I'm using this polyfill for ES6 promises and Mocha / Chai.
My assertions for the promises are not working. The following is a sample test:
it('should fail', function(done) {
new Promise(function(resolve, reject) {
resolve(false);
}).then(function(result) {
assert.equal(result, true);
done();
}).catch(function(err) {
console.log(err);
});
});
When I run this test it fails due to timeout. The assertion failure that was thrown in the then block is caught in the catch block. How can I avoid this and just throw it straight to Mocha?
I could just throw it from the catch function, but then how would I make assertions for the catch block?
If your Promise has a failure, it will only call your catch callback. As a result, Mocha's done callback is never called, and Mocha never figures out that the Promise failed (so it waits and eventually times out).
You should replace console.log(err);
with done(err);
. Mocha should automatically display the error message when you pass an error to the done callback.
I ended up solving my problem by using Chai as Promised .
It allows you to make assertions about the resolution and rejections of promises:
return promise.should.become(value)
return promise.should.be.rejected
A pattern I use in my Mocha/Chai/es6-promise tests is the following:
it('should do something', function () {
aPromiseReturningMethod(arg1, arg2)
.then(function (response) {
expect(response.someField).to.equal("Some value")
})
.then(function () {
return anotherPromiseReturningMethod(arg1, arg2)
})
.then(function (response) {
expect(response.something).to.equal("something")
})
.then(done).catch(done)
})
The last line is odd looking, but calls Mocha's done on success or on error.
One problem is if the last then returns something, I then need to noop()* before both the then
and the catch
:
it('should do something', function () {
aPromiseReturningMethod(arg1, arg2)
.then(function (response) {
expect(response.someField).to.equal("Some value")
})
.then(function () {
return anotherPromiseReturningMethod(arg1, arg2)
})
.then(_.noop).then(done).catch(done)
})
*Lodash's noop().
Would love to hear any critiques of this pattern.
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