DevNexus 2017: Functional Programming Basics in ES6

Transcript

1. FUNCTIONAL
   PROGRAMMING
   Jeremy Fairbank
   blog.jeremyfairbank.com
   @elpapapollo / jfairbank
   BASICS IN ES6
   

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2. sigient.com
   

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4. ¯\_(ϑ)_/¯
   WHAT IS FUNCTIONAL
   PROGRAMMING?
   

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5. ¯\_(ϑ)_/¯
   WHY FUNCTIONAL
   PROGRAMMING?
   

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7. Domain to Range
   

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10. Domain Range
    

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11. CALCULUS
    

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12. PRINCIPLES
    

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13. PURE & DECLARATIVE
    PREDICTABLE
    

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14. IMMUTABLE STATE
    SAFE
    

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15. FIRST CLASS STATE
    TRANSPARENT
    

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16. COMPOSABLE FIRST
    CLASS CLOSURES
    MODULAR
    

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17. ES2015
    (ES6)
    

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18. let age = 10;
    age = 11;
    const name = 'Tucker';
    name = 'Sally';
    All good
    Syntax error
    

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19. const add = (x, y) => {
    return x + y;
    };
    const identity = x => x;
    

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20. const add = (x, y) => {
    return x + y;
    };
    const identity = x => x;
    

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21. const add = (x, y) => {
    return x + y;
    };
    const identity = x => x;
    

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22. const add = (x, y) => {
    return x + y;
    };
    const identity = x => x;
    

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23. const add = (x, y) => {
    return x + y;
    };
    const identity = x => x;
    

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24. const add = (x, y) => {
    return x + y;
    };
    const identity = x => x;
    

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25. const array = (...elements) => {
    return elements;
    };
    array(1, 2, 3); // [1, 2, 3]
    

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26. const array = (...elements) => {
    return elements;
    };
    array(1, 2, 3); // [1, 2, 3]
    

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27. const array = (...elements) => {
    return elements;
    };
    array(1, 2, 3); // [1, 2, 3]
    

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28. const array = (...elements) => {
    return elements;
    };
    array(1, 2, 3); // [1, 2, 3]
    

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29. const log = (...args) => {
    console.log(...args);
    };
    log('Hello', 'DevNexus');
    // Hello DevNexus
    

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30. const log = (...args) => {
    console.log(...args);
    };
    log('Hello', 'DevNexus');
    // Hello DevNexus
    

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31. const langs = [
    'JavaScript',
    'Elm',
    'Haskell',
    ];
    const [js, ...rest] = langs;
    js === 'JavaScript';
    rest[0] === 'Elm';
    rest[1] === 'Haskell';
    

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32. const langs = [
    'JavaScript',
    'Elm',
    'Haskell',
    ];
    const [js, ...rest] = langs;
    js === 'JavaScript';
    rest[0] === 'Elm';
    rest[1] === 'Haskell';
    

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33. const langs = [
    'JavaScript',
    'Elm',
    'Haskell',
    ];
    const [js, ...rest] = langs;
    js === 'JavaScript';
    rest[0] === 'Elm';
    rest[1] === 'Haskell';
    

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34. const langs = [
    'JavaScript',
    'Elm',
    'Haskell',
    ];
    const [js, ...rest] = langs;
    js === 'JavaScript';
    rest[0] === 'Elm';
    rest[1] === 'Haskell';
    

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35. const head = ([x]) => x;
    head([1, 2, 3]) === 1;
    

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36. const head = ([x]) => x;
    head([1, 2, 3]) === 1;
    

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37. const head = ([x]) => x;
    head([1, 2, 3]) === 1;
    

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38. const greet = (name, greeting = 'Hi') => {
    console.log(greeting, name);
    };
    greet('DevNexus', 'Hello');
    // Hello DevNexus
    greet('Atlanta');
    // Hi Atlanta
    

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39. const greet = (name, greeting = 'Hi') => {
    console.log(greeting, name);
    };
    greet('DevNexus', 'Hello');
    // Hello DevNexus
    greet('Atlanta');
    // Hi Atlanta
    

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40. const greet = (name, greeting = 'Hi') => {
    console.log(greeting, name);
    };
    greet('DevNexus', 'Hello');
    // Hello DevNexus
    greet('Atlanta');
    // Hi Atlanta
    

