In our mocha tests, sometimes we have bugs and throw exceptions, and when those exceptions are thrown, the subsequent asserts are not called and our tests might pass even though there was something wrong.
// this would pass
it('adsf', async () => {
assert.equal(1,1)
var foo = undefined;
foo.hi(); // throw exception
assert.equal(1,2) // assert not called
});
We tried wrapping this in a try catch, like the following
// this would fail, but not say why
it('adsf', async () => {
try {
assert.equal(1,1)
// var foo = undefined;
// foo.hi();
assert.equal(1,2)
} catch (err) {
assert.fail(err) // if fail, such as the 1,2 case above, the line number of the error is not shown
}
});
but the catch case hides some of the failed assert info. If anyone has any suggestions, it would be appreciated.
The test in your example will not pass. In mocha, the test would fail if the function that is called in a test throws an exception. Example:
const assert = require('assert');
function foo() {
throw new Error('err');
}
describe('test', () => {
it('works', async () => {
foo();
console.log('came here');
});
});
$ npx mocha test.js
test
1) works
0 passing (6ms)
1 failing
1) test
works:
Error: err
at foo (test.js:8:9)
at Context.<anonymous> (test.js:13:11)
at processImmediate (internal/timers.js:439:21)
So, in your example, since foo.hi throws TypeError, it would be caught by mocha and shown as a test failure (the execution won't reach asserts indeed but the test will be shown as failed anyway).
What I suspect happens in your case is throwing in a promise or rejecting a promise, like in one of these examples:
function throwing() {
return new Promise((resolve, reject) => { throw new Error('err'); });
}
function rejecting() {
return new Promise((resolve, reject) => { reject(new Error('err')); });
}
describe('test', () => {
it('works', async () => {
throwing();
rejecting();
console.log('came here');
});
});
$ npx mocha test.js
test
came here
✓ works
[some UnhandledPromiseRejectionWarnings here]
1 passing (6ms)
Both will not be caught by a test because the function executes successfully returning a promise and the test block completes, but the failure occurs later. If your function returns a promise, just await for it in your test to make sure you get the promise result:
describe('test', () => {
it('works', async () => {
await throwing();
await rejecting();
console.log('came here');
});
});
$ npx mocha test.js
test
1) works
0 passing (6ms)
1 failing
1) test
works:
Error: err
at test.js:4:51
at new Promise (<anonymous>)
at throwing (test.js:4:10)
at Context.<anonymous> (test.js:13:11)
at processImmediate (internal/timers.js:439:21)
Related
I have the following function in something.js, a browser script.
async function somethingFunction(value) {
try {
//Lots of lovely code manipulating 'value',
//an improper assignment here by passing 'undefined'
//to value argument of function would cause a TypeError
//When trying to access property of value (cannot access .test of undefined)
let somethingVar = value.test;
return somethingVar;
} catch (e) {
throw new Error();
}
}
And I've written as test as such:
describe(`#somethingFunction()`, async function () {
context(`Passing undefined as value parameter should be caught be try catch block`, function () {
it(`Should return error (TypeError)`, async function () {
expect(async function () {
await somethingFunction(undefined);
}).to.throw(TypeError);
});
});
});
On this test, Mocha give:
AssertionError: expected [Function] to throw 'TypeError'
But passing undefined to this function does in fact cause a error - A TypeError!
Are TypeErrors caught? How would Mocha test this?
Tl;dr - How do I catch TypeErrors and get Mocha to test for them?
This is a design question that came up to me while unit testing.
Let's dive into the example:
Imagine this:
async function foo() {
try {
return apiCall()
}
catch (e) {
throw new CustomError(e);
}
}
async function bar() {
return foo()
}
async function main() {
try {
await bar()
}catch(e) {
console.error(e)
}
}
main()
What do we see here? That the only function that hasn't got a try-catch block is bar.
But if foo fails, it should get catched by the main catch.
While unittesting this like
describe('testing bar', () => {
it('foo should throw', () => {
foo.mockImplementantion(() => { throw new CustomError('error')});
bar()
.then((result) => console.log(result))
.catch((err) => { exepect(err).toBeInstanceOf(CustomError)}) // this is what we are testing
})
})
The output we see is that an Unhandled promise rejection is logged in the console.
So, my question is... even if I know that the main() will catch the error, should I use try-catch block inside all async functions?
try..catch may be necessary if a function is able to recover from an error, do a side effect like logging, or re-throw a more meaningful error.
