关于Mocha中it()方法参数函数及Chai断言库返回值的问询

Great question! Let's break this down clearly, since there's a common misconception here about how assertion libraries like Chai work with Mocha.

Mocha's it() Callback Return Values #

First, let's set the context: Mocha cares about two key scenarios from your it() callback:

• Synchronous tests: Mocha checks if the callback throws an error. If no error is thrown, the test passes; if an error is thrown, it fails. Any return values (other than Promises) are completely ignored here.
• Asynchronous tests: Mocha expects either a returned Promise (it will wait for it to resolve or reject) or a call to the done() callback. If the Promise rejects, or done() is called with an error, the test fails.

What Chai Assertions Actually Return #

Your assumption that Chai methods return true or false is incorrect—this is a super common mix-up! Here's the real behavior:

• When an assertion passes: Chai's methods (like expect(foo).to.equal(bar) or assert.equal(foo, bar)) return the Chai assertion object itself. This is what enables chained assertions like expect(foo).to.be.a('number').and.be.greaterThan(5). They do not return true.
• When an assertion fails: Chai throws an Error object with a detailed message (e.g., "expected 3 to equal 2" or "expected 'foo' to be a number"). This error is what Mocha catches to mark the test as failed.

Is Returning Chai Assertions a Valid Pattern? #

Let's say you wrote something like this:

it('checks if values are equal', () => {
  return expect(1 + 1).to.equal(2);
});

Will this work? Technically, yes—but not for the reason you might think.

Mocha ignores the returned Chai assertion object (since it's not a Promise). The test passes because the assertion doesn't throw an error. If the assertion failed (e.g., expect(1 + 1).to.equal(3)), Chai would throw an error, which Mocha catches and uses to mark the test as failed.

That said, this pattern is unnecessary. There's no benefit to returning the assertion—you can just run the assertion directly:

it('checks if values are equal', () => {
  expect(1 + 1).to.equal(2); // No return needed!
});

This is the standard, recommended approach for synchronous Mocha tests.

Key Takeaways #

• Chai assertions do not return boolean values. They either return a chainable assertion object (on pass) or throw an error (on fail).
• Mocha uses error throwing (for sync tests) or Promise state/done callbacks (for async tests) to determine test pass/fail—returned non-Promise values are irrelevant.
• Returning Chai assertions isn't "wrong," but it's redundant and doesn't align with how Mocha and Chai are designed to work together.

内容的提问来源于stack exchange，提问作者Leviathan_the_Great