Mocha / Chai expect.to.throw not catching thrown errors

Function binding: Keeping context & arguments intact

When testing for exceptions with expect.to.throw, use function binding if it needs specific context or some arguments. It helps maintain correct context and enables precise error assertion:

expect(myMethod.bind(context, arg1, arg2)).to.throw();


The use of .bind() ensures the function retains its context when passed to expect.to.throw, avoiding any "I lost my this" scenario and missing arguments.

Asserting specific error messages: Get precise

To assert a specific error message, wrap your code in a function or utilise an ES6 arrow function for enhanced syntax and controlled scope:

expect(() => { throw new Error("Specific error message"); }).to.throw("Specific error message");


It's vital to test for specific error messages. Not only does it save you from "Oops, not the error I expected" moments, but also helps in efficient debugging and robust code maintenance.

Managing uncaught exceptions: Catch 'em all

During testing, uncaught exceptions can break your flow and might lead to misleading results. To efficiently handle uncaught exceptions, encapsulate your assertions and use try/catch within asynchronous tests for promises, or you can simply rely on Mocha’s native support for Promises:

it('should handle and assert uncaught exceptions', async () => {
  try {
    await expect(Promise.reject(new Error("Error"))).to.be.rejected;
    // Oops! We rejected a promise! As expected, though. 
  } catch (error) {
    expect(error).to.be.an('error');
    // Gotcha! Caught the error red-handed.
  }
});


Handling these exceptions properly can reassure that expect.to.throw functions as expected, even in complex situations involving asynchronous operations and promises.

Checking exceptions: The alternatives

In addition to expect.to.throw, Chai provides other assertion styles such as should.throw and assert.throws. These can be used based on your style preference or specific requirement:

should(() => myFunction()).throw("Error message");
assert.throws(() => myFunction(), "Error message");


These methods ensure you have the right tool, in the right syntax for every situation, talking about flexibility, eh?

Adapting test patterns: Different scenarios demand adaptation

The strategy of testing error handling isn't a one-size-fits-all. Based on the requirements of your specific test scenario, you need to adapt your use of expect.to.throw. This includes scenarios handling async functions, constructors, or methods which throw synchronously versus those that reject promises.

Testing constructors and methods: Keeping an eye on 'new' constructions

When testing constructors or methods that throw, wrap the invocation to prevent immediate execution:

class CustomError extends Error {}
function createErrorInstance() {
  new CustomError();
}

expect(() => createErrorInstance()).to.throw(CustomError);
// Are you constructing the problem?


Take note, you are testing the construction process, not the instance, a common pitfall.

Working with async functions and Promises: Catching up with async debugging

For async functions and Promises, Mocha and Chai can play very well with async/await patterns:

it('should catch errors from promises', async () => {
  await expect(asyncFunctionThatRejects()).to.eventually.be.rejectedWith(Error);
  // Promise me you will reject me?
});


This is how you assure that an async function eventually throws or rejects with a specific error.

Best practices for error testing: The path to reliable testing

To ensure high reliability of your error testing, follow best practices including:

• Consistency: Stick to a decided pattern, whether it's expect.to.throw, should.throw, or assert.throws.
• Exactness: Always test for specific error types and messages. Lifesaver for future debugging!
• Up-to-date: Regularly check the Chai documentation for updates or new features to mould better tests.

Combining these principles will strive for robust, consistent, and readable tests.