Setting up Jest for Typescript

It takes a few steps to run typescript jest tests.

npm init # to create package.json. also create an empty yarn.lock file

yarn add -D jest ts-jest

jest --init # to initialise jest

yarn add -D babel-jest @babel/core @babel/preset-env
yarn add -D @babel/preset-typescript
yarn add -D ts-node
yarn add -D @types/jest

The jest.config.ts would be like this

export default {
  clearMocks: true,
  collectCoverage: true,
  coverageDirectory: "coverage",
  coveragePathIgnorePatterns: [
    "/node_modules/"
  ],
  coverageProvider: "v8",
	preset: 'ts-jest',
  transform: {
    '^.+\\.(ts|tsx)?$': 'ts-jest',
    "^.+\\.(js|jsx)$": "babel-jest",
  }
};

babel needs config file too

module.exports = {
  presets: [
    ['@babel/preset-env', {targets: {node: 'current'}}],
    '@babel/preset-typescript',
  ],

};

Finally “test” to npm script

"scripts": {
  "test": "jest"
},

A sample test file, /src/reduce.test.ts

it('should calculate average', () => {
  const results = [
    { id: 1, result: 64 },
    { id: 2, result: 87 },
    { id: 3, result: 89 },
  ]

  const average = results.map(x => x.result).reduce((prev, curr) => prev + curr, 0) / results.length
  expect(average).toBe(80)
})

Often you have an error, Cannot use import statement outside a module

Make sure you have babel.config.ts and jest.config.ts

https://stackoverflow.com/questions/58613492/how-to-resolve-cannot-use-import-statement-outside-a-module-in-jest