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!");
}
Related
I'm having an issue catching an error thrown from the Node-Postgres NPM package.
The issue seems simple on the surface, but I've tried everything I can think of.
My code is like the following:
import { Pool } from 'pg' // Import postgres connection pool
const pgPool = new Pool()
async function queryDatabase() {
try {
// Force TypeError by passing undefined
let queryResult = await pgPool.query( undefined )
if ( queryResult.rows.length > 0 ) {
return queryResult.rows[0]
}
return false
} catch( err ) {
// Never Reached
return new Error( 'Test error' )
}
}
queryDatabase()
And the error is as follows:
TypeError: Client was passed a null or undefined query
at Client.query (~/.../node_modules/pg/lib/client.js:479:11)
The error itself is pretty self-explanatory. I'm forcing the error here, for the sake of trying to handle it in the event that undefined gets passed by mistake. I realize that I can simply perform a check to make sure the input is never null or undefined, but that's not my main concern.
My worry is if I can't catch this error thrown from this package, how many other unforeseen cases am I going to encounter where I simply can't catch and handle a thrown error.
I've tried numerous different approaches - The Async/Await Try/Catch method, shown above - I've tried pgPool.query().then().catch() - Combinations of the two. I've even tried running the catch against the Pool instance itself. No matter what I do, I can't handle the exception without using Node's process.on('unhandledRejection', ...), which is of course a bad idea.
I've been racking my brain on this for hours. Is there any way that I can catch and handle errors like this, so it's not crashing my server every time? Thanks in advance!
I was able to reproduce this and it seems to be an actual bug in the pg-library.
According to the source if you call .query on a pool instance, this instance will attempt to connect and get a client. In this connect-callback the actual query is dispatched to the client-module, which will throw the mentioned type error if the query is nil.
This error is thrown synchronously (i.e. the error is not passed to the callback argument, e.g. callback(new TypeError("...")) and since there's no try/catch around the client.query call in the pool's connect-callback, the error will not be caught by your try/catch.
A potential fix would be to wrap the client.query call in a try catch:
client.once('error', onError)
this.log('dispatching query')
try {
client.query(text, values, (err, res) => {
this.log('query dispatched')
client.removeListener('error', onError)
if (clientReleased) {
return
}
clientReleased = true
client.release(err)
if (err) {
return cb(err)
} else {
return cb(undefined, res)
}
})
}catch(err) {
return cb(err)
}
So for now, you probably should create an issue on github and wait for the bugfix or fork the repo and use above workaround, I'm afraid.
I'm calling this function in my code and throwing an error:
myFunction(message) {
if (!message) {
throw new Error('No Message')
}
}
and the test:
it.only('should throw error if no message', () => {
library.myFunction() // no message
expect(global.opener.postMessage).not.toHaveBeenCalled()
})
But Jest is just pointing at the word new and the test is failing with no other message. How can I fix this?
Your test is crashing, rather than failing, because the error thrown by myFunction is never caught. There are two broad solutions to this:
Let Jest catch it, using the built-in toThrow expectation for errors:
it.only('should throw error if no message', () => {
expect(() => library.myFunction()).toThrow()
expect(global.opener.postMessage).not.toHaveBeenCalled()
})
Note that in this case you need to wrap the call to myFunction in a function, to defer execution so that Jest can handle the error (otherwise the error is thrown before expect can be called and you're back in the same position).
Catch it yourself, using try/catch, and explicitly ensure that one expectation is reached during the test:
it.only('should throw error if no message', () => {
expect.assertions(1)
try {
library.myFunction()
catch (err) {
expect(global.opener.postMessage).not.toHaveBeenCalled()
}
})
Without expect.assertions, if a future change means that the call to library.myFunction does not throw an error, then no expectation is ever reached inside the test and it silently passes. It's important to ensure that either all logical paths through your test have expectations, or you explicitly check the expected number are reached.
Some time ago I was curious on how to execute async tests using Mocha and those tests to expect an error as a result (see Mocha async test handle errors)
Now I tried the same for the newer version of nodejs using await/async. Just a simple case but as I need to wrap the code in a try / catch block, things get out of hand.
After long hours of testing I ended up with a solution but it doesn't look good.
So I'm looking for a better way to do this.
