書籍「テスト駆動開発」の内容でTypeScriptとJestを使ってTDD（テスト駆動開発）をしてみる

$ mkdir tdd-with-typescript-and-jest
$ cd tdd-with-typescript-and-jest
$ yarn init

$ yarn add -D typescript @types/node

$ tsc --init

message TS6071: Successfully created a tsconfig.json file.

$ tsc

$ tsc -w

$ yarn add -D jest ts-jest @types/jest

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

$ yarn test

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

import { sum } from '../src/sum'

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

$ yarn test

yarn run v1.22.0
warning ../../package.json: No license field
$ jest
 FAIL  test/sum.test.ts
  ● Test suite failed to run

    Jest encountered an unexpected token

    This usually means that you are trying to import a file which Jest cannot parse, e.g. it's not plain JavaScript.

    By default, if Jest sees a Babel config, it will use that to transform your files, ignoring "node_modules".

    Here's what you can do:
     • If you are trying to use ECMAScript Modules, see https://jestjs.io/docs/en/ecmascript-modules for how to enable it.
     • To have some of your "node_modules" files transformed, you can specify a custom "transformIgnorePatterns" in your config.
     • If you need a custom transformation specify a "transform" option in your config.
     • If you simply want to mock your non-JS modules (e.g. binary assets) you can stub them out with the "moduleNameMapper" config option.

    You'll find more details and examples of these config options in the docs:
    https://jestjs.io/docs/en/configuration.html

    Details:

    /Users/ryosuke/Project/tdd-with-typescript-and-jest/test/sum.test.ts:1
    ({"Object.<anonymous>":function(module,exports,require,__dirname,__filename,global,jest){import { sum } from '../src/sum';
                                                                                             ^^^^^^

    SyntaxError: Cannot use import statement outside a module

      at Runtime.createScriptFromCode (node_modules/jest-runtime/build/index.js:1350:14)

Test Suites: 1 failed, 1 total
Tests:       0 total
Snapshots:   0 total
Time:        1.876 s, estimated 3 s
Ran all test suites.
error Command failed with exit code 1.
info Visit https://yarnpkg.com/en/docs/cli/run for documentation about this command.

{
・・・
  "license": "MIT",
  "jest": {
    "preset": "ts-jest",
    "testEnvironment": "node"
  },
・・・
}

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

$ yarn test

$ jest
 PASS  test/sum.test.ts
  ✓ adds 1 + 2 to equal 3 (4 ms)

Test Suites: 1 passed, 1 total
Tests:       1 passed, 1 total
Snapshots:   0 total
Time:        1.138 s
Ran all test suites.
✨  Done in 2.74s.

{
  ・・・
  "[typescript]": {
    "editor.formatOnSave": true
  },
  ・・・
}

class Money {
  protected amount: number
  protected cur: string

  constructor(amount: number, currency: string) {
    this.amount = amount
    this.cur = currency
  }

  times(multiplier: number): Expression {
    return new Money(this.amount * multiplier, this.cur)
  }

  static dollar(amount: number): Money {
    return new Money(amount, 'USD')
  }
}

times(multiplier: number): Expression {
  return new Money(this.amount * multiplier, this.cur)
}

import { Money } from '../src/money'

describe('Money module', () => {
  test('times()', () => {
    const five = Money.dollar(5)
    expect(five.times(2)).toEqual(Money.dollar(10))
  })
})