If CustomError is more preferable than an error that apiCall can throw then try..catch necessary, otherwise it doesn't. Also the problem with foo is that it handles only synchronous errors. In order to handle rejected promises, it should be return await apiCall(), this is a known pitfall of async.
Uncaught rejections are unwanted, they currently result in UnhandledPromiseRejectionWarning and are expected to crash Node in future versions. It's preferable to handle an error in a meaningful way at top level, so main needs to catch the error. This can be delegated to process uncaughtRejection event handler but it may be beneficial for it to stay extra level of error handling that should be never reached.
The output we see is that an Unhandled promise rejection is logged in the console.
This shouldn't happen. A rejection needs to be handled by the test. One possible point of failure is explained above, foo can return original error from apiCall instead of CustomError in case it wasn't correctly mocked, this will fail the expectation and result in unhandled rejection in catch(). Another point of failure is that the test has unchained promise because it wasn't returned, the test always passes.
Asynchronous test that uses promises should always return a promise. This can be improved by using async..await. foo is async, it's expected to always return a promise:
it('foo should throw', async () => {
foo.mockImplementantion(() => { return Promise.reject(new CustomError('error')) });
await expect(bar()).rejects.toThrow(CustomError);
})
Now even if foo mock fails (foo mock won't affect bar if they are defined in the same module as shown) and bar rejects with something that is not CustomError, this will be asserted.
No. You don't need to use try/catch in every async/await. You only need to do it at the top level. In this case your main function which you are already doing.
Weather you should is a matter of opinion. The go language designers feel strongly enough about this that is has become the standard in go to always handle errors at each function call. But this is not the norm in javascript or most other languages.
Unhandled promise rejection
Your unhandled promise rejection is thrown by your it() function because you are not telling it to wait for the promise to complete.
I assume you are using something like mocha for the unit test (other frameworks may work differently). In mocha there are two ways to handle asynchronous tests:
Call the done callback - the it() function will always be called with a done callback. It is up to you weather you want to use it or like in your posted code to not use it:
describe('testing bar', () => {
it('foo should throw', (done) => {
foo.mockImplementantion(() => { throw new CustomError('error')});
bar()
.then((result) => {
console.log(result);
done(); // ------------- THIS IS YOUR ACTUAL BUG
})
.catch((err) => {
exepect(err).toBeInstanceOf(CustomError);
done(); // ------------- THIS IS YOUR ACTUAL BUG
})
})
})
Return a Promise. If you return a promise to the it() function mocha will be aware that your code is asynchronous and wait for completion:
describe('testing bar', () => {
it('foo should throw', (done) => {
foo.mockImplementantion(() => { throw new CustomError('error')});
return bar() // <----------- THIS WOULD ALSO FIX IT
.then((result) => {
console.log(result);
})
.catch((err) => {
exepect(err).toBeInstanceOf(CustomError);
})
})
})
In short, there is nothing wrong with your code. But you have a bug in your unit test.
As #Bergi told me I will post some solutions right here
I wrap the function in a try catch block
1.
async function bar() {
try{
return foo()
} catch (e) {
throw e
}
}
Rewrite the test
describe('testing bar', () => {
it('foo should throw', (done) => {
foo.mockImplementantion(() => { throw new CustomError('error')});
bar()
.then((result) => { throw result }) // this is because we are expecting an error, so if the promise resolves it's actually a bad sign.
.catch((err) => {
exepect(err).toBeInstanceOf(CustomError)}) // this is what we are testing
done();
})
})
Use return in the test case
describe('testing bar', () => {
it('foo should throw', () => {
foo.mockImplementantion(() => { throw new CustomError('error')});
return bar()
.then((result) => { throw result })
.catch((err) => { exepect(err).toBeInstanceOf(CustomError)}) // this is what we are testing
})
})
I am hard time writing test to assert something happened inside catch block which is executed inside forEach loop.
Prod code
function doSomething(givenResourceMap) {
givenResourceMap.forEach(async (resourceUrl) => {
try {
await axios.delete(resourceUrl);
} catch (error) {
logger.error(`Error on deleting resource ${resourceUrl}`);
logger.error(error);
throw error;
}
});
I am wanting to assert logger.error is being called twice and called with right arguments each time. So I wrote some test like this
describe('Do Something', () => {
it('should log message if fail to delete the resource', function() {
const resource1Url = chance.url();
const givenResourceMap = new Map();
const thrownError = new Error('Failed to delete');
givenResourceMap.set(resource1Url);
sinon.stub(logger, 'error');
sinon.stub(axios, 'delete').withArgs(resource1Url).rejects(thrownError);
await doSomething(givenResourceMap);
expect(logger.error).to.have.callCount(2);
expect(logger.error.getCall(0).args[0]).to.equal(`Error deleting resource ${resource1Url}`);
expect(logger.error.getCall(1).args[0]).to.equal(thrownError);
// Also need to know how to assert about `throw error;` line
});
});
I am using Mocha, sinon-chai, expect tests. Above test is failing saying logger.error is being 0 times.
