Why Does `expect.to.throw` Fail to Assert Thrown Errors in Node.js Tests?

Chai: Addressing the Elusive expect.to.throw in Node.js

Chai's expect.to.throw can prove perplexing in Node.js testing. Despite its intended purpose of asserting thrown errors, it often fails when directly applied to code snippets.

Understanding the Issue:

Consider the example test case:

it('should throw an error if you try to get an undefined property', function (done) {
  // Passing the result of model.get('z') directly fails
  expect(model.get('z')).to.throw('Property does not exist in model schema.');
});

This test fails, although the error is actually thrown. A common misconception is that expect.to.throw handles the retrieval and assertion of the thrown error.

Solution: Embracing Function-Passing:

The key to resolving this issue lies in passing a function to expect.to.throw instead of the result. This function will be executed by expect, triggering the retrieval and validation of the thrown error:

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

In this modified code, model.get is bound to the context of the model object and 'z' is set as its argument. Then, the resulting function is passed to expect.to.throw, ensuring that the intended error is captured and asserted.

By following this approach, you can harness the full power of expect.to.throw in Node.js testing, effectively asserting the occurrence of thrown errors and ensuring the robustness of your code.

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