jest test function code example

Example 1: jest mock implementation

describe("mockImplementation", () => {
  test("function", () => {
    const mockFn1 = jest.fn().mockImplementation(() => 42);
    const mockFn2 = jest.fn(() => 42);

    expect(mockFn1()).toBe(42);
    expect(mockFn2()).toBe(42);
  });

Example 2: testing with jest

npm i jest --save -dev
-> In Package.json change the Script object(watchAll - jest will run auto)
"test" : "jest --watchAll"
-> test file should be named such way that re used by jest can recognize it
like filename.<test> or <spec> or skip it.js
Ex: sum.test.js or sum.spec.js or sum.js
-> require the target for testing file in this test file then enjoy testing.
-> visit jestWebsite-> Docs.

Example 3: write unit test jest first before json function

const axios = require('axios');
const Users = require('./users');

jest.mock('axios');

test('should fetch users', () => {

    const users = [{
        "id": 1,
        "first_name": "Robert",
        "last_name": "Schwartz",
        "email": "[email protected]"
    }, {
        "id": 2,
        "first_name": "Lucy",
        "last_name": "Ballmer",
        "email": "[email protected]"
    }];

    const resp = { data : users };

    axios.get.mockImplementation(() => Promise.resolve(resp));

    Users.all().then(resp => expect(resp.data).toEqual(users));
});