Thanks.
The problem is that you are using await on a function that doesn't return a Promise. Note that doSomething is not async and does not return a Promise object.
The forEach function is async but that means they'll return right away with an unresolved Promise and you don't ever await on them.
In reality, doSomething will return before the work inside of the forEach is complete, which is probably not what you intended. To do that you could use a regular for-loop like this:
async function doSomething(givenResourceMap) {
for (const resourceUrl of givenResourceMap) {
try {
await axios.delete(resourceUrl);
} catch (error) {
logger.error(`Error on deleting resource ${resourceUrl}`);
logger.error(error);
throw error;
}
}
}
Note that it changes the return type of doSomething to be a Promise object rather than just returning undefined as it originally did. But it does let you do an await on it as you want to in the test (and presumably in production code also).
However since you re-throw the exception caught in the loop, your test will exit abnormally. The test code would have to also change to catch the expected error:
it('should log message if fail to delete the resource', function(done) {
// ... the setup stuff you had before...
await doSomething(givenResourceMap).catch(err => {
expect(logger.error).to.have.callCount(2);
expect(logger.error.getCall(0).args[0]).to.equal(`Error deleting resource ${resource1Url}`);
expect(logger.error.getCall(1).args[0]).to.equal(thrownError);
done();
});
});
I'm writing an async test that expects the async function to throw like this:
it("expects to have failed", async () => {
let getBadResults = async () => {
await failingAsyncTest()
}
expect(await getBadResults()).toThrow()
})
But jest is just failing instead of passing the test:
FAIL src/failing-test.spec.js
● expects to have failed
Failed: I should fail!
If I rewrite the test to looks like this:
expect(async () => {
await failingAsyncTest()
}).toThrow()
I get this error instead of a passing test:
expect(function).toThrow(undefined)
Expected the function to throw an error.
But it didn't throw anything.
You can test your async function like this:
it('should test async errors', async () => {
await expect(failingAsyncTest())
.rejects
.toThrow('I should fail');
});
'I should fail' string will match any part of the error thrown.
I'd like to just add on to this and say that the function you're testing must throw an actual Error object throw new Error(...). Jest does not seem to recognize if you just throw an expression like throw 'An error occurred!'.
await expect(async () => {
await someAsyncFunction(someParams);
}).rejects.toThrowError("Some error message");
We must wrap the code in a function to catch the error. Here we are expecting the Error message thrown from someAsyncFunction should be equal to "Some error message". We can call the exception handler also
await expect(async () => {
await someAsyncFunction(someParams);
}).rejects.toThrowError(new InvalidArgumentError("Some error message"));
Read more https://jestjs.io/docs/expect#tothrowerror
Custom Error Class
The use of rejects.toThrow will not work for you. Instead, you can combine the rejects method with the toBeInstanceOf matcher to match the custom error that has been thrown.
Example
it("should test async errors", async () => {
await expect(asyncFunctionWithCustomError()).rejects.toBeInstanceOf(
CustomError
)
})
To be able to make many tests conditions without having to resolve the promise every time, this will also work:
it('throws an error when it is not possible to create an user', async () => {
const throwingFunction = () => createUser(createUserPayload)
// This is what prevents the test to succeed when the promise is resolved and not rejected
expect.assertions(3)
await throwingFunction().catch(error => {
expect(error).toBeInstanceOf(Error)
expect(error.message).toMatch(new RegExp('Could not create user'))
expect(error).toMatchObject({
details: new RegExp('Invalid payload provided'),
})
})
})
I've been testing for Firebase cloud functions and this is what I came up with:
test("It should test async on failing cloud functions calls", async () => {
await expect(async ()=> {
await failingCloudFunction(params)
})
.rejects
.toThrow("Invalid type"); // This is the value for my specific error
});
This is built on top of lisandro's answer.
If you want to test that an async function does NOT throw:
it('async function does not throw', async () => {
await expect(hopefullyDoesntThrow()).resolves.not.toThrow();
});
The above test will pass regardless of the value returned, even if undefined.
