Creating a Mocha+Chai test that anticipates an exception being thrown by setTimeout

Question

Creating a Mocha+Chai test that anticipates an exception being thrown by setTimeout

Here is what I have:

it('invalid use', () => {
  Matcher(1).case(1, () => {});
});

I am trying to ensure that the case method throws an exception after a delay. How can I specify this for Mocha/Chai so that the test passes only if an exception is thrown (and should fail otherwise)?

I need to emphasize that the case method cannot be modified in any way.

To simulate the behavior for testing purposes, consider the following equivalent code:

it('setTimeout throw', _ => {
  setTimeout(() => { throw new Error(); }, 1); // This line must remain unchanged
});

I attempted the following approach:

it('invalid use', done => {
  Matcher(1).case(1, () => {});
  // Call the done callback after 'case' may throw an error
  setTimeout(() => done(), MatcherConfig.execCheckTimeout + 10);
});

However, this solution is not helping me as it produces opposite results - passing when no exception is thrown from case (setTimeout) and failing when an exception is thrown.

I remember reading about utilizing a global error handler, but I prefer to find a clean solution using Mocha and/or Chai, if possible (I assume Mocha already incorporates something similar behind the scenes).

javascript typescript mocha.js chai

Answer 1

Answer №1

Handling exceptions within an asynchronous callback can be tricky, as discussed in this Stack Overflow thread. The execution model of ECMAScript plays a role in this limitation. One workaround is to implement global error handling specific to your environment, such as using

process.on('uncaughtException', ...)

in Node.js.

If you convert your function to Promises, you can simplify testing by utilizing the Chai plugin called 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;
});

Answer 2

Handling exceptions within an asynchronous callback can be tricky, as discussed in this Stack Overflow thread. The execution model of ECMAScript plays a role in this limitation. One workaround is to implement global error handling specific to your environment, such as using

process.on('uncaughtException', ...)

in Node.js.

If you convert your function to Promises, you can simplify testing by utilizing the Chai plugin called 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;
});

Answer 3

Answer №2

To make Mocha statements such as before, after, or it work asynchronously, you can utilize promises. Below is an example of how to implement async tests with promises. Remember to set a timeout using this.timeout(...) if the async function may take more than 2 seconds to complete.

it('some test', () => {
    return new Promise(function(resolve,reject){
        SomeAsyncFunction(function(error,vals) {
            if(error) {
                 return reject(error);    
            } else {
                try {
                    //perform chai tests here
                } catch(e) {
                    return reject(e);
                }
                return resolve();
            }
        });
    });
});

In your specific case, where you expect an error to be thrown after a certain time period, you can structure your test like this:

it('invalid use', () => {
    let prom = new Promise(function(resolve,reject){
        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');
            //additional checks to verify the error
        } catch(e) {
            return Promise.reject(e);
        }
        return Promise.resolve();
    });
    return prom;
});

Answer 4

To make Mocha statements such as before, after, or it work asynchronously, you can utilize promises. Below is an example of how to implement async tests with promises. Remember to set a timeout using this.timeout(...) if the async function may take more than 2 seconds to complete.

it('some test', () => {
    return new Promise(function(resolve,reject){
        SomeAsyncFunction(function(error,vals) {
            if(error) {
                 return reject(error);    
            } else {
                try {
                    //perform chai tests here
                } catch(e) {
                    return reject(e);
                }
                return resolve();
            }
        });
    });
});

In your specific case, where you expect an error to be thrown after a certain time period, you can structure your test like this:

it('invalid use', () => {
    let prom = new Promise(function(resolve,reject){
        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');
            //additional checks to verify the error
        } catch(e) {
            return Promise.reject(e);
        }
        return Promise.resolve();
    });
    return prom;
});

Answer 5

Answer №3

According to information from the Chai documentation:

When no arguments are provided, .throw will execute the target function and verify that it throws an error.

For example, you could do something like this:

expect(Matcher(1).case(1, () => {})).to.throw

Answer 6

According to information from the Chai documentation:

When no arguments are provided, .throw will execute the target function and verify that it throws an error.

For example, you could do something like this:

expect(Matcher(1).case(1, () => {})).to.throw

Answer 7

Answer №4

If the code being tested includes a call to setTimeout with a callback that throws an uncaught exception, then:

1) the code is considered broken

2) one way to identify this issue is through a platform global exception handler such as process.on('uncaughtException', as suggested by user ComFreek

If all else fails, you can temporarily replace setTimeout during testing (e.g., using sinon.stub) or manually stub it.

When using a stubbed setTimeout, you can modify the timeout handler to detect exceptions and trigger appropriate assertions.

It's important to note that this approach should be a last resort - ideally, your application code should handle errors properly to ensure smooth operation, not just for testing purposes but for overall code quality.

Here is some pseudocode as an example:

it('test', (done) => {

    const originalSetTimeout = setTimeout;
    setTimeout = (callback, timeout) => {
        originalSetTimeout(() => {
            try {
                callback();
            } catch(error) {
                 // Exception intercepted in _SOME_ setTimeout handler
            }
        }, timeout)
    }
    yourTestCodeThatTriggersErrorInSomeSetTimeoutCallback(done);
})

Please note that I have deliberately omitted proper asynchronous cleanup code. This is left as an exercise for the reader. Check out sinon.js and its sandbox feature for assistance.

Keep in mind that this approach will intercept all setTimeout calls during the test duration. Proceed with caution!

Answer 8

If the code being tested includes a call to setTimeout with a callback that throws an uncaught exception, then:

1) the code is considered broken

2) one way to identify this issue is through a platform global exception handler such as process.on('uncaughtException', as suggested by user ComFreek

If all else fails, you can temporarily replace setTimeout during testing (e.g., using sinon.stub) or manually stub it.

When using a stubbed setTimeout, you can modify the timeout handler to detect exceptions and trigger appropriate assertions.

It's important to note that this approach should be a last resort - ideally, your application code should handle errors properly to ensure smooth operation, not just for testing purposes but for overall code quality.

Here is some pseudocode as an example:

it('test', (done) => {

    const originalSetTimeout = setTimeout;
    setTimeout = (callback, timeout) => {
        originalSetTimeout(() => {
            try {
                callback();
            } catch(error) {
                 // Exception intercepted in _SOME_ setTimeout handler
            }
        }, timeout)
    }
    yourTestCodeThatTriggersErrorInSomeSetTimeoutCallback(done);
})

Please note that I have deliberately omitted proper asynchronous cleanup code. This is left as an exercise for the reader. Check out sinon.js and its sandbox feature for assistance.

Keep in mind that this approach will intercept all setTimeout calls during the test duration. Proceed with caution!

Creating a Mocha+Chai test that anticipates an exception being thrown by setTimeout

Answer №1

Answer №2

Answer №3

Answer №4

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