05-jest.md

Jest

Jest seems to be the most popular test framework in JavaScript.

There's a few ways you can run jest with TypeScript. We will use ts-jest.

yarn add --dev jest
yarn add --dev ts-jest @types/jest
yarn ts-jest config:init

Configure the coverage settings in jest.config.js

{
  // ---
  "jest": {
    "coverageThreshold": {
      "global": {
        "branches": 90,
        "functions": 90,
        "lines": 90,
        "statements": 90
      }
    }
  }
}

See more here

Create some tests:

mkdir -p src/__tests__
touch src/__tests__/index.test.ts

Add some scripts to package.json

{
  // ---
  "scripts": {
    // ---
    "test": "jest --coverage",
    "test:watch": "jest --watch",
    // ---
  },
  // ---
}

Optimise the build

yarn build

You will notice that __tests__ are also getting transpiled. We dont need these.

We can try to add exclude key into tsconfig.json. However, you will soon notice this will break eslint. We are using type-aware rules, tsconfig.json needs to include the test files.

Instead, we can split tsconfig into a few files.

• tsconfig.base.json

// this tsconfig is used the language server and for testing
{
  "extends": "@tsconfig/node14/tsconfig.json",
  "compilerOptions": {
    "noUncheckedIndexedAccess": true,
    "noImplicitOverride": true,
  }
}

• tsconfig.json

// this tsconfig is used the language server and for testing
{
  "extends": "tsconfig.base.json",
  "compilerOptions": {
    "noEmit": true // no output is required
  },
  "include": ["./src/**/*.ts"]
}

• tsconfig.build.json

// this tsconfig is used for transpile the code when building
{
  "extends": "tsconfig.base.json",
  "compilerOptions": {
    "outDir": "dist",
    "noEmitOnError": true // don't transpile the code if there's an error
  },
  "include": ["./src/**/*.ts"],
  "exclude": ["./src/**/*.test.ts"]
}

Update package.json.

{
  // ---
  "scripts": {
    //"build": "rimraf dist && tsc --project tsconfig.json",
    "build": "rimraf dist && tsc --project tsconfig.build.json",
  }
  // ---
}