lupomontero

const double = require('./double-1');

describe('double', () => {
  it('should create new array without mutating ref declared in outer scope', () => {
    const a = [1, 2, 3];
    const b = double(a)
    expect(a).toEqual([1, 2, 3]);
    expect(b).toEqual([2, 4, 6]);
    expect(a).not.toBe(b);
  });

lupomontero

const double = arr => arr.map(item => item * 2);

module.exports = double;

lupomontero

const double = require('./double-0');

describe('double', () => {
  it('should modify array in place mutating ref declared in outer scope', () => {
    const a = [1, 2, 3];
    const b = double(a);
    expect(a).toEqual([2, 4, 6]);
    expect(b).toEqual([2, 4, 6]);
    expect(a).toBe(b);
  });

lupomontero

const double = (arr) => {
  arr.forEach((item, idx) => {
    arr[idx] = item * 2;
  });
  return arr;
};

module.exports = double;

lupomontero

const reducer = require('./reducer-2');

describe('reducer()', () => {
  it('should not be affected by previous call', () => {
    expect(reducer(undefined, { type: 'INCREMENT' })).toEqual({ count: 1 });
    expect(reducer(undefined, { type: 'INCREMENT' })).toEqual({ count: 1 });
    expect(reducer(undefined, { type: 'DECREMENT' })).toEqual({ count: -1 });
    expect(reducer({ count: 99 }, { type: 'INCREMENT' }))
      .toEqual({ count: 100 });
  });

lupomontero

const reducer = (state = { count: 0 }, action) => {
  switch (action.type) {
    case 'INCREMENT':
      return { count: state.count + 1 };
    case 'DECREMENT':
      return { count: state.count - 1 };
    default:
      return state;
  }
};

lupomontero

const reducer = require('./reducer-1');

describe('reducer()', () => {
  it('should not be affected by previous calls', () => {
    expect(reducer(undefined, { type: 'INCREMENT' })).toEqual({ count: 1 });
    expect(reducer(undefined, { type: 'INCREMENT' })).toEqual({ count: 1 });
    expect(reducer(undefined, { type: 'DECREMENT' })).toEqual({ count: -1 });
  });

  it('should handle increment action', () => {

lupomontero

const reducer = (state = { count: 0 }, action) => {
  switch (action.type) {
    case 'INCREMENT':
      state.count++;
      break;
    case 'DECREMENT':
      state.count--;
      break;
  }
  return state;

lupomontero

const reducer = require('./reducer-0');

describe('reducer()', () => {
  it('should handle increment and decrement actions', () => {
    expect(reducer({ type: 'INCREMENT' })).toEqual({ count: 1 });
    expect(reducer({ type: 'INCREMENT' })).toEqual({ count: 2 });
    expect(reducer({ type: 'DECREMENT' })).toEqual({ count: 1 });
  });

  it('should depend on prevous invocations (history) - unpredictable!', () => {

lupomontero

const state = { count: 0 };

const reducer = (action) => {
  switch (action.type) {
    case 'INCREMENT':
      state.count++;
      break;
    case 'DECREMENT':
      state.count--;
      break;