What's this?
This a synchronous implementation od JavaScript Promise. JestMockPromise implements the same API as native JavaScript Promise, with two added benefits:
- it works synchronously - it calls the handler functions (
then
, catch
, finally
) right away (examples below clarify this) - it exposes the
resolve
and reject
functions as instance methods allowig the promise to be resolved outside of executor function
Although its originally created to make writing unit tests easier, it can also be used outside the unit testing context.
What's in this document?
TL; DR
Why is it then called jest-mock-promise?
Because it was originally written as a part of the jest-mock-fetch project and still is it's integral part (as an external dependency). So it's name is a legacy of it's humble beginnings :)
Can it be used in unit testing?
The answer is Yes! Since it's not married with Jest it can also be used with other JavaScript testing frameworks (i.e. Mocha, Jasmine).
API
The API of this synchronous promise matches the one of the regular Promise, with two additional instance methods (attached to an instance of the Promise):
resolve
- resolves a promise instancereject
- rejects a promise instance
This methods do the same job as the ones passed to the main callback function:
new Promise<string>((resolve, reject) => { resolve("some string value"); });
Having resolve
and reject
attached as instance methods enables us to call them outside the callback function, which makes our code much more readable:
let promise = new Promise<string>();
promise.resolve("some string value");
JestMockPromise vs native Promise
Let's jump right in and see an example:
import JestMockPromise from "../lib/jest-mock-promise";
const promise = new JestMockPromise((resolve, reject) => {
});
console.log('1. Promise is ready');
promise.then(() => console.log('3. Promise is resolved');
console.log('2. Will resolve the promise');
promise.resolve();
console.log('4. Last line of code');
Let's now have a look at what the console output will look:
1. Promise is ready
2. Will resolve the promise
3. Promise is resolved
4. Last line of code
As you can see, the then
handler was executed before the last line of code!
The crutial thing here is that this promise exposes the resolve function as an instance method, which means that you can call it directly on the instance of the promise object. This becomes escpecially useful in unit testing when you need to mock a component which returns a promise (i.e. jest-mock-fetch
).
Let's now try to implement something similar using regular Promise object:
let resolveFn;
let promise = new Promise((resolve, reject) => {
resolveFn = resolve;
console.log('1. Promise is ready');
});
promise.then(() => console.log('3. Promise is resolved'));
console.log('2. Will resolve the promise');
resolveFn();
console.log('4. Last line of code');
The regular promise would produce the following console output:
1. Promise is ready
2. Will resolve the promise
4. Last line of code
3. Promise is resolved
If you compare this with the first example you can notice the following:
- the order of execution is different - "3. Promise is resolved" is logged to the console AFTER "4. Last line of code"
- in order to access the
resolve
function you need to store to an outside variable within the Promise callback function
An unit testing example
Synchronous Promise was created to simplify unit testing an async component. So in the next two examples we'll have a look at how we can do just that - simplify a unit test.
What we'll be testing
We are going to test a component, which multiplies two numbers provided as a payload of a promise. The result is returned a call to a callback function.
The following snippet shows implementation of that component:
import Promise from 'es6-promise';
const onPromiseMultiply = (promise, callback) => {
promise.then(([a,b]) => {
callback(a*b);
})
};
export {onPromiseMultiply};
Now let's write some Jest tests.
First example - Traditional async test
In our first example we'll create a test in a traditional async way ... just to show how terible it is. Then, in the second example, we'll improve on the original idea by introducing jest-mock-promise
.
The next snippet contains a test written in traditional async way:
import {onPromiseMultiply} from '../component.js';
describe('testing the multiply component', () => {
it('should multiply two numbers and provide the result to the callback function', () => {
let callbackFn = jest.fn();
let promise = new Promise<[number,number]>((resolve, reject) {
resolve([1,2]);
});
onPromiseMultiply(promise, callbackFn);
return(promise.then(() => {
expect(callbackFn).toHaveBeenCalledWith(3);
}));
});
})
As we can see, it's not easy to see the order in which our code gets executed. Can we make this better? Yes we can! In the following section we'll see how ...
Second example - Applying the synchronous Promise
The first thing we need to do is install this component: npm i --save-dev jest-mock-promise
Since our component uses es6-promise
, we'll manually mock this dependency (if you don't know what manual mocking is, have a look at Manual Mocks @ Jest ). We'll create a __mocks__
directory inside our project root. There we'll create a es6-promise.js
file with the following content:
import JestMockPromise from 'jest-mock-promise';
export { JestMockPromise as Promise };
Now that's set up, we can modify our test:
import {onPromiseMultiply} from '../component.js';
describe('testing the multiply component', () => {
it('should multiply two numbers and provide the result to the callback function', () => {
let callbackFn = jest.fn();
let promise = new Promise<[number,number]>();
onPromiseMultiply(promise, callbackFn);
promise.resolve([1,2]);
expect(callbackFn).toHaveBeenCalledWith(3);
});
})
As we can see, reading our code just became much easier! Hooray!
Third example - Mocking fetch
As the final example we can have a look source code of jest-mock-fetch
, which is based on jest-mock-promise
.
License
MIT License, http://www.opensource.org/licenses/MIT