The test looks like this:
it('myTest', async function () {
// THIS WORKS => CODE OPTION 1:
// this works, it uses promises but not await/async
return somePromiseFunction()
.then(() => Promise.reject(new Error('Expected method to reject.')))
.catch(err => assert.ok(err instanceof Error, 'This should be an error!'));
// THIS WORKS => CODE OPTION 2:
// this works, it uses await/async
let forceFail = false;
try {
await somePromiseFunction();
forceFail = true;
} catch (err) {}
if (forceFail) assert.ok(false, 'Expected method to reject.');
// WONT WORK => CODE OPTION 3:
try {
await somePromiseFunction();
assert.ok(false, 'you shouln\'t be here');
} catch (err) {
assert.ok(err instanceof Error, 'This should be an error!');
}
});
Both option 1 and 2 work. Option 1 uses classic Promise.then.catch syntax, and that's ok. Option 2 was the only way for me to make things work but is very complex to understand / maintain. It relies on a sort of global variable and handling states, and it just doesn't look good.
There is an option 3 in the code, but it does not work. Something like that would be easy to read but it does not work at all, not the first assert nor the second one.
If you remove the try / catch block then it won't work either.
Thanks.
In the third option, err would always be undefined since you're catching the exception as e, not as err. And the assert would always fail because undefined is not an instance of Error.
I found a way, but you'll need to upgrade to node version 10. I was using 8, but this new feature included in assert solves my problem in an elegant way.
Instead of trying this:
// WONT WORK => CODE OPTION 3:
try {
await somePromiseFunction();
assert.ok(false, 'you shouln\'t be here');
} catch (err) {
assert.ok(err instanceof Error, 'This should be an error!');
}
Do this:
// this works!
await assert.rejects(
async () => somePromiseFunction(),
Error
);
You can also verify the type of error being thrown, but in my case Error will suffice.
EDIT: async () => was not really necessary, I ended up with a one line assert (second parameter is optional, but I'd rather keep it):
await assert.rejects(somePromiseFunction(), Error);
I am trying to understand how the asynchronous code for Mocha (at http://mochajs.org/#getting-started) works.
describe('User', function() {
describe('#save()', function() {
it('should save without error', function(done) {
var user = new User('Luna');
user.save(function(err) {
if (err) throw err;
done();
});
});
});
});
I want to know how Mocha decides whether a test has succeeded or failed behind the scenes.
I can understand from the above code that user.save() being asynchronous would return immediately. So Mocha would not decide if the test has succeeded or failed after it executes it(). When user.save() ends up calling done() successfully, that's when Mocha would consider it to be a successful test.
I cannot understand how it Mocha would ever come to know about a test failure in the above case. Say, user.save() calls its callback with the err argument set, then the callback throws an error. None of Mocha's function was called in this case. Then how would Mocha know that an error occurred in the callback?
Mocha is able to detect failures that prevent calling the callback or returning a promise because it uses process.on('uncaughtException', ...); to detect exceptions which are not caught. Since it runs all tests serially, it always knows to which test an uncaught exception belongs. (Sometimes people are confused by this: telling Mocha a test is asynchronous does not mean Mocha will run it in parallel with other tests. It just tells Mocha it should wait for a callback or a promise.)
Unless there is something that intervenes to swallow exceptions, Mocha will know that the test failed and will report the error as soon as it detects it. Here is an illustration. The first test fails due to a generic exception thrown. The 2nd one fails due to an expect check that failed. It also raises an unhandled exception.
var chai = require("chai");
var expect = chai.expect;
it("failing test", function (done) {
setTimeout(function () {
throw new Error("pow!");
done();
}, 1000);
});
it("failing expect", function (done) {
setTimeout(function () {
expect(1).to.equal(2);
done();
}, 1000);
});
This is the output on my console:
1) failing test
2) failing expect
0 passing (2s)
2 failing
1) failing test:
Uncaught Error: pow!
at null._onTimeout (test.js:6:15)
2) failing expect:
Uncaught AssertionError: expected 1 to equal 2
+ expected - actual
-1
+2
at null._onTimeout (test.js:13:22)
The stack traces point to the correct code lines. If the exceptions happened deeper, the stack would be fuller.