Keep in mind that if an async function throws an Error, its really coming back as a Promise Rejection in Node, not an error (thats why if you don't have try/catch blocks you will get an UnhandledPromiseRejectionWarning, slightly different than an error). So, like others have said, that is why you use either:
.rejects and .resolves methods, or a
try/catch block within your tests.
Reference:
https://jestjs.io/docs/asynchronous#asyncawait
This worked for me
it("expects to have failed", async () => {
let getBadResults = async () => {
await failingAsyncTest()
}
expect(getBadResults()).reject.toMatch('foo')
// or in my case
expect(getBadResults()).reject.toMatchObject({ message: 'foo' })
})
You can do like below if you want to use the try/catch method inside the test case.
test("some test case name with success", async () => {
let response = null;
let failure = null;
// Before calling the method, make sure someAsyncFunction should be succeeded
try {
response = await someAsyncFunction();
} catch(err) {
error = err;
}
expect(response).toEqual(SOME_MOCK_RESPONSE)
expect(error).toBeNull();
})
test("some test case name with failure", async () => {
let response = null;
let error = null;
// Before calling the method, make sure someAsyncFunction should throw some error by mocking in proper way
try {
response = await someAsyncFunction();
} catch(err) {
error = err;
}
expect(response).toBeNull();
expect(error).toEqual(YOUR_MOCK_ERROR)
})
Edit:
As my given solution is not taking the advantage of inbuilt jest tests with the throwing feature, please do follow the other solution suggested by #Lisandro https://stackoverflow.com/a/47887098/8988448
it('should test async errors', async () => {
await expect(failingAsyncTest())
.rejects
.toThrow('I should fail');
});
test("It should test async on failing cloud functions calls", async () => {
failingCloudFunction(params).catch(e => {
expect(e.message).toBe('Invalid type')
})
});
So, I'm testing a component that relies on an event-emitter. To do so I came up with a solution using Promises with Mocha+Chai:
it('should transition with the correct event', (done) => {
const cFSM = new CharacterFSM({}, emitter, transitions);
let timeout = null;
let resolved = false;
new Promise((resolve, reject) => {
emitter.once('action', resolve);
emitter.emit('done', {});
timeout = setTimeout(() => {
if (!resolved) {
reject('Timedout!');
}
clearTimeout(timeout);
}, 100);
}).then((state) => {
resolved = true;
assert(state.action === 'DONE', 'should change state');
done();
}).catch((error) => {
assert.isNotOk(error,'Promise error');
done();
});
});
On the console I'm getting an 'UnhandledPromiseRejectionWarning' even though the reject function is getting called since it instantly shows the message 'AssertionError: Promise error'
(node:25754) UnhandledPromiseRejectionWarning: Unhandled promise
rejection (rejection id: 2): AssertionError: Promise error: expected
{ Object (message, showDiff, ...) } to be falsy
should transition with the correct event
And then, after 2 sec I get
Error: timeout of 2000ms exceeded. Ensure the done() callback is
being called in this test.
Which is even weirder since the catch callback was executed(I think that for some reason the assert failure prevented the rest of the execution)
Now the funny thing, if I comment out the assert.isNotOk(error...) the test runs fine without any warning in the console. It stills 'fails' in the sense that it executes the catch.
But still, I can't understand these errors with promise. Can someone enlighten me?
The issue is caused by this:
.catch((error) => {
assert.isNotOk(error,'Promise error');
done();
});
If the assertion fails, it will throw an error. This error will cause done() never to get called, because the code errored out before it. That's what causes the timeout.
The "Unhandled promise rejection" is also caused by the failed assertion, because if an error is thrown in a catch() handler, and there isn't a subsequent catch() handler, the error will get swallowed (as explained in this article). The UnhandledPromiseRejectionWarning warning is alerting you to this fact.
In general, if you want to test promise-based code in Mocha, you should rely on the fact that Mocha itself can handle promises already. You shouldn't use done(), but instead, return a promise from your test. Mocha will then catch any errors itself.
Like this:
it('should transition with the correct event', () => {
...
return new Promise((resolve, reject) => {
...
