Jest is a JavaScript unit testing framework which is a super version of Jasmine.

Jest is a framework and not a library. It comes with a test runner, assertion library, and good mocking support. A pretty comprehensive all-in-one testing framework which is kind of equivalent to Mocha (test runner) + Chai (assertion library) + Sinon (spies, mocks) + Istanbul (coverage).

Place your tests in a tests folder, or name your test files with a .spec.js or .test.js extension. Whatever you prefer, Jest will find and run your tests.

• Getting Started
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• Jest Crash Course - Unit Testing in JavaScript

$ npm i --save-dev jest jest-dashboard

# If using "React-Native":
$ npm i --save-dev jest-react-native

# If using babel, install as below and don't forget to add a ".babelrc" file in your project's root folder.
$ npm i --save-dev babel-jest babel-core regenerator-runtime

# If using enzyme:
$ npm i --save-dev jest-enzyme

/**
 * jest.config.js
 * Remove jest configuration from package.json and create this config file
 */
module.exports = {
    preset: "react-native", // Only if using react-native
    verbose: true,
    setupTestFrameworkScriptFile: "<rootDir>src/test/setupTests.js",
    testPathIgnorePatterns: [
        "/node_modules/",
        "/src/modules/",
        "/src/common/utils/"
    ],
    "coveragePathIgnorePatterns": [
        "/node_modules/",
        "/src/modules/",
        "/src/common/utils/"
    ],
    transform: {
        "^.+\\.js?$": "babel-jest"
    },
    automock: false
}

$ npm run jest

# Watch mode
$ npm run jest --watch

# Coverage Reporting. Also check "coverage/lcov-report/index.html" file for report
$ npm run jest --coverage

The Jest API focusses more on the ability to define tests, make assertions, and create mocks.

• describe: defines a test suite.
• it: defines a test.
• beforeEach: defines an entry hook before running each test.
• expect: makes an assertion.
• jest.fn(): creates a mock function.

Several methods are available for assertions.

• toEqual: checks if two objects have the same value.
• toBe: checks if two objects have the same value and type.
• toBeDefined: checks if the object is defined.
• toContain: checks if an item is present in a list.
• toBeCalled: checks if a mock function is called.

There are useful methods on the mock.

• mockImplementation: provides an implementation for the mock function.
• mockReturnValue: returns a value when the mock function is called.