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41. const greet = (name, greeting = 'Hi') => {
    console.log(greeting, name);
    };
    greet('DevNexus', 'Hello');
    // Hello DevNexus
    greet('Atlanta');
    // Hi Atlanta
    

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42. Object.assign(
    {},
    { hello: 'Atlanta' },
    { hi: 'DevNexus' }
    );
    // {
    // hello: 'Atlanta',
    // hi: 'DevNexus'
    // }
    

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43. Object.assign(
    {},
    { hello: 'Atlanta' },
    { hi: 'DevNexus' }
    );
    // {
    // hello: 'Atlanta',
    // hi: 'DevNexus'
    // }
    

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44. Object.assign(
    {},
    { hello: 'Atlanta' },
    { hi: 'DevNexus' }
    );
    // {
    // hello: 'Atlanta',
    // hi: 'DevNexus'
    // }
    

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45. class Point {
    constructor(x, y) {
    this.x = x;
    this.y = y;
    }
    moveBy(dx, dy) {
    this.x += dx;
    this.y += dy;
    }
    }
    function Point(x, y) {
    this.x = x;
    this.y = y;
    }
    Point.prototype.moveBy =
    function(dx, dy) {
    this.x += dx;
    this.y += dy;
    };
    

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46. PURE
    

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47. const add = (x, y) => x + y;
    add(2, 3) === 5;
    add(2, 3) === 5;
    add(2, 3) === 5;
    

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48. const add = (x, y) => x + y;
    add(2, 3) === 5;
    add(2, 3) === 5;
    add(2, 3) === 5;
    Referentially
    transparent
    

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49. × let name = 'Jeremy';
    const getName = () => name;
    const setName = (newName) => {
    name = newName;
    };
    const printUpperName = () => {
    console.log(name.toUpperCase());
    };
    

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50. × let name = 'Jeremy';
    const getName = () => name;
    const setName = (newName) => {
    name = newName;
    };
    const printUpperName = () => {
    console.log(name.toUpperCase());
    };
    

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51. × let name = 'Jeremy';
    const getName = () => name;
    const setName = (newName) => {
    name = newName;
    };
    const printUpperName = () => {
    console.log(name.toUpperCase());
    };
    

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52. × let name = 'Jeremy';
    const getName = () => name;
    const setName = (newName) => {
    name = newName;
    };
    const printUpperName = () => {
    console.log(name.toUpperCase());
    };
    

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53. describe('api', () => {
    beforeEach(() => mockConsoleLog());
    afterEach(() => restoreConsoleLog());
    it('sets and prints the name', () => {
    printUpperName();
    expect(console.log).calledWith('JEREMY');
    setName('Jet');
    printUpperName();
    expect(console.log).calledWith('JET');
    });
    });
    ×
    

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54. HIDDEN STATE IS
    UNCERTAIN STATE
    

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55. const upperName = (name) => name.toUpperCase();
    describe('api', () => {
    it('returns an uppercase name', () => {
    expect(upperName('Jeremy')).to.equal('JEREMY');
    expect(upperName('Jet')).to.equal('JET');
    });
    });
    ✓
    

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56. HOW TO ACHIEVE THE
    RESULT
    IMPERATIVE
    

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57. function doubleNumbers(numbers) {
    const doubled = [];
    const l = numbers.length;
    for (let i = 0; i < l; i++) {
    doubled.push(numbers[i] * 2);
    }
    return doubled;
    }
    doubleNumbers([1, 2, 3]);
    // [2, 4, 6]
    

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58. DECLARE WHAT THE
    DESIRED RESULT IS
    DECLARATIVE
    

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59. function doubleNumbers(numbers) {
    return numbers.map(n => n * 2);
    }
    doubleNumbers([1, 2, 3]);
    // [2, 4, 6]
    

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60. function doubleNumbers(numbers) {
    return numbers.map(n => n * 2);
    }
    doubleNumbers([1, 2, 3]);
    // [2, 4, 6]
    

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61. 1
    2
    3
    numbers.map(n => n * 2)
    2
    4
    6
    Domain Range
    

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62. CREATE STATE, DON’T
    MUTATE IT
    IMMUTABLE
    

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63. const hobbies = [
    'programming',
    'reading',
    'music',
    ];
    const firstTwo = hobbies.splice(0, 2);
    console.log(firstTwo);
    // ['programming', 'reading']
    console.log(hobbies);
    // ['music']
    

