Expect a function to throw an exception in Jest

Question

I have a function (hostelService.book) that returns a promise: return Promise.resolve(response); and I made this test:

const action = async () => {
        await hostelService.book(id);
    };


  await expect(action).rejects.toThrow();

but I have this error:

 Matcher error: received value must be a promise

Answer 1

Jest is throwing this error Matcher error: received value must be a promise because in expect you are just passing the function reference. Without () - action is just a function reference it will not return anything.

To fix this issue you have to call the function in expect like action() so it will return the promise object.

And the second part, you have to throw error like. Promise.reject(new Error('some error')); in the reject so the tothrow condition can be full filled.

Example:

function hostelService() {
   return Promise.reject(new Error('some error'));
}

const action = async () => {
   await hostelService();
};

it( "Should throw the error", async () => {
   await expect(action()).rejects.toThrow('some error');
});

Hope this will be helpful.

Answer 2

Your test fails because you're awaiting the promise (and therefore, it is no longer a Promise when you call .rejects on it). If you remove await from the last statement, it should do the trick:

const action = async () => {
  await hostelService.book(id);
};

expect(action()).rejects.toThrow();

Here is some rationale behind it. async automatically wraps your return type into a Promise. However, when you call await, it synchronously "unwraps" the value of a given Promise and therefore it can't call .rejects on the result (unless you wrap a Promise into another Promise).

I've composed a small example to illustrate what I mean.