

  // FINDME
  it(`should throw when too short`, () => {
    return expect(testuser.password('1a')).to.eventually.throw();
  });

  // ...

});

Look for the // FINDME ment in the above code to get the context I'm referring to.

So what happens is that the test(s) do not wait for the async password() function to finish. As I found out ECMAScript7 async functions immediatelly return a thenable, so it should be fine since chai-as-promised use exactly this feature. My guess is, that async functions don't throw errors into this said promise rather than throwing it into the enclosing function itself, not being able to capture it through expect().

For the eager folks: I transpile this code on-the-fly with babel using the following mand:

babel-node --experimental node_modules/mocha/bin/_mocha --watch --recursive -R spec --check-leaks

Update for Babel 6:

Using babel-node 6.4.0, the following worked:

npm install --save-dev babel-preset-stage-0

Then run:

./node_modules/.bin/babel-node --presets stage-0 node_modules/mocha/bin/_mocha --    
recursive -R spec --check-leaks

• javascript
• testing
• async-await
• bdd
• ecmascript-2016

Share Improve this question Follow edited Jan 19, 2016 at 14:22 arcseldon 37.2k1717 gold badges125125 silver badges126126 bronze badges asked Mar 29, 2015 at 21:00 kernelkernel 3,75333 gold badges2828 silver badges3434 bronze badges 1

• Take a look at my code at stackoverflow./questions/33369389/… maybe it can help you a bit? – egeland Commented Oct 28, 2015 at 2:00

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1 Answer 1

Sorted by: Reset to default 8

We solved this issue by replacing eventually.throw() with .be.rejected provided by chai-as-promised, since an async function always returns a Promise. That wasn't clear to me in the first place. Have a look at the discussion on github if you like.