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.
Related
I am new to asynchronous programming. I am writing a test case for the following example code.
someAsync(text) {
try {
//do something and resolve result
return Promise.resolve(result);
} catch (err) {
return Promise.reject(new Error(`Failure ${err}`));
}
}
I am testing it with the following code:
it('should throw error when called', (done) => {
const mymodule = new MyModule(args);
mymodule.someAsync('something that causes failure').catch((err) => {
expect(err).to.exist;
expect(err.message).to.contains('This should pass');
done(err);
});
});
This test case fails, assertions pass and then after done again it throws error.
Please tell me where am I going wrong?
done(err) makes a spec to fail. Since it's expected error, it shouldn't fail a spec, it should be done() instead.
Mocha doesn't need done to handle promises, a promise can be returned from a spec.
It likely should be:
it('should throw error when called', () => {
const mymodule = new MyModule(args);
return mymodule.someAsync('something that causes failure').catch((err) => {
expect(err).to.exist;
expect(err.message).to.contain('This should pass');
});
});
Also, it's not evident from the listed code that expect(err.message).to.contain('This should pass') assertion is true.
This might or might not have anything to do with your error, but that does not really look like asynchronous code. Something like this might make more sense:
const someAsync = (text) => new Promise((resolve, reject)) => {
try {
// do something and capture result
resolve(result);
} catch (err) {
reject(new Error(`Failure ${err}`));
}
})
As estus' comment said, we probably need the actual error and the test framework to diagnose further.
I'm working on adding test coverage to a Node project I'm working on using Jest. The code I'm testing is throwing errors within promises resulting in an UnhandledPromiseRejectionWarning message being logged to the console.
While writing tests, I can pretty easily identify these issues and resolve them, but these warnings aren't actually causing Jest to mark the tests as failed, so our CI won't catch it. I've searched around for any suggestions and haven't found much.
I did find in Node's documentation that you can catch these warnings and handle them...
process.on('unhandledRejection', (error) => {
throw error; // Or whatever you like...
});
So it seems like it would be pretty straightforward to add this code into my test cases. After all, an Error thrown within the test should cause the test to fail...
describe('...', () => {
it('...', () => {
process.on('uncaughtRejection', (error) => {
throw error;
});
// the rest of my test goes here
});
});
Unfortunately the behavior I'm seeing is that the error does get thrown, but Jest doesn't catch it and fail the test. Instead, Jest crashes with this error and the tests don't continue to run. This isn't really desirable, and seems like incorrect behavior.
Throwing an error outside of the uncaughtRejection handler works as expected: Jest logs the thrown error and fails the test, but doesn't crash. (i.e. the test watcher keeps watching and running tests)
The way I've approached this is very much tied into the way I write my functions - basically, any function that uses promises should return a promise. This allows whatever code calls that function to handle catching errors in any way it sees fit. Note that this is my approach and I'm not going to claim this is the only way to do things.
For example... Imagine I'm testing this function:
const myFunction = () => {
return doSomethingWithAPromise()
.then(() => {
console.log('no problems!');
return true;
});
};
The test will look something like this:
describe('...', () => {
it('...', () => {
return myFunction()
.then((value) => {
expect(value).toBe(true);
});
});
});
Which works great. Now what happens if the promise is rejected? In my test, the rejected promise is passed back to Jest (because I'm returning the result of my function call) and Jest can report on it.
If, instead, your function does not return a promise, you might have to do something like this:
const myOtherFunction = () => {
doSomethingWithAPromise()
.then(() => {
console.log('no problems!');
return true;
})
.catch((err) => {
// throw the caught error here
throw err;
});
};
Unlike the example above, there is no (direct) way for Jest to handle a rejected promise because you're not passing the promise back to Jest. One way to avoid this might be to ensure there is a catch in the function to catch & throw the error, but I haven't tried it and I'm not sure if it would be any more reliable.
