Jest

Updated 12/01/2024

Jest is a javaScript testing framework.

Pre Cursor

  • Install Node so we can run the JS locally https://nodejs.org/en/download, check its installed with node -v
  • npm init, name jesttesting and enter defaults, for test type jest
  • this creates package.json
  • npm install --save-dev jest to install jest
  • create our root module index.js and test for it index.test.js

The package.json file should look like 

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{
  "name": "jesttesting",
  "version": "1.0.0",
  "description": "",
  "main": "index.js",
  "scripts": {
    "test": "jest"
  },
  "author": "",
  "license": "ISC",
  "devDependencies": {
    "jest": "^29.7.0"
  }
}

  • Then to create a js file that the browser can use run npm install -g webpack webpack-cli
  • create webpack.config.js (it had to have the absolute path)
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module.exports = {
    entry: './index.js',
    output: {
      filename: 'bundle.js',
      path: 'C:\\dev\\jest\\dist'
    }
  };
  • Run webpack, this creates the bundle in dist which should be browser friendly.

Life Cycle

Note that it and test are the same thing and you nest a describe in a describe

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describe('unit of work should', () => {
	beforeAll(() => {
		//setup spyOn
	});
	it(`do x under y predicate "${allowsVariableInName}"`, () => {
        //arrange, act, expect
	});
	it('do foo under baz predicate', () => {
        //arrange, act, expect
	});
	afterAll(() => {
		//mockRestore
	});
});

Update index.js

This is a simple example that redirects a user to a new version of the page when the js file is loaded in the master (so avalible on all pages)

Data driven

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describe('unit of work', () => {
    [
        { abName: true, expected: true },
        { abName: false, expected: false }
    ].forEach(({ abName, expected }) =>
        it(`should ${expected ? '' : 'not'} use 'child-component-to-maybe-show' if abName feature is ${abName}`, () => {

TBA

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expect(spy).toHaveBeenCalledTimes(1);

toHaveBeenCalledWith

toBeInstanceOf

toBe

toMatchSnapshot

References