Jest
Painless JavaScript Unit Testing
Familiar Approach
Built on top of the Jasmine test framework, using familiar expect(value).toBe(other) assertions
Mock by Default
Automatically mocks CommonJS modules returned by require(), making most existing code testable
Short Feedback Loop
DOM APIs are mocked and tests run in parallel via a small node.js command line utility
Why not just use vanilla Jasmine?
Jest provides you with multiple layers on top of Jasmine:
- Automatically finds tests to execute in your repo
- Automatically mocks dependencies for you when running your tests
- Allows you to test asynchronous code synchronously
- Runs your tests with a fake DOM implementation (via jsdom) so that your tests can run on the command line
- Runs tests in parallel processes so that they finish sooner
Getting Started
Consider a scenario where you want to test the following sum.js file:
// sum.js
function sum(value1, value2) {
return value1 + value2;
}
module.exports = sum;
We can get up and running with the following 4 steps:
Create a directory
__tests__/with a filesum-test.js// __tests__/sum-test.js jest.dontMock('../sum'); describe('sum', function() { it('adds 1 + 2 to equal 3', function() { var sum = require('../sum'); expect(sum(1, 2)).toBe(3); }); });Run
npm install jest-cli --save-devAdd the following to your
package.json{ ... "scripts": { "test": "jest" } ... }Run
npm test[PASS] __tests__/sum-test.js (0.015s)