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64. const hobbies = [
    'programming',
    'reading',
    'music',
    ];
    const firstTwo = hobbies.splice(0, 2);
    console.log(firstTwo);
    // ['programming', 'reading']
    console.log(hobbies);
    // ['music']
    Slight typo + mutability
    

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65. Object.freeze
    Immutable.js
    

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66. const hobbies = Object.freeze([
    'programming',
    'reading',
    'music',
    ]);
    const firstTwo = hobbies.splice(0, 2);
    // TypeError
    

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67. const hobbies = Object.freeze([
    'programming',
    'reading',
    'music',
    ]);
    const firstTwo = hobbies.splice(0, 2);
    // TypeError
    

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68. FREE YOUR
    STATE
    

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69. class Point {
    constructor(x, y) {
    this.x = x;
    this.y = y;
    }
    moveBy(dx, dy) {
    this.x += dx;
    this.y += dy;
    }
    }
    const point = new Point(0, 0);
    point.moveBy(5, 5);
    point.moveBy(-2, 2);
    console.log([point.x, point.y]);
    // [3, 7]
    ×
    

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70. const createPoint = (x, y) => Object.freeze([x, y]);
    const movePointBy = ([x, y], dx, dy) => {
    return Object.freeze([x + dx, y + dy]);
    };
    let point = createPoint(0, 0);
    point = movePointBy(point, 5, 5);
    point = movePointBy(point, -2, 2);
    console.log(point);
    // [3, 7]
    ✓
    (*or object-oriented
    without mutation)
    

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71. const createPoint = (x, y) => Object.freeze([x, y]);
    const movePointBy = ([x, y], dx, dy) => {
    return Object.freeze([x + dx, y + dy]);
    };
    let point = createPoint(0, 0);
    point = movePointBy(point, 5, 5);
    point = movePointBy(point, -2, 2);
    console.log(point);
    // [3, 7]
    ✓
    (*or object-oriented
    without mutation)
    

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72. const createPoint = (x, y) => Object.freeze([x, y]);
    const movePointBy = ([x, y], dx, dy) => {
    return Object.freeze([x + dx, y + dy]);
    };
    let point = createPoint(0, 0);
    point = movePointBy(point, 5, 5);
    point = movePointBy(point, -2, 2);
    console.log(point);
    // [3, 7]
    ✓
    (*or object-oriented
    without mutation)
    

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73. const createPoint = (x, y) => Object.freeze([x, y]);
    const movePointBy = ([x, y], dx, dy) => {
    return Object.freeze([x + dx, y + dy]);
    };
    let point = createPoint(0, 0);
    point = movePointBy(point, 5, 5);
    point = movePointBy(point, -2, 2);
    console.log(point);
    // [3, 7]
    ✓
    (*or object-oriented
    without mutation)
    

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74. PROS
    SAFETY FROM ACCIDENTAL MUTATION
    FREE UNDO/REDO LOGS — REDUX
    EXPLICIT FLOW OF DATA
    CONCURRENCY SAFETY
    

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75. CONS
    VERBOSE
    MORE OBJECT CREATION*
    MORE GARBAGE COLLECTION*
    MORE MEMORY USAGE*
    *Alleviated with libs like Immutable.js
    

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76. FIRST CLASS
    FUNCTIONS
    

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77. const multiply = (x, y) => x * y;
    function add(x, y) {
    return x + y;
    }
    const addAlias = add;
    const evens = [1, 2, 3].map(n => n * 2);
    

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78. const multiply = (x, y) => x * y;
    function add(x, y) {
    return x + y;
    }
    const addAlias = add;
    const evens = [1, 2, 3].map(n => n * 2);
    

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79. const multiply = (x, y) => x * y;
    function add(x, y) {
    return x + y;
    }
    const addAlias = add;
    const evens = [1, 2, 3].map(n => n * 2);
    

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80. const multiply = (x, y) => x * y;
    function add(x, y) {
    return x + y;
    }
    const addAlias = add;
    const evens = [1, 2, 3].map(n => n * 2);
    

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81. ENCAPSULATION
    CLOSURES
    

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82. const createAdder = (x) => {
    return (y) => x + y;
    };
    const add3 = createAdder(3);
    add3(2) === 5;
    add3(3) === 6;
    

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83. const createAdder = (x) => {
    return (y) => x + y;
    };
    const add3 = createAdder(3);
    add3(2) === 5;
    add3(3) === 6;
    

