Chai's rejectedWith method fails to properly chain

After reviewing various posts and documentation, I am struggling to understand why this code is not functioning as expected.

const chai = require('chai');
const expect = chai.expect;
const chaiAsPromised = require('chai-as-promised');
chai.use(chaiAsPromised);

async function foo(val) {

    if (!val) {
        const err = new Error();
        err.status = 404;
        throw err;
    }

    return new Promise(resolve => sleepSetTimeout_ctrl = setTimeout(resolve, 10));
}

describe('does not work', () => {
    it('throws', async () => {
        await expect(foo()).to.be.rejectedWith(Error).and.have.property('status');
    });
});

When I stop the expect at rejectedWith(Error), everything works fine. However, when trying to test for the existence of a property, it fails with: 

AssertionError: expected {} to have property 'status'

javascriptchai-as-promised

Answer №1

The reason behind this is that when you use the rejectedWith method, it returns a PromisedAssertion, which checks if the promise-like object has a status property. A more detailed error message can help clarify this:

AssertionError: expected Promise{…} to have property 'status'

If you want to test for a property that the object does have, such as then:

await expect(foo()).to.be.rejectedWith(Error).and.have.property('then');  // ✅

To assert on the actual error object, you will need to utilize chainers from chai-as-promised once again, like this:

await expect(foo()).to.be.rejectedWith(Error).and.eventually.have.property("status");  // ✅
                                              // ^---------^

Alternatively, if you have multiple assertions to make on the resolved or rejected value, you can chain using .then:

await expect(foo()).to.be.rejectedWith(Error).then((err) => {
    expect(err).to.have.property("status", 404);
}); // ✅

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