How to test Promise catch with Mocha - unit-testing

Question

I'm trying to test the GET HTTP method from a requests module:
const get = (host, resource, options) => {
...
return new Promise((resolve, reject) => fetch(url, opts)
.then(response => {
if (response.status >= 400) {
reject({
message: `[API request error] response status: ${response.status}`,
status: response.status });
}
resolve(response.json());
})
.catch(error => reject(error)));
};
And here is how I tested the .then part:
it('Wrong request should return a 400 error ', (done) => {
let options = { <parameter>: <wrong value> };
let errorJsonResponse = {
message: '[API request error] response status: 400',
status: 400,
};
let result = {};
result = get(params.hosts.api, endPoints.PRODUCTS, options);
result
.then(function (data) {
should.fail();
done();
},
function (error) {
expect(error).to.not.be.null;
expect(error).to.not.be.undefined;
expect(error).to.be.json;
expect(error).to.be.jsonSchema(errorJsonResponse);
done();
}
);
});
However I didn't find a way to test the catch part (when it gives an error and the response status is not >= 400).
Any suggestions?
It would also help me solve the problem a simple example with another code that tests the catch part of a Promise.

Answer 1

I've ended up writing the following code in order to test the catch:
it('Should return an error with invalid protocol', (done) => {
const host = 'foo://<host>';
const errorMessage = 'only http(s) protocols are supported';
let result = {};
result = get(host, endPoints.PRODUCTS);
result
.then(
() => {
should.fail();
done();
},
(error) => {
expect(error).to.not.be.null;
expect(error).to.not.be.undefined;
expect(error.message).to.equal(errorMessage);
done();
}
);
});