Why Does `expect.to.throw` Fail to Catch Thrown Errors in Chai?

Mocha / Chai expect.to.throw: Understanding How It Works

In testing, it's crucial to ensure that errors are thrown as expected. However, users often encounter challenges with Chai's expect.to.throw, failing to catch thrown errors correctly. Let's delve into why this occurs and how to resolve it.

The problem arises when directly passing the result of a function call to expect.to.throw. For example, expect(model.get('z')).to.throw(...). In this case, you're asserting on the return value of model.get('z'), not the actual throwing of an error.

To fix this issue, you need to pass a function expecting a call to expect. This function would then call model.get with appropriate arguments. Consider the following code:

expect(model.get.bind(model, 'z')).to.throw('Property does not exist in model schema.');

Here, model.get.bind(model, 'z') creates a new function that, when called, invokes model.get with the 'z' argument, resolving the problem.

Understanding this behavior is essential to effectively use expect.to.throw. Remember to pass functions that will induce the expected error, ensuring accurate testing of error handling scenarios.

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