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84. const request = (options) => {
    return fetch(options.url, options)
    .then(resp => resp.json());
    };
    const usersPromise = request({
    url: '/users',
    headers: { 'X-Custom': 'mykey' }
    });
    const tasksPromise = request({
    url: '/tasks',
    headers: { 'X-Custom': 'mykey' }
    });
    

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85. const request = (options) => {
    return fetch(options.url, options)
    .then(resp => resp.json());
    };
    const usersPromise = request({
    url: '/users',
    headers: { 'X-Custom': 'mykey' }
    });
    const tasksPromise = request({
    url: '/tasks',
    headers: { 'X-Custom': 'mykey' }
    });
    Repetitive
    

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86. const createRequester = (options) => {
    return (otherOptions) => {
    return request(Object.assign(
    {}, options, otherOptions
    ));
    };
    };
    const customRequest = createRequester({
    headers: { 'X-Custom': 'mykey' }
    });
    const usersPromise = customRequest({ url: '/users' });
    const tasksPromise = customRequest({ url: '/tasks' });
    

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87. const createRequester = (options) => {
    return (otherOptions) => {
    return request(Object.assign(
    {}, options, otherOptions
    ));
    };
    };
    const customRequest = createRequester({
    headers: { 'X-Custom': 'mykey' }
    });
    const usersPromise = customRequest({ url: '/users' });
    const tasksPromise = customRequest({ url: '/tasks' });
    

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88. FOUNDATION FOR
    HIGHER ORDER
    PATTERNS
    FIRST CLASS
    CLOSURES
    

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89. PARTIAL
    APPLICATION
    

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90. const createAdder = (x) => {
    return (y) => x + y;
    };
    const createRequester = (options) => {
    return (otherOptions) => {
    return request(Object.assign(
    {}, options, otherOptions
    ));
    };
    };
    RECALL
    

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91. const add = (x, y) => x + y;
    const add3 = partial(add, 3);
    add3(2) === 5;
    

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92. const add = (x, y) => x + y;
    const add3 = partial(add, 3);
    add3(2) === 5;
    

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93. const request = (defaults, options) => {
    options = Object.assign({}, defaults, options);
    return fetch(options.url, options)
    .then(resp => resp.json());
    };
    const customRequest = partial(request, {
    headers: { 'X-Custom': 'mykey' }
    });
    const usersPromise = customRequest({ url: '/users' });
    const tasksPromise = customRequest({ url: '/tasks' });
    

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94. const request = (defaults, options) => {
    options = Object.assign({}, defaults, options);
    return fetch(options.url, options)
    .then(resp => resp.json());
    };
    const customRequest = partial(request, {
    headers: { 'X-Custom': 'mykey' }
    });
    const usersPromise = customRequest({ url: '/users' });
    const tasksPromise = customRequest({ url: '/tasks' });
    

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95. const partialFromBind = (fn, ...args) => {
    return fn.bind(null, ...args);
    };
    const partial = (fn, ...args) => {
    return (...otherArgs) => {
    return fn(...args, ...otherArgs)
    };
    };
    

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96. const partialFromBind = (fn, ...args) => {
    return fn.bind(null, ...args);
    };
    const partial = (fn, ...args) => {
    return (...otherArgs) => {
    return fn(...args, ...otherArgs)
    };
    };
    

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97. const partialFromBind = (fn, ...args) => {
    return fn.bind(null, ...args);
    };
    const partial = (fn, ...args) => {
    return (...otherArgs) => {
    return fn(...args, ...otherArgs)
    };
    };
    

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98. const partialFromBind = (fn, ...args) => {
    return fn.bind(null, ...args);
    };
    const partial = (fn, ...args) => {
    return (...otherArgs) => {
    return fn(...args, ...otherArgs)
    };
    };
    

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99. const partialFromBind = (fn, ...args) => {
    return fn.bind(null, ...args);
    };
    const partial = (fn, ...args) => {
    return (...otherArgs) => {
    return fn(...args, ...otherArgs)
    };
    };
    

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100. const partialFromBind = (fn, ...args) => {
     return fn.bind(null, ...args);
     };
     const partial = (fn, ...args) => {
     return (...otherArgs) => {
     return fn(...args, ...otherArgs)
     };
     };
     

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101. CURRYING
     

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102. const add3 = add(3);
     add3(2) === 5;
     const customRequest = request({
     headers: { 'X-Custom': 'mykey' }
     });
     const usersPromise = customRequest({ url: '/users' });
     const tasksPromise = customRequest({ url: '/tasks' });
     