}).then((state) => {
assert(state.action === 'DONE', 'should change state');
})
.catch((error) => {
assert.isNotOk(error,'Promise error');
});
});
For those who are looking for the error/warning UnhandledPromiseRejectionWarning outside of a testing environment, It could be probably because nobody in the code is taking care of the eventual error in a promise:
For instance, this code will show the warning reported in this question:
new Promise((resolve, reject) => {
return reject('Error reason!');
});
(node:XXXX) UnhandledPromiseRejectionWarning: Unhandled promise rejection (rejection id: 1): Error: Error reason!
and adding the .catch() or handling the error should solve the warning/error
new Promise((resolve, reject) => {
return reject('Error reason!');
}).catch(() => { /* do whatever you want here */ });
Or using the second parameter in the then function
new Promise((resolve, reject) => {
return reject('Error reason!');
}).then(null, () => { /* do whatever you want here */ });
I got this error when stubbing with sinon.
The fix is to use npm package sinon-as-promised when resolving or rejecting promises with stubs.
Instead of ...
sinon.stub(Database, 'connect').returns(Promise.reject( Error('oops') ))
Use ...
require('sinon-as-promised');
sinon.stub(Database, 'connect').rejects(Error('oops'));
There is also a resolves method (note the s on the end).
See http://clarkdave.net/2016/09/node-v6-6-and-asynchronously-handled-promise-rejections
The assertion libraries in Mocha work by throwing an error if the assertion was not correct. Throwing an error results in a rejected promise, even when thrown in the executor function provided to the catch method.
.catch((error) => {
assert.isNotOk(error,'Promise error');
done();
});
In the above code the error objected evaluates to true so the assertion library throws an error... which is never caught. As a result of the error the done method is never called. Mocha's done callback accepts these errors, so you can simply end all promise chains in Mocha with .then(done,done). This ensures that the done method is always called and the error would be reported the same way as when Mocha catches the assertion's error in synchronous code.
it('should transition with the correct event', (done) => {
const cFSM = new CharacterFSM({}, emitter, transitions);
let timeout = null;
let resolved = false;
new Promise((resolve, reject) => {
emitter.once('action', resolve);
emitter.emit('done', {});
timeout = setTimeout(() => {
if (!resolved) {
reject('Timedout!');
}
clearTimeout(timeout);
}, 100);
}).then(((state) => {
resolved = true;
assert(state.action === 'DONE', 'should change state');
})).then(done,done);
});
I give credit to this article for the idea of using .then(done,done) when testing promises in Mocha.
I faced this issue:
(node:1131004) UnhandledPromiseRejectionWarning: Unhandled promise rejection (re
jection id: 1): TypeError: res.json is not a function
(node:1131004) DeprecationWarning: Unhandled promise rejections are deprecated.
In the future, promise rejections that are not handled will terminate the Node.j
s process with a non-zero exit code.
It was my mistake, I was replacing res object in then(function(res), so changed res to result and now it is working.
Wrong
module.exports.update = function(req, res){
return Services.User.update(req.body)
.then(function(res){//issue was here, res overwrite
return res.json(res);
}, function(error){
return res.json({error:error.message});
}).catch(function () {
console.log("Promise Rejected");
});
Correction
module.exports.update = function(req, res){
return Services.User.update(req.body)
.then(function(result){//res replaced with result
return res.json(result);
}, function(error){
return res.json({error:error.message});
}).catch(function () {
console.log("Promise Rejected");
});
Service code:
function update(data){
var id = new require('mongodb').ObjectID(data._id);
userData = {
name:data.name,
email:data.email,
phone: data.phone
};
return collection.findAndModify(
{_id:id}, // query
[['_id','asc']], // sort order
{$set: userData}, // replacement
{ "new": true }
).then(function(doc) {
if(!doc)
throw new Error('Record not updated.');
return doc.value;
});
}
module.exports = {
update:update
}
Here's my take experience with E7 async/await:
In case you have an async helperFunction() called from your test... (one explicilty with the ES7 async keyword, I mean)
→ make sure, you call that as await helperFunction(whateverParams) (well, yeah, naturally, once you know...)
And for that to work (to avoid ‘await is a reserved word’), your test-function must have an outer async marker:
it('my test', async () => { ...
I had a similar experience with Chai-Webdriver for Selenium.
I added await to the assertion and it fixed the issue:
Example using Cucumberjs:
Then(/I see heading with the text of Tasks/, async function() {
await chai.expect('h1').dom.to.contain.text('Tasks');
});
Just a heads-up that you can get a UnhandledPromiseRejectionWarning if you accidentally put your test code outside of the it-function. 😬
describe('My Test', () => {
context('My Context', () => {
it('should test something', () => {})
const result = testSomething()
assert.isOk(result)
})
})