Include the following content in Jest's setupFiles:
if (!process.env.LISTENING_TO_UNHANDLED_REJECTION) {
process.on('unhandledRejection', reason => {
throw reason
})
// Avoid memory leak by adding too many listeners
process.env.LISTENING_TO_UNHANDLED_REJECTION = true
}
Courtesy of stipsan in https://github.com/facebook/jest/issues/3251#issuecomment-299183885.
module:
export function myPromise() {
return new Promise((resolve, reject) => {
const error = new Error('error test');
reject(error);
});
}
test:
import { myPromise } from './module';
it('should reject the promise', () => {
expect.assertions(1);
const expectedError = new Error('error test');
myPromise().catch((error) => {
expect(error).toBe(expectedError);
});
From the node documentation site we can see that The process object is an instance of EventEmitter.
Using the emit function from process we can trigger the errors like uncaughtRejection and uncaughtException programmatically when needed.
it("should log the error", () => {
process.emit("unhandledRejection");
...
const loggerInfo = jest.spyOn(logger, "info");
expect(loggerInfo).toHaveBeenCalled();
});
Not sure if this helps, but you can also assert for promise rejections as such
index.js
module.exports = () => {
return Promise.reject('it didnt work');
}
index.spec.js
const thing = require('../src/index');
describe('rejected promise', () => {
it('should reject with a reason', ()=> {
return expect(thing()).rejects.toEqual('it didnt work');
});
});
I have following:
it('invalid use', () => {
Matcher(1).case(1, () => {});
});
The case method is supposed to throw after some delay, how can I describe it for Mocha/Chai that's what I want - the test should pass (and must not pass when exception is not thrown)?
Consider case method off limits, it cannot be changed.
For testing purposes it should be equivalent to:
it('setTimeout throw', _ => {
setTimeout(() => { throw new Error(); }, 1); // this is given, cannot be modified
});
I tried:
it('invalid use', done => {
Matcher(1).case(1, () => {});
// calls done callback after 'case' may throw
setTimeout(() => done(), MatcherConfig.execCheckTimeout + 10);
});
But that's not really helping me, because the test behavior is exactly reverted - when an exception from case (setTimeout) is not thrown, it passes (should fail) and when an exception is thrown the test fails (should succeed).
I read somewhere someone mentioning global error handler, but I would like to solve this cleanly using Mocha and/or Chai, if it is possible (I guess Mocha is already using it in some way).
You cannot handle exceptions from within a asynchronous callback, e.g. see Handle error from setTimeout. This has to do with the execution model ECMAScript uses. I suppose the only way to catch it is in fact to employ some environment-specific global error handling, e.g. process.on('uncaughtException', ...) in Node.js.
If you convert your function to Promises, however, you can easily test it using the Chai plugin chai-as-promsied:
import * as chai from 'chai';
import chaiAsPromised = require('chai-as-promised');
chai.use(chaiAsPromised);
const expect = chai.expect;
it('invalid use', async () => {
await expect(Matcher(1).case(1, () => {})).to.eventually.be.rejected;
});
Any Mocha statements like before, after or it will work asynchronously if you return a promise. I generally use something like the below for async tests.
Also don't forget to set timeout this.timeout(...) if you expect the async function to take more than 2 seconds.
it('some test', () => {
return new Promise(function(resolve,reject){
SomeAsyncFunction(function(error,vals) {
if(error) {
return reject(error);
} else {
try {
//do some chai tests here
} catch(e) {
return reject(e);
}
return resolve();
}
});
});
});
Specifically for your case, since we expect some error to be thrown after a period of time (assuming the empty callback you have provided to .case should not run due to the exception being thrown) then you can write it something like:
it('invalid use', () => {
//define the promise to run the async function
let prom = new Promise(function(resolve,reject){
//reject the promise if the function does not throw an error
//note I am assuming that the callback won't run if the error is thrown
//also note this error will be passed to prom.catch so need to do some test to make sure it's not the error you are looking for.
Matcher(1).case(1, () => {return reject(new Error('did not throw'))});
});
prom.catch(function(err){
try {
expect(err).to.be.an('error');
expect(err.message).to.not.equal('did not throw');
//more checks to see if err is the error you are looking for
} catch(e) {
//err was not the error you were looking for
return Promise.reject(e);
}
//tests passed
return Promise.resolve();
});
//since it() receives a promise as a return value it will pass or fail the test based on the promise.
return prom;
});
From Chai documentation :
When no arguments are provided, .throw invokes the target function and asserts that an error is thrown.
So you could something like
expect(Matcher(1).case(1, () => {})).to.throw
If your tested code calls setTimeout with a callback that throws and no-one is catching this is exception then:
1) this code is broken
2) the only way to see that problem is platform global exception handler like process.on('uncaughtException' mentioned by user ComFreek
The last resort chance is to stub setTimeout for duration of test (for example using sinon.stub) or just manually.
In such stubbed setTimeout you can decorate timeout handler, detect exception and call appropriate asserts.
NOTE, this is last resort solution - your app code is broken and should be fixed to properly propagate errors, not only for testing but ... well, to be good code.
Pseudocode example:
it('test', (done) => {
const originalSetTimeout = setTimeout;
setTimeout = (callback, timeout) => {
originalSetTimeout(() => {
try {
callback();
} catch(error) {
// CONGRATS, you've intercepted exception
// in _SOME_ setTimeout handler
}
}, timeout)
}
yourTestCodeThatTriggersErrorInSomeSetTimeoutCallback(done);
})
NOTE2: I intentionally didn't wrote proper async cleanup code, it's a homework. Again, see sinon.js and its sandbox
NOTE3: It will catch all setTimeout calls during test duration. Beware, there are dragons.
How to get asserts in mocha before blocks to work? If I am not suppose to do this let me know. Currently, using promises, if I get an error using an catch block, I'll add an assert to fail the before block. WHat I want is it to fail the describe block, but instead I get two possible outcomes. 1. My test suite completely crashes, 2. I have to wait for each timeout to hit for each test because the before block failed.
before(function (done) {
promise()
.then(function () {
done();
})
.catch(function (error) {
assert(!error);
done();
});
});
I even tried this, thinking, maybe the done was never called.
before(function (done) {
promise()
.then(function () {
//no done here
})
.catch(function (error) {
assert(!error);
});
.finally(function () {
done();
});
});
So far to avoid crashing and waiting, and to make it work, I have done this:
var beforeError;
before(function (done) {
promise()
.then(function () {
done();
})
.catch(function (error) {
beforeError = error;
done();
});
});
it('test something', function () {
assert(beforeError, 'Before block failed with error.');
});
I am really curious if there is a better way to go about this so that if my before/beforeEach/after/afterEach blocks fail, it doesn't cause me to wait ages or my suite to crash! Thanks S/O community! :)
I can't speak to your use of the done callback, but mocha 3.0 supports promises in before hooks now. Were I to write this, I would let the returned promise throw its own error, which will fail the before hook without breaking the suite.
before(function () {
return promise(<async behavior here>);
});
I have several integration tests in my app:
it('creating vehicle', function (done) {
createVehicle()
.then(() => {
done();
})
.catch((err) => {
done(err);
});
});
createVehicle makes post request and returns promise:
return request.json('post', '/api/vehicle/')
.send(obj)
.expect(200)
.then((res) => {
expect(res.body).toExist("Body is empty");
expect(res.body.id).toExist("Id is empty");
return res.body;
});
Now all works fine, but if I rewrite first snippet in the following way:
it('creating vehicle', function (done) {
createVehicle()
.then(done) //*
.catch(done); //*
});
I get error from Mocha
done() invoked with non-Error
I understand why. The createVehicle return res.body and it's passed to then callback, in result done run as done(arg) and I get the error, because mocha done callback has to be called without arg when there is no error and with argument when there is error.
Is it possible to use this variant:
.then(done)
.catch(done);
How to achieve this?
Of course, I can delete return statement, but createVehicle is used in several places and I need returned value:
it('creating vehicle with media', function (done) {
createVehicle()
.then(createMedia) //createMedia will get returned value
//....
});
The easiest solution would be to just return the promise instead of having to deal with callbacks, because Mocha supports promises out-of-the-box:
it('creating vehicle', function() {
return createVehicle();
});