​Pro egghead lesson on Jest / TypeScript​

No testing solution out there is perfect. That said, jest is an excellent unit testing option which provides great TypeScript support.

Note: We assume you start off with a simple node package.json setup. Also all TypeScript files should be in a src folder which is always recommended (even without Jest) for a clean project setup.

Step 1: Install

Install the following using npm:

npm i jest @types/jest ts-jest typescript -D

Explanation:

• Install jest framework (jest)

• Install the types for jest (@types/jest)

• Install the TypeScript preprocessor for jest (ts-jest) which allows jest to transpile TypeScript on the fly and have source-map support built in.

• Install the TypeScript compiler ('typescript') which is prerequisite for 'ts-jest'.

• Save all of these to your dev dependencies (testing is almost always a npm dev-dependency)

Step 2: Configure Jest

Add the following jest.config.js file to the root of your project:

module.exports = {
  "roots": [
    "<rootDir>/src"
  ],
  "testMatch": [
    "**/__tests__/**/*.+(ts|tsx|js)",
    "**/?(*.)+(spec|test).+(ts|tsx|js)"
  ],
  "transform": {
    "^.+\\.(ts|tsx)$": "ts-jest"
  },
}

(If your package.json file contains "type": "module", which causes Node to assume modules are in es6 format, you can convert the above to es6 format by replacing the top line to export default { .)

Explanation:

• We always recommend having all TypeScript files in a src folder in your project. We assume this is true and specify this using the roots option.

• The testMatch config is a glob pattern matcher for discovering .test / .spec files in ts / tsx / js format.

• The transform config just tells jest to use ts-jest for ts / tsx files.

Step 3: Run tests

Run npx jest from your project root and jest will execute any tests you have.

Optional: Add script target for npm scripts

Add package.json:

{
  "test": "jest"
}

• This allows you to run the tests with a simple npm t.

• And even in watch mode with npm t -- --watch.

Optional: Run jest in watch mode

• npx jest --watch

Example

• For a file foo.ts:

    export const sum
      = (...a: number[]) =>
        a.reduce((acc, val) => acc + val, 0);

• A simple foo.test.ts:

    import { sum } from '../foo';
  ​
    test('basic', () => {
      expect(sum()).toBe(0);
    });
  ​
    test('basic again', () => {
      expect(sum(1, 2)).toBe(3);
    });

Notes:

• Jest provides the global test function.

• Jest comes prebuilt with assertions in the form of the global expect.

Example async

Jest has built-in async/await support. e.g.

test('basic',async () => {
  expect(sum()).toBe(0);
});
​
test('basic again', async () => {
  expect(sum(1, 2)).toBe(3);
}, 1000 /* optional timeout */);

Example enzyme

​Pro egghead lesson on Enzyme / Jest / TypeScript​

Enzyme allows you to test react components with dom support. There are three steps to setting up enzyme:

1. Install enzyme, types for enzyme, a better snapshot serializer for enzyme, enzyme-adapter-react for your react version npm i enzyme @types/enzyme enzyme-to-json enzyme-adapter-react-16 -D

2. Add "snapshotSerializers" and "setupTestFrameworkScriptFile" to your jest.config.js:

    module.exports = {
      // OTHER PORTIONS AS MENTIONED BEFORE
   ​
      // Setup Enzyme
      "snapshotSerializers": ["enzyme-to-json/serializer"],
      "setupFilesAfterEnv": ["<rootDir>/src/setupEnzyme.ts"],
    }

3. Create src/setupEnzyme.ts file.

    import { configure } from 'enzyme';
    import EnzymeAdapter from 'enzyme-adapter-react-16';
    configure({ adapter: new EnzymeAdapter() });

Now here is an example react component and test:

• checkboxWithLabel.tsx:

    import * as React from 'react';
  ​
    export class CheckboxWithLabel extends React.Component<{
      labelOn: string,
      labelOff: string
    }, {
        isChecked: boolean
      }> {
      constructor(props) {
        super(props);
        this.state = { isChecked: false };
      }
  ​
      onChange = () => {
        this.setState({ isChecked: !this.state.isChecked });
      }
  ​
      render() {
        return (
          <label>
            <input
              type="checkbox"
              checked={this.state.isChecked}
              onChange={this.onChange}
            />
            {this.state.isChecked ? this.props.labelOn : this.props.labelOff}
          </label>
        );
      }
    }

• checkboxWithLabel.test.tsx:

    import * as React from 'react';
    import { shallow } from 'enzyme';
    import { CheckboxWithLabel } from './checkboxWithLabel';
  ​
    test('CheckboxWithLabel changes the text after click', () => {
      const checkbox = shallow(<CheckboxWithLabel labelOn="On" labelOff="Off" />);
  ​
      // Interaction demo
      expect(checkbox.text()).toEqual('Off');
      checkbox.find('input').simulate('change');
      expect(checkbox.text()).toEqual('On');
  ​
      // Snapshot demo
      expect(checkbox).toMatchSnapshot();
    });

Reasons why we like jest

​For details on these features see jest website​

• Built-in assertion library.

• Great TypeScript support.

• Very reliable test watcher.

• Snapshot testing.

• Built-in coverage reports.

• Built-in async/await support.