jest.md

Using Jest with TypeScript

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.