Unit Test with jest

npm install --save-dev jest

//create file sum.js
function sum(a, b) {
  return a + b;
}
module.exports = sum;

//create sum.test.js
const sum = require('./sum');

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

//in the package.json
{
  "scripts": {
    "test": "jest"
  }
}

npm test

PASS  ./sum.test.js
✓ adds 1 + 2 to equal 3 (5ms)


//numbers
test('two plus two', () => {
  const value = 2 + 2;
  expect(value).toBeGreaterThan(3);
  expect(value).toBeGreaterThanOrEqual(3.5);
  expect(value).toBeLessThan(5);
  expect(value).toBeLessThanOrEqual(4.5);

  // toBe and toEqual are equivalent for numbers
  expect(value).toBe(4);
  expect(value).toEqual(4);
});

//string
test('there is no I in team', () => {
  expect('team').not.toMatch(/I/);
});

test('but there is a "stop" in Christoph', () => {
  expect('Christoph').toMatch(/stop/);
});

//array
const shoppingList = [
  'diapers',
  'kleenex',
  'trash bags',
  'paper towels',
  'milk',
];

test('the shopping list has milk on it', () => {
  expect(shoppingList).toContain('milk');
  expect(new Set(shoppingList)).toContain('milk');
});

//exceptions
function compileAndroidCode() {
  throw new Error('you are using the wrong JDK!');
}

test('compiling android goes as expected', () => {
  expect(() => compileAndroidCode()).toThrow();
  expect(() => compileAndroidCode()).toThrow(Error);

  // You can also use a string that must be contained in the error message or a regexp
  expect(() => compileAndroidCode()).toThrow('you are using the wrong JDK');
  expect(() => compileAndroidCode()).toThrow(/JDK/);

  // Or you can match an exact error message using a regexp like below
  expect(() => compileAndroidCode()).toThrow(/^you are using the wrong JDK$/); // Test fails
  expect(() => compileAndroidCode()).toThrow(/^you are using the wrong JDK!$/); // Test pass
});

//DOM MANIPULATION
npm install --save-dev jest-environment-jsdom

//Emule Localstorage
npm i --save-dev jest-localstorage-mock

/* script in the package.json
"jest": {
    "resetMocks": false,
    "setupFiles": ["jest-localstorage-mock"]
  }
  */