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103. const add3 = add(3);
     add3(2) === 5;
     const customRequest = request({
     headers: { 'X-Custom': 'mykey' }
     });
     const usersPromise = customRequest({ url: '/users' });
     const tasksPromise = customRequest({ url: '/tasks' });
     

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104. const add = x => y => x + y;
     function add(x) {
     return function(y) {
     return x + y;
     };
     }
     

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105. const add = x => y => x + y;
     function add(x) {
     return function(y) {
     return x + y;
     };
     }
     

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106. const add = x => y => x + y;
     function add(x) {
     return function(y) {
     return x + y;
     };
     }
     

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107. const request = defaults => options => {
     options = Object.assign(
     {}, defaults, options
     );
     return fetch(options.url, options)
     .then(resp => resp.json());
     };
     

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108. const request = defaults => options => {
     options = Object.assign(
     {}, defaults, options
     );
     return fetch(options.url, options)
     .then(resp => resp.json());
     };
     

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109. PIECING IT
     TOGETHER
     

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110. const map = fn => array => array.map(fn);
     const multiply = x => y => x * y;
     const pluck = key => object => object[key];
     const discount = multiply(0.98);
     const tax = multiply(1.0925);
     const customRequest = request({
     headers: { 'X-Custom': 'mykey' }
     });
     customRequest({ url: '/cart/items' })
     .then(map(pluck('price')))
     .then(map(discount))
     .then(map(tax));
     

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111. const map = fn => array => array.map(fn);
     const multiply = x => y => x * y;
     const pluck = key => object => object[key];
     const discount = multiply(0.98);
     const tax = multiply(1.0925);
     const customRequest = request({
     headers: { 'X-Custom': 'mykey' }
     });
     customRequest({ url: '/cart/items' })
     .then(map(pluck('price')))
     .then(map(discount))
     .then(map(tax));
     

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112. const map = fn => array => array.map(fn);
     const multiply = x => y => x * y;
     const pluck = key => object => object[key];
     const discount = multiply(0.98);
     const tax = multiply(1.0925);
     const customRequest = request({
     headers: { 'X-Custom': 'mykey' }
     });
     customRequest({ url: '/cart/items' })
     .then(map(pluck('price')))
     .then(map(discount))
     .then(map(tax));
     

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113. const map = fn => array => array.map(fn);
     const multiply = x => y => x * y;
     const pluck = key => object => object[key];
     const discount = multiply(0.98);
     const tax = multiply(1.0925);
     const customRequest = request({
     headers: { 'X-Custom': 'mykey' }
     });
     customRequest({ url: '/cart/items' })
     .then(map(pluck('price')))
     .then(map(discount))
     .then(map(tax));
     

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114. const map = fn => array => array.map(fn);
     const multiply = x => y => x * y;
     const pluck = key => object => object[key];
     const discount = multiply(0.98);
     const tax = multiply(1.0925);
     const customRequest = request({
     headers: { 'X-Custom': 'mykey' }
     });
     customRequest({ url: '/cart/items' })
     .then(map(pluck('price')))
     .then(map(discount))
     .then(map(tax));
     

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115. const map = fn => array => array.map(fn);
     const multiply = x => y => x * y;
     const pluck = key => object => object[key];
     const discount = multiply(0.98);
     const tax = multiply(1.0925);
     const customRequest = request({
     headers: { 'X-Custom': 'mykey' }
     });
     customRequest({ url: '/cart/items' })
     .then(map(pluck('price')))
     .then(map(discount))
     .then(map(tax));
     

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116. const map = fn => array => array.map(fn);
     const multiply = x => y => x * y;
     const pluck = key => object => object[key];
     const discount = multiply(0.98);
     const tax = multiply(1.0925);
     const customRequest = request({
     headers: { 'X-Custom': 'mykey' }
     });
     customRequest({ url: '/cart/items' })
     .then(map(pluck('price')))
     .then(map(discount))
     .then(map(tax));
     

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117. customRequest({ url: '/cart/items' })
     .then(map(pluck('price')))
     .then(map(discount))
     .then(map(tax));
     [
     { price: 5 },
     { price: 10 },
     { price: 3 },
     ]
     

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118. [
     { price: 5 },
     { price: 10 },
     { price: 3 },
     ]
     map(pluck('price'))
     [
     5,
     10,
     3,
     ]
     item.price
     

