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

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sigient.com Your website, SimplyBuilt. simplybuilt.com

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

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¯\_(ϑ)_/¯ WHY FUNCTIONAL PROGRAMMING?

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Domain to Range

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Domain Range

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CALCULUS

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PRINCIPLES

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PURE & DECLARATIVE PREDICTABLE

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IMMUTABLE STATE SAFE

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FIRST CLASS STATE TRANSPARENT

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COMPOSABLE FIRST CLASS CLOSURES MODULAR

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ES2015 (ES6)

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All good let age = 28; age = 29; const name = 'Jeremy'; name = 'Jet'; Syntax error

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

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

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const array = (...elements) => { return elements; }; array(1, 2, 3); // [1, 2, 3] const log = (...args) => { console.log(...args); }; log('hello', 'scenic city summit'); // hello scenic city summit

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const array = (...elements) => { return elements; }; array(1, 2, 3); // [1, 2, 3] const log = (...args) => { console.log(...args); }; log('hello', 'scenic city summit'); // hello scenic city summit

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const langs = ['JavaScript', 'Ruby', 'Haskell']; const [js, ...rest] = langs; js === 'JavaScript'; rest[0] === 'Ruby'; rest[1] === 'Haskell'; const head = ([x]) => x; head([1, 2, 3]) === 1;

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const langs = ['JavaScript', 'Ruby', 'Haskell']; const [js, ...rest] = langs; js === 'JavaScript'; rest[0] === 'Ruby'; rest[1] === 'Haskell'; const head = ([x]) => x; head([1, 2, 3]) === 1;

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const greet = (name, greeting = 'Hi') => { console.log(greeting, name); }; greet('Scenic City Summit', 'Hello'); // Hello Scenic City Summit greet('Chattanooga'); // Hi Chattanooga

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Object.assign( {}, { hello: 'Chattanooga' }, { hi: 'Scenic City Summit' } ); // { // hello: 'Chattanooga', // hi: 'Scenic City Summit' // }

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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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PURE

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

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

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

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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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HIDDEN STATE IS UNCERTAIN STATE

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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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HOW TO ACHIEVE THE RESULT IMPERATIVE

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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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DECLARE WHAT THE DESIRED RESULT IS DECLARATIVE

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

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

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CREATE STATE, DON’T MUTATE IT IMMUTABLE

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

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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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Object.freeze Immutable.js

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

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FREE YOUR STATE

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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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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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PROS SAFETY FREE UNDO/REDO LOGS — REDUX EXPLICIT FLOW OF DATA LESS MEMORY USAGE* CONCURRENCY SAFETY* *In certain cases

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

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FIRST CLASS FUNCTIONS

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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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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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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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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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ENCAPSULATION CLOSURES

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

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

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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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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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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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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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FOUNDATION FOR HIGHER ORDER PATTERNS FIRST CLASS CLOSURES

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PARTIAL APPLICATION

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

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

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

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

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

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

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

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

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CURRYING

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

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

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

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

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

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PIECING IT TOGETHER

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

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

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

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

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COMPOSING CLOSURES

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

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

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

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

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const compose = (...fns) => (...args) