Develop Reference

Is there any way to break out of a function by running another function in JS

I am working on a simple project and I would like to create a simple helper function that checks for a error in a callback. If there is a error then it should break the whole function that called it. Code example:
//Makes call to database and tries to insert element
db.collection("data").insertOne(
{
key: 'some-data'
}, (error, result) => {
//Return error if something goes wrong - else error is empty
checkError(error, "Unable to load database");
console.log("Succes item added")
}
);
Note: Yes this is node.js but this whole principle could be repeated in js with other callbacks - very simple repeatable error principle.
So in the insertOne function the first argument is some data I am adding to the database. The second argument is the callback function that is called after this async operation is finished. It returns a error which I could just handle by adding this if statement to the callback:
if (error) {
console.error(error);
return;
}
Buuut thats disrespecting the dry principle (bc I write the exact same if statement everywhere with no syntax being changed except the message) and is also distracting when reading the callback function. Now my issue is in the function checkError() even tho I can just print the error with the message or throw the error, I dont actually have a way to break the original callback so that it doesnt cause any more havoc in my database. I will go on to promisify this callback which is a solution. BUT I want to know if there is a way to this in the way I presented it here. Note: I dont want to use the try catch block bc thats replacing a if statement with another two blocks.
My checkError function:
const checkError = function (error, msg = "Something went wrong") {
if (error) console.error(`${msg}: error`);
//Break original block somehow ¯\_(ツ)_/¯
};
If I were to compress my question it would be: how to break a function with another function. Is there any way to achieve this?

I don't think this is possible. But you could achieve something similar with this:
function checkError (error, msg = "Something went wrong") {
if (!error) return false;
console.error(`${msg}: error`);
return true;
};
db.collection("data").insertOne(
{
key: 'some-data'
}, (error, result) => {
//Return error if something goes wrong - else error is empty
if (checkError(error, "Unable to load database")) return;
console.log("Succes item added")
}
);

Things become easier when you use promises.
Often asynchronous APIs provide a promise interface, and this is also the case for mongodb/mongoose, where you can chain a .exec() call to execute the database query and get a promise in return. This gives you access to the power of JavaScript's async/await syntax. So you can then do like this:
async function main() {
// Connect to database
// ...
// Other db transactions
// ...
let result = await db.collection("data").insertOne({ key: 'some-data'}).exec();
console.log("Item added successfully");
// Any other database actions can follow here using the same pattern
// ...
}
main().catch(err => {
console.log(err);
});
The idea here is that await will throw an exception if the promise returned by .exec() eventually rejects. You can either put a standard try...catch construct around it to deal with that error, or you can just let it happen. In the latter case the promise returned by the wrapping async function will reject. So you can deal with the error at a higher level (like done above).
This way of working also removes the need for numerous nested callbacks. Often you can keep the nesting to just one of two levels by using promises.

In Jest, how can I make a test fail?

I know I could throw an error from inside the test, but I wonder if there is something like the global fail() method provided by Jasmine?

Jest actually uses Jasmine, so you can use fail just like before.
Sample call:
fail('it should not reach here');
Here's the definition from the TypeScript declaration file for Jest:
declare function fail(error?: any): never;
If you know a particular call should fail you can use expect.
expect(() => functionExpectedToThrow(param1)).toThrow();
// or to test a specific error use
expect(() => functionExpectedToThrow(param1)).toThrowError();
See Jest docs for details on passing in a string, regex, or an Error object to test the expected error in the toThrowError method.
For an async call use .rejects
// returning the call
return expect(asyncFunctionExpectedToThrow(param1))
.rejects();
// or to specify the error message
// .rejects.toEqual('error message');
With async/await you need to mark the test function with async
it('should fail when calling functionX', async () => {
await expect(asyncFunctionExpectedToThrow(param1))
.rejects();
// or to specify the error message
// .rejects.toEqual('error message');
}
See documentation on .rejects and in the tutorial.
Also please note that the Jasmine fail function may be removed in a future version of Jest, see Yohan Dahmani's comment. You may start using the expect method above or do a find and replace fail with throw new Error('it should not reach here'); as mentioned in other answers. If you prefer the conciseness and readability of fail you could always create your own function if the Jasmine one gets removed from Jest.
function fail(message) {
throw new Error(message);
}

You can do it by throwing an error. For example:
test('Obi-Wan Kenobi', () => {
throw new Error('I have failed you, Anakin')
})

Copy/pasta failing test:
it('This test will fail', done => {
done.fail(new Error('This is the error'))
})

Here are certain scenarios where some of the answers won't work. In a world of async-await, it is quite common to have try-catch logic like so.
try {
await someOperation();
} catch (error) {
expect(error.message).toBe('something');
}
Now imagine if someOperation() somehow passed, but you were expecting it to fail, then this test will still pass because it never went to the catch block. So what we want is to make sure that the test fails if someOperation does not throw an error.
So now let's see which solutions will work and which won't.
Accepted answer won't work here because the throw will be catched again.
try {
await someOperation();
throw new Error('I have failed you, Anakin');
} catch (error) {
console.log('It came here, and so will pass!');
}
The answer with true === false also won't work because, assertions too throw an error like above which will be catched.
try {
await someOperation();
expect(true).toBe(false); // This throws an error which will be catched.
} catch (error) {
console.log('It came here, and so will pass!');
}
The one solution that DOES WORK (as shown in #WhatWouldBeCool's answer) for this case is below. Now it explicitly fails the test.
try {
await someOperation();
fail('It should not have come here!')
} catch (error) {
console.log('It never came here!');
}
Update May-2022
The fail() function is not officially supported by Jest anymore. Instead, you can do a couple of things to fail explicitly.
Method-1
You can wrap your promise function within expect and tell jest the function should reject with the given error. If the someOperation() somehow passes, jest will throw an error. If the someOperation() fails for any other reason other than the one you specified, it will throw an error. There are also different methods other than toThrowError() that you can use.
await expect(someOperation()).rejects.toThrowError('error!')
Method-2
You can declare explicitly how many assertions you expect in your test. If that doesn't match because someOperation() never failed, jest would throw an error.
expect.assertions(1)
try {
await someOperation();
} catch (error) {
expect(error.message).toBe('something');
}

Dont think there is, discussed here: https://github.com/facebook/jest/issues/2129

A lot of good ideas here. Only to add extra info about testing async code which may lead to trying to make Jest explicitly fail, check the docs for Testing Asynchronous Code https://jestjs.io/docs/en/asynchronous
To test a function that returns a Promise that resolves, it's important to return the Promise, so Jest knows that the test is done only when the Promise is resolved or it'll time out:
test('the data is peanut butter', () => {
return fetchData().then(data => {
expect(data).toBe('peanut butter')
})
})
To test a function that returns a Promise that rejects, it's important to return the Promise, so Jest knows that the test is done only when the Promise is rejected or it'll time out. And also have to say how many assertions Jest needs to count or it won't fail if the Promise is resolved - which is wrong in this case -:
test('the fetch fails with an error', () => {
expect.assertions(1)
return fetchData().catch(e => expect(e).toMatch('some specific error'))
})

You can always do something like this :)
expect(true).toBe(false);

The done callback passed to every test will throw an error if you pass a string to it.
for instance
it('should error if the promise fails', async (done) => {
try {
const result = await randomFunction();
expect(result).toBe(true);
done();
} catch (e) {
done('it should not be able to get here');
}
});
In this following code if the randomFunction throws an error it will be caught in the catch and with auto fail due to the string being passed to done.

Add jest-fail-on-console npm package, then on your jest.config.js
import failOnConsole from 'jest-fail-on-console'
failOnConsole();
This will fail a test once there is a console error or warning done by jest because of an error or warning thrown in the test item.

I just ran into this one, and after some digging, I found the root of the issue.
Jest, since its inception, has been compatible with Jasmine. Jasmine provided a fail function for programmatically fail the test. This is very useful for cases where throwing an error would cause the test to pass incorrectly (overly-simplified example, but hopefully illustrates the use-case):
function alwaysThrows() {
throw new Error();
}
describe('alwaysThrows', () => {
it('should throw', () => {
try {
alwaysThrows();
// here if there is nothing to force a failure, your
// test could "pass" as there are no failed expectations
// even though no error was thrown. If you just put the
// following to prevent that, you actually force the test
// to always pass:
throw new Error('it should have failed');
// that's why instead you use Jasmine's `fail(reason)` function:
fail('it should have failed');
} catch(err) {
expect(err).toBeDefined();
}
});
)
});
So, what has happened is this:
originally Jest did have a fail() function defined, because its default test runner was jest-jasmine2, which provided fail().
In Jest version 27 (or thereabouts), Jest replaced jest-jasmine2 with jest-circus as the default test runner. jest-circus does not implement a fail() function. This was reported as a bug on July 28th 2021: https://github.com/facebook/jest/issues/11698
Jest's type definitions (maintained in DefinitelyTyped) did not remove the fail() function, so autocompletion and the TypeScript compiler still think that it exists and can be used. There is an issue going on in DefinitelyTyped as well: https://github.com/DefinitelyTyped/DefinitelyTyped/discussions/55803
The issue with this thread is that they have decided not to remove it from the type definitions as it is marked as a "regression" in the Jest repository. Unfortunately, the Jest repository's thread has no official response about whether or not they will support this in the future, so the type definitions are in limbo.
So, long story short, Jest doesn't support fail() by default, but knowing that it's a matter of the default task runner, you can restore the fail() functionality by telling Jest to use the jest-jasmine2 runner instead of the default jest-circus runner:
npm i -D jest-jasmine2
configure the Jest config:
module.exports = {
testRunner: "jest-jasmine2"
};
P.S.: usually there is a better way than try/catch to account for errors in your actual test cases. You can see an example of different ways to handle errors without requiring try/catch in both synchronous and asynchronous contexts here: https://gist.github.com/joeskeen/d9c053b947e5e7462e8d978286311e83

You can throw an error simulating an error thrown by the application and then expect its message to be different from what it actually is.
try {
await somthingYouExpectToFail();
throw new Error("Fail!");
} catch (error) {
expect(error.message).not.toBe("Fail!");
}

Force-failing a Jasmine test

If I have code in a test that should never be reached (for example the fail clause of a promise sequence), how can I force-fail the test?
I use something like expect(true).toBe(false); but this is not pretty.
The alternative is waiting for the test to timeout, which I want to avoid (because it is slow).

Jasmine provides a global method fail(), which can be used inside spec blocks it() and also allows to use custom error message:
it('should finish successfully', function (done) {
MyService.getNumber()
.success(function (number) {
expect(number).toBe(2);
done();
})
.fail(function (err) {
fail('Unwanted code branch');
});
});
This is built-in Jasmine functionality and it works fine everywhere in comparison with the 'error' method I've mentioned below.
Before update:
You can throw an error from that code branch, it will fail a spec immediately and you'll be able to provide custom error message:
it('should finish successfully', function (done) {
MyService.getNumber()
.success(function (number) {
expect(number).toBe(2);
done();
})
.fail(function (err) {
throw new Error('Unwanted code branch');
});
});
But you should be careful, if you want to throw an error from Promise success handler then(), because the error will be swallowed in there and will never come up. Also you should be aware of the possible error handlers in your app, which might catch this error inside your app, so as a result it won't be able to fail a test.

Thanks TrueWill for bringing my attention to this solution. If you are testing functions that return promises, then you should use the done in the it(). And instead of calling fail() you should call done.fail(). See Jasmine documentation.
Here is an example
describe('initialize', () => {
// Initialize my component using a MOCK axios
let axios = jasmine.createSpyObj<any>('axios', ['get', 'post', 'put', 'delete']);
let mycomponent = new MyComponent(axios);
it('should load the data', done => {
axios.get.and.returnValues(Promise.resolve({ data: dummyList }));
mycomponent.initialize().then(() => {
expect(mycomponent.dataList.length).toEqual(4);
done();
}, done.fail); // <=== NOTICE
});
});

How to prevent false positives with mocha, promises, and catch blocks

I want to take advantage of Mocha's built in promise support, but i'm having difficulty dealing with false positives when I want to test catch branches in my promise chains.
This question gets closest to what I want, but the solution requires every developer on my project to add a then that will throw an error, and ensure that that error doesn't accidentally pass the test.
Because of this, i've had to revert to using the done style of tests, which relies on the built in timeout to catch errors instead of an assertion. This is less than ideal, but removes the chance of false positives.
var RSVP = require('RSVP');
function request (shouldSucceed) {
if (shouldSucceed) {
return RSVP.Promise.resolve('success');
} else {
return RSVP.Promise.reject(new Error('failure'));
}
}
describe('request', function () {
it('throws an error if shouldSucceed is not provided', function () {
// this test is incorrectly calling `request`
// which leads to it passing without actually testing
// the behaviour it wants to
return request(true)
.catch(function (err) {
expect(err).to.be.an.instanceof(Error)
});
});
it('throws an error if shouldSucced is not provided (manual then block)', function () {
// this test tacks a `then` onto the chain, to ensure `request`
// actually throws an error as expected
// unfortunately, it still passes since the assertion needs to do
// a little extra work to ensure the type of error thrown is the
// expected error
return request(true)
.then(function () {
throw new Error('Not expected');
})
.catch(function (err) {
expect(err).to.be.an.instanceof(Error)
});
});
it('throws an error if shouldSucceed is not provided (done syntax)', function (done) {
// this assertion fails (as it should)
// due to a timeout
return request(true)
.catch(function () {
expect(err).to.be.an.instanceof(Error);
done()
});
});
});
Output:
request
✓ throws an error if shouldSucceed is not provided
✓ throws an error if shouldSucced is not provided (manual then block)
1) throws an error if shouldSucceed is not provided (done syntax)
2 passing (2s)
1 failing
1) request throws an error if shouldSucceed is not provided (done syntax):
Error: timeout of 2000ms exceeded. Ensure the done() callback is being called in this test.
Is there a cleaner way to tell mocha that I am expecting something to happen in a catch block, and that a successful resolution of the promise should be a test failure?

You're looking for
it('request(false) should throw an error', function () {
return request(false).then(function() {
throw new Error('unexpected success');
}, function(err) {
expect(err).to.be.an.instanceof(Error)
});
});
See When is .then(success, fail) considered an antipattern for promises? for an explanation of the difference to the code that you currently have.

Possibly unhandled Error with passing tests

I have following method:
EventHandler.prototype.handle = function (event) {
var me = this;
return me.processEvent(event).then(function () {
return me.saveLastSeenEventRevision(event);
}).catch(function (err) {
me.logger.debug("First attempt of event process failed, trying recovery");
return me.recoveryStrategy.execute(event,me.processEvent.bind(me)).then(function() {
return me.saveLastSeenEventRevision(event);
});
});
};
And I have this test written:
describe('when processEvent fails', function() {
beforeEach(function () {
instance.processEvent.returns(Bluebird.reject(new Error('rejection')));
});
describe('when recovery fails', function() {
beforeEach(function () {
instance.recoveryStrategy.execute.returns(Bluebird.reject(new Error('recovery rejected')));
});
it('should not save the revision', function(done) {
instance.handle(event).catch(function() {
sinon.assert.notCalled(instance.saveLastSeenEventRevision);
done();
}).done();
});
});
});
I simulate rejection of processEvent method, and in catching function I simulate final rejection of my recovery implementation. I have more test written for this method, but the setup is the same. All test passes as they are supposed to, but in console I see this message:
Possibly unhandled Error: rejection
Is there any other way of handling these promises to pass this test?
Test setup - mocha / chai / sinon, as promise library is used bluebird.

Well, you're creating an explicit rejection but are not handling it here:
instance.processEvent.returns(Bluebird.reject(new Error('rejection')));
Bluebird sees you're creating an unhandled rejection here so it's letting you know. This is because a real function would return the rejection when called but sinon is creating it early - so the app has a pending place of error that is not handled. You can turn that off with:
Bluebird.onPossiblyUnhandledRejection(function(){});
Or better, use a tool like sinon-as-promised which lets you do:
instance.processEvent.rejects('rejection');
On a side note - don't use the silly done syntax, Mocha has built in promises support. You can return promises from test.

So, problem was with sinon stubs and setting up return values. It seems, that passing rejected promise as return value is some how evaluated inside sinon itself.
There are two options:
to not set return value directly, but implement function which will be used instead of the stub one
sinon.stub(instance, 'processEvent', function() {
return Bluebird.reject(new Error('rejected');
});
to use sinon-as-promised library, which allows following:
sinon.stub(instance, 'processEvent').rejects(new Error('rejected'));