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119. [
     5,
     10,
     3,
     ]
     map(discount)
     [
     4.9,
     9.8,
     2.94,
     ]
     price * 0.98
     

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120. [
     5.35,
     10.71,
     3.21,
     ]
     map(tax)
     [
     4.9,
     9.8,
     2.94,
     ]
     price * 1.0925
     

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121. COMPOSING
     CLOSURES
     

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122. View Slide

123. const processWord =
     compose(hyphenate, reverse, toUpperCase);
     const words = [
     'hello', 'functional', 'programming'
     ];
     const newWords = words.map(processWord);
     console.log(newWords);
     // ['OL-LEH, 'LANOI-TCNUF', 'GNIMM-ARGORP']
     

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124. const processWord =
     compose(hyphenate, reverse, toUpperCase);
     const words = [
     'hello', 'functional', 'programming'
     ];
     const newWords = words.map(processWord);
     console.log(newWords);
     // ['OL-LEH, 'LANOI-TCNUF', 'GNIMM-ARGORP']
     

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125. const processWord =
     compose(hyphenate, reverse, toUpperCase);
     const processWordExplicit = (word) => {
     return hyphenate(reverse(toUpperCase(word)));
     };
     

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126. const compose = (...fns) => arg => (
     fns.reduceRight(
     (result, fn) => fn(result),
     arg
     )
     );
     

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127. const compose = (...fns) => arg => (
     fns.reduceRight(
     (result, fn) => fn(result),
     arg
     )
     );
     

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128. const compose = (...fns) => arg => (
     fns.reduceRight(
     (result, fn) => fn(result),
     arg
     )
     );
     

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129. const compose = (...fns) => arg => (
     fns.reduceRight(
     (result, fn) => fn(result),
     arg
     )
     );
     

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130. const compose = (...fns) => arg => (
     fns.reduceRight(
     (result, fn) => fn(result),
     arg
     )
     );
     

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131. const compose = (...fns) => arg => (
     fns.reduceRight(
     (result, fn) => fn(result),
     arg
     )
     );
     

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132. const compose = (...fns) => arg => (
     fns.reduceRight(
     (result, fn) => fn(result),
     arg
     )
     );
     fns = [hyphenate, reverse, toUpperCase]
     arg = result = 'hello'
     

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133. const compose = (...fns) => arg => (
     fns.reduceRight(
     (result, fn) => fn(result),
     arg
     )
     );
     fns = [hyphenate, reverse]
     result = 'HELLO'
     

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134. const compose = (...fns) => arg => (
     fns.reduceRight(
     (result, fn) => fn(result),
     arg
     )
     );
     fns = [hyphenate]
     result = 'OLLEH'
     

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135. const compose = (...fns) => arg => (
     fns.reduceRight(
     (result, fn) => fn(result),
     arg
     )
     );
     fns = []
     result = 'OL-LEH'
     

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136. customRequest({ url: '/cart/items' })
     .then(map(pluck('price')))
     .then(map(discount))
     .then(map(tax));
     RETURNING TO PRICES EXAMPLE
     

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137. customRequest({ url: '/cart/items' })
     .then(map(pluck('price')))
     .then(map(discount))
     .then(map(tax)); Triple
     iteration
     RETURNING TO PRICES EXAMPLE
     

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138. customRequest({ url: '/cart/items' })
     .then(map(
     compose(
     tax,
     discount,
     pluck('price')
     )
     ));
     Single
     iteration
     

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139. RECURSION SOLVE A PROBLEM
     IN TERMS OF ITSELF
     

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140. FACTORIAL
     

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141. const factorial = (n) => {
     let result = 1;
     while (n > 1) {
     result *= n;
     n--;
     }
     return result;
     };
     

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142. const factorial = (n) => {
     if (n < 2) {
     return 1;
     }
     return n * factorial(n - 1);
     };
     

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143. const factorial = (n) => {
     if (n < 2) {
     return 1;
     }
     return n * factorial(n - 1);
     };
     Recursive call
     

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144. const factorial = (n) => {
     if (n < 2) {
     return 1;
     }
     return n * factorial(n - 1);
     };
     Base case
     

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145. factorial(4);
     

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146. factorial(4);
     4 * factorial(3);
     

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147. factorial(4);
     4 * factorial(3);
     4 * 3 * factorial(2);
     

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148. factorial(4);
     4 * factorial(3);
     4 * 3 * factorial(2);
     4 * 3 * 2 * factorial(1);
     

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149. factorial(4);
     4 * factorial(3);
     4 * 3 * factorial(2);
     4 * 3 * 2 * factorial(1);
     4 * 3 * 2 * 1;
     

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150. factorial(4);
     4 * factorial(3);
     4 * 3 * factorial(2);
     4 * 3 * 2 * factorial(1);
     4 * 3 * 2 * 1;
     4 * 3 * 2;
     

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151. factorial(4);
     4 * factorial(3);
     4 * 3 * factorial(2);
     4 * 3 * 2 * factorial(1);
     4 * 3 * 2 * 1;
     4 * 3 * 2;
     4 * 6;
     

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152. factorial(4);
     4 * factorial(3);
     4 * 3 * factorial(2);
     4 * 3 * 2 * factorial(1);
     4 * 3 * 2 * 1;
     4 * 3 * 2;
     4 * 6;
     24;
     

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153. STEPS
     Find the recurrence

     (n × n-1 × n-2 × … 1)

     Find the base case

     (n < 2)
     

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154. PERFORMANCE
     RECURSION
     

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155. const value = factorial(100000);
     console.log(value); // ???
     WHAT IS THE RESULT?
     

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156. const value = factorial(100000);
     console.log(value); // ???
     WHAT IS THE RESULT?
     RangeError: Maximum call
     stack size exceeded
     

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157. factorial(4);
     4 * factorial(3);
     4 * 3 * factorial(2);
     4 * 3 * 2 * factorial(1);
     4 * 3 * 2 * 1;
     4 * 3 * 2;
     4 * 6;
     24;
     
     Call Stack
     

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158. factorial(4);
     4 * factorial(3);
     4 * 3 * factorial(2);
     4 * 3 * 2 * factorial(1);
     4 * 3 * 2 * 1;
     4 * 3 * 2;
     4 * 6;
     24;
     
     factorial(4)
     Call Stack
     

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159. factorial(4);
     4 * factorial(3);
     4 * 3 * factorial(2);
     4 * 3 * 2 * factorial(1);
     4 * 3 * 2 * 1;
     4 * 3 * 2;
     4 * 6;
     24;
     
     factorial(4)
     factorial(3)
     Call Stack
     

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160. factorial(4);
     4 * factorial(3);
     4 * 3 * factorial(2);
     4 * 3 * 2 * factorial(1);
     4 * 3 * 2 * 1;
     4 * 3 * 2;
     4 * 6;
     24;
     
     factorial(4)
     factorial(3)
     factorial(2)
     Call Stack
     

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161. factorial(4);
     4 * factorial(3);
     4 * 3 * factorial(2);
     4 * 3 * 2 * factorial(1);
     4 * 3 * 2 * 1;
     4 * 3 * 2;
     4 * 6;
     24;
     
     factorial(4)
     factorial(3)
     factorial(2)
     factorial(1)
     Call Stack
     

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162. factorial(4);
     4 * factorial(3);
     4 * 3 * factorial(2);
     4 * 3 * 2 * factorial(1);
     4 * 3 * 2 * 1;
     4 * 3 * 2;
     4 * 6;
     24;
     
     factorial(4)
     factorial(3)
     factorial(2)
     Call Stack
     

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163. factorial(4);
     4 * factorial(3);
     4 * 3 * factorial(2);
     4 * 3 * 2 * factorial(1);
     4 * 3 * 2 * 1;
     4 * 3 * 2;
     4 * 6;
     24;
     
     factorial(4)
     factorial(3)
     Call Stack
     

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164. factorial(4);
     4 * factorial(3);
     4 * 3 * factorial(2);
     4 * 3 * 2 * factorial(1);
     4 * 3 * 2 * 1;
     4 * 3 * 2;
     4 * 6;
     24;
     
     factorial(4)
     Call Stack
     

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165. factorial(4);
     4 * factorial(3);
     4 * 3 * factorial(2);
     4 * 3 * 2 * factorial(1);
     4 * 3 * 2 * 1;
     4 * 3 * 2;
     4 * 6;
     24;
     
     Call Stack
     

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166. const value = factorial(100000);
     console.log(value); // ???
     100,000 calls = 100,000 stack frames
     1 stack frame ≈ 48B
     Max stack usage ≈ 1MB
     100,000 x 48 / 1024 / 1024 = 4.58MB > 1MB
     

