Introduction to ts-jest

ts-jest is an extremely useful TypeScript preprocessor with source map support for Jest, allowing seamless testing with TypeScript. It simplifies the process of testing TypeScript applications and provides powerful features to make the development workflow more efficient.

Installation

npm install --save-dev ts-jest @types/jest

Configuration

After installation, configure Jest to use ts-jest. Create or update the jest.config.js file:

 module.exports = {
  preset: 'ts-jest',
  testEnvironment: 'node',
}; 

Basic Usage

Here is an example showing how to write and test a simple TypeScript function:

Function Implementation (math.ts)

 export const sum = (a: number, b: number): number => {
  return a + b;
}; 

Test (math.test.ts)

 import { sum } from './math';
test('adds 1 + 2 to equal 3', () => {
  expect(sum(1, 2)).toBe(3);
}); 

Advanced Features

Mocking Functions

 jest.mock('./moduleToMock', () => ({
  functionToMock: jest.fn().mockReturnValue('mockValue'),
})); 

Testing Asynchronous Code

 test('the data is peanut butter', async () => {
  const data = await fetchData();
  expect(data).toBe('peanut butter');
});
test('the fetch fails with an error', async () => {
  expect.assertions(1);
  try {
    await fetchData();
  } catch (e) {
    expect(e).toMatch('error');
  }
}); 

Snapshot Testing

 test('renders correctly', () => {
  const tree = renderer.create().toJSON();
  expect(tree).toMatchSnapshot();
}); 

Example Application

Let’s create an example application using the covered APIs.

String Util (stringUtil.ts)

 export const capitalize = (str: string): string => {
  if (!str || typeof str !== 'string') return '';
  return str.charAt(0).toUpperCase() + str.slice(1);
};
export const reverse = (str: string): string => {
  if (!str || typeof str !== 'string') return '';
  return str.split('').reverse().join('');
}; 

Test String Util (stringUtil.test.ts)

 import { capitalize, reverse } from './stringUtil';
describe('capitalize', () => {
  it('should capitalize the first letter', () => {
    expect(capitalize('hello')).toBe('Hello');
  });

  it('should return empty string on non-string input', () => {
    expect(capitalize(null)).toBe('');
  });
});
describe('reverse', () => {
  it('should reverse the string', () => {
    expect(reverse('hello')).toBe('olleh');
  });

  it('should return empty string on non-string input', () => {
    expect(reverse(null)).toBe('');
  });
}); 

By using ts-jest, you can easily test TypeScript applications, ensuring better code quality and reducing errors in the production environment.

Hash: 79ee710a6ce528a371aa0f750dbef43f00f322ad5be9a38b32bbd287bfa4566b