How to stub promises using Sinon.JS

Promises are a great way of doing async programming. But testing with promises can be a bit cumbersome, even with the use of the arrow functions syntax.

Recently I've found a small and nice package to help to stub tests using Sinon, and the library is sinon-stub-promise.

npm i sinon-stub-promise -D

So, imagine you have a code like the following:

function fetchMovieData() {
	return fetch("/movies")
		.then(res => {
			return res.json();
		})
		.then(movies => {
			return movies;
		});
}

function printMovies() {
	fetchMovieData().then(movies => {
		movies.forEach(movie => {
			console.log(movie);
		});
	});
}

A test for this code would be something like:

describe("printMovies", () => {
	let stubedFetch;
	beforeEach(() => {
		stubedFetch = sinon.stub(window, "fetch");
	});
	afterEach(() => {
		sinon.restore(window.fetch);
	});

	it("should work", () => {
		stubedFetch
			.returnsPromise()
			.resolves(["Star Wars", "The Matrix", "Forrest Gump"]);

		// do something
	});

	it("should handle errors", () => {
		stubedFetch.returnsPromise().rejects("a reason");
		// assert that error was handled
	});
});

Very simple, one just have to stub the function that will return the Promise, use the function returnsPromise. After that, you just have to the if the Promise will resolve and reject.

Oh yeah! And if you're using karma to run your tests there's even a plugin for that karma-sinon-stub-promise.