Javascript - Jest Test Runner


Jest is a test runner from Facebook for client-side JavaScript applications and React applications specifically.

Jest ships with jsdom for DOM manipulation testing.


Jest and Typescript installation / configuration


See IDEA - Javascript Test with Jest



let oneObj = {name: 'nico', id: 2};
expect(oneObj).toEqual({name: 'nico'}) // false, should be exactly equal all Obj keys and values  
expect(oneObj).toMatchObject({name: 'nico'}) // true


Jest - Mock


The assertion library is jasmine


test('Crawler Base Url Test', async () => {
        expect(() => {myFunction()}).toThrow(Error);

File Structure

Jest will look for test files with any of the following popular naming conventions:

  • Files with .js suffix in __tests__ folders.
  • Files with .test.js suffix.
  • Files with .spec.js suffix.

The .test.js / .spec.js files (or the __tests__ folders) can be located at any depth under the src top level folder.

It's recommended to collocate the test file with the source file (ie put the test files (or __tests__ folders) next to the code they are testing ) because of:

  • shorter relative imports (ie Source)
  • searching the test is easier



test is a global variable injected by Jest. You need to run your tests through jest

  • Test
test('3 equal 3', () => {
  • Run
jest my-test --notify --config=config.json




<script src="[email protected]/dist/core.js"></script>
expect = jestLite.core.expect;

try {
} catch (e) {
   // If the test is not good an exception is fired.

Documentation / Reference

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