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167. IN ES2015!
     TAIL CALL
     OPTIMIZATION
     

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168. const factorial = (n) => {
     if (n < 2) {
     return 1;
     }
     return n * factorial(n - 1);
     };
     UNOPTIMIZABLE
     

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169. const factorial = (n) => {
     if (n < 2) {
     return 1;
     }
     return n * factorial(n - 1);
     };
     1
     UNOPTIMIZABLE
     

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170. const factorial = (n) => {
     if (n < 2) {
     return 1;
     }
     return n * factorial(n - 1);
     };
     1
     UNOPTIMIZABLE
     2
     

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171. const factorial = (n) => {
     if (n < 2) {
     return 1;
     }
     return n * factorial(n - 1);
     };
     1
     UNOPTIMIZABLE
     2
     3
     

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172. const factorial = (n, accum = 1) => {
     if (n < 2) {
     return accum;
     }
     return factorial(n - 1, n * accum);
     };
     OPTIMIZABLE
     

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173. const factorial = (n, accum = 1) => {
     if (n < 2) {
     return accum;
     }
     return factorial(n - 1, n * accum);
     };
     OPTIMIZABLE
     

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174. const factorial = (n, accum = 1) => {
     if (n < 2) {
     return accum;
     }
     return factorial(n - 1, n * accum);
     };
     OPTIMIZABLE
     

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175. const factorial = (n, accum = 1) => {
     if (n < 2) {
     return accum;
     }
     return factorial(n - 1, n * accum);
     };
     OPTIMIZABLE
     

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176. const factorial = (n, accum = 1) => {
     if (n < 2) {
     return accum;
     }
     return factorial(n - 1, n * accum);
     };
     1
     OPTIMIZABLE
     

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177. const factorial = (n, accum = 1) => {
     if (n < 2) {
     return accum;
     }
     return factorial(n - 1, n * accum);
     };
     1
     2
     OPTIMIZABLE
     

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178. const factorial = (n, accum = 1) => {
     if (n < 2) {
     return accum;
     }
     return factorial(n - 1, n * accum);
     };
     1
     2
     3
     OPTIMIZABLE
     

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179. const value = factorial(100000);
     console.log(value);
     // Infinity
     

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180. factorial(4 /*, 1 */);
     factorial(3, 4);
     factorial(2, 12);
     factorial(1, 24);
     24;
     
     Call Stack
     

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181. factorial(4 /*, 1 */);
     factorial(3, 4);
     factorial(2, 12);
     factorial(1, 24);
     24;
     
     factorial(4, 1)
     Call Stack
     

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182. factorial(4 /*, 1 */);
     factorial(3, 4);
     factorial(2, 12);
     factorial(1, 24);
     24;
     
     factorial(3, 4)
     Call Stack
     

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183. factorial(4 /*, 1 */);
     factorial(3, 4);
     factorial(2, 12);
     factorial(1, 24);
     24;
     
     factorial(2, 12)
     Call Stack
     

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184. factorial(4 /*, 1 */);
     factorial(3, 4);
     factorial(2, 12);
     factorial(1, 24);
     24;
     
     factorial(1, 24)
     Call Stack
     

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185. factorial(4 /*, 1 */);
     factorial(3, 4);
     factorial(2, 12);
     factorial(1, 24);
     24;
     
     Call Stack
     

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186. RECAP
     PREDICTABLE
     SAFE
     TRANSPARENT
     MODULAR
     

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187. DEMO
     

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188. RESOURCES
     

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189. drboolean.gitbooks.io/mostly-adequate-guide
     Brian Lonsdorf
     

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190. ES6/7/LATER
     babeljs.io
     

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191. LANGUAGES
     Elm (elm-lang.org)
     Clojurescript (github.com/clojure/clojurescript)
     Purescript (purescript.org)
     

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192. LIBRARIES
     Lodash (lodash.com)
     Ramda (ramdajs.com)
     Rx (reactivex.io)
     Bacon.js (baconjs.github.io)
     Immutable.js (facebook.github.io/immutable-js)
     

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193. “MV*”
     React (facebook.github.io/react)
     Redux (redux.js.org)
     

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194. THANKS!
     Jeremy Fairbank
     blog.jeremyfairbank.com
     @elpapapollo / jfairbank
     Code: github.com/jfairbank/fp-basics-in-es6
     Slides: bit.ly/devnexus-fp-basics
     

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