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 -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.
• 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"
  ],
  "transform": {
    "^.+\\.tsx?$": "ts-jest"
  },
  "testRegex": "(/__tests__/.*|(\\.|/)(test|spec))\\.tsx?$",
  "moduleFileExtensions": [
    "ts",
    "tsx",
    "js",
    "jsx",
    "json",
    "node"
  ],
}

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 transform config just tells jest to use ts-jest for ts / tsx files.
• The testRegex tells Jest to look for tests in any __tests__ folder AND also any files anywhere that use the (.test|.spec).(ts|tsx) extension e.g. asdf.test.tsx etc.
• The moduleFileExtensions tells jest to recognize our file extensions. This is needed as we add ts/tsx into the defaults (js|jsx|json|node).

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 '../';

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"],
  "setupTestFrameworkScriptFile": "<rootDir>/src/setupEnzyme.ts",
}

3. Create src/setupEnzyme.ts file.

import { configure } from 'enzyme';
import * as 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.