Synchronous Promise Mock for testing with Jest
npm install jest-mock-promisethen, catch, finally) right away (examples below clarify this)
resolve and reject functions as instance methods allowig the promise to be resolved outside of _executor function_
fetch
resolve - resolves a promise instance
reject - rejects a promise instance
javascript
new Promise((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:
`javascript
let promise = new Promise();
// resolving a promise
promise.resolve("some string value");
`
JestMockPromise vs native Promise
Let's jump right in and see an example:
`javascript
import JestMockPromise from "../lib/jest-mock-promise";
const promise = new JestMockPromise((resolve, reject) => {
// do you business normal here as normaly
});
console.log('1. Promise is ready');
promise.then(() => console.log('3. Promise is resolved');
console.log('2. Will resolve the promise');
promise.resolve(); // resolving the promise directly
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:
`javascript
// here we'll store the resolve Function
let resolveFn;
// creating a new promise
let promise = new Promise((resolve, reject) => {
// assigning the resolve function a variable from outter scope
resolveFn = resolve;
console.log('1. Promise is ready');
});
// attaching a then handler
promise.then(() => console.log('3. Promise is resolved'));
// resolving the promise right away
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:
1. the order of execution is different - "3. Promise is resolved" is logged to the console AFTER "4. Last line of code"
2. 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:
`javascript
// ./src/component.js
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:
`typescript
// ./src/__test__/component.spec.js
import {onPromiseMultiply} from '../component.js';
import Promise from 'jest-mock-promise';
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) {
// providing two numbers which need to be multiplied
// as we know, although we have resolved the promise right away,
// then handlers will be called asnyc at later time
resolve([1,2]);
});
// calling the function we want to test
onPromiseMultiply(promise, callbackFn);
// Altought promise is already resolved, then handlers will
// be called async at later time. That's why we need to put
// our expectation inside a then handler
// + we need to return a promise to the Jest, so it knows
// we're doing some async testing
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:
`javascript
// ./__mocks__/es6-promise.js
import JestMockPromise from 'jest-mock-promise';
// mocking the es6-promise, which is used by component we are testing
export { JestMockPromise as Promise };
`
Now that's set up, we can modify our test:
`javascript
// ./src/__test__/component.spec.js
import {onPromiseMultiply} from '../component.js';
import Promise from 'jest-mock-promise';
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]>();
// calling the function we want to test
onPromiseMultiply(promise, callbackFn);
// resolving our promise
promise.resolve([1,2]);
// testing to see if our function is working correctly
expect(callbackFn).toHaveBeenCalledWith(3);
});
})
`
As we can see, reading our code just became much easier! Hooray!
A working implementation of this example can be found in the example directory of this project.
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`.