When Mocha cannot report what went wrong exactly, that's usually because there is intervening code that swallows the exception that was raised. Or when you use promises the problem may be that someone forgot to call a method that indicates whether the promise is supposed to be completely processed and unhandled exceptions should be thrown. (How you do this depends on the promise implementation you use.)
It won't, it's a shame. It has no way to know that your callback is executing. It's an easier way to do asynchronous testing, where you just tell the test when you are finished. The downside, as you have noticed, is that errors in asynchronous callbacks won't be detected. Nevermind, Mocha hooks to process.on('uncaughtException',...) as mentioned by Louis. Note that if you use done instead of waitsFor and runs in jasmine, then you will have the problem.
Other frameworks like js-test-driver force to you wrap callbacks so the the testing framework can put a try catch around your callbacks (and you don't need to call done). Your test would look like the following:
var AsynchronousTest = AsyncTestCase('User');
AsynchronousTest.prototype.testSave = function(queue) {
queue.call('Saving user', function(callbacks) {
var user = new User('Luna');
user.save(callbacks.add(function(err) {
if (err) throw err;
// Run some asserts
}));
});
};
One of the things that I find frustrating about Mocha is that when tests fail, they don't give the actual error message of the failing line, Instead, they just end with Error: timeout of 2000ms exceeded. Ensure the done() callback is being called in this test.
Take this test for example:
describe("myTest", function() {
it("should return valid JSON.", function(done) {
api.myCall("valid value").then(function(result) {
console.log(result);
var resultObj = JSON.parse(result);
assert.isFalse(resultObj.hasOwnProperty("error"), "result has an error");
done();
});
});
});
The output is:
myTest
{"error":null,"status":403}
1) should return valid JSON.
0 passing (2s)
1 failing
1) myTest should return valid JSON.:
Error: timeout of 2000ms exceeded. Ensure the done() callback is being called in this test.
The assert.isFalse is failing, but the message that should be displayed ("result has an error") isn't displayed. In fact, processing seems to stop right there because done() is never called. Take that line out and the test passes because done() is called.
So, what am I missing? Why do Mocha tests behave this way? The actual test library I'm using is:
var assert = require("chai").assert;
Does anyone know what I'm doing wrong or why this behaves this way?
It looks like your API is using promises. Before trying anything else, I would suggest checking what the documentation of the API says about promises and how to deal with unhandled exceptions because this may be what is happening here. Some promise implementations require that you call .done() at the end of your call chain to ensure that uncaught exceptions are going to be processed. Some require that some global promise setting be properly configured. The Bluebird documentation gives a good discussion of the issues.
Mocha is capable of handling uncaught exceptions in run-of-the-mill code:
var chai = require("chai");
var assert = chai.assert;
chai.config.includeStack = true;
describe("foo", function() {
it("let the exception be caught by Mocha", function(done) {
setTimeout(function () {
assert.isFalse(true, "foo");
done();
}, 1000);
});
});
This will result in the output:
foo
1) let the exception be caught by Mocha
0 passing (1s)
1 failing
1) foo let the exception be caught by Mocha:
Uncaught AssertionError: foo: expected true to be false
at Assertion.<anonymous> (/tmp/t7/node_modules/chai/lib/chai/core/assertions.js:286:10)
at Assertion.Object.defineProperty.get (/tmp/t7/node_modules/chai/lib/chai/utils/addProperty.js:35:29)
at Function.assert.isFalse (/tmp/t7/node_modules/chai/lib/chai/interface/assert.js:297:31)
at null._onTimeout (/tmp/t7/test.js:8:20)
at Timer.listOnTimeout (timers.js:119:15)
I've encountered the same in my code, using Q for promises.
What happened was:
The assertion inside the then block failed.
The rest of the then block, including the done() statement, was not executed.
Q went looking for a catch block, which wasn't there.
This led to a 'hanging' promise, and thus to a Mocha 2000 ms timeout.
I worked around it by doing something like this:
describe("myTest", function() {
it("should return valid JSON.", function(done) {
api.myCall("valid value").then(function(result) {
console.log(result);
var resultObj = JSON.parse(result);
assert.isFalse(resultObj.hasOwnProperty("error"), "result has an error");
done();
})
.catch(function(err) {
console.error(err);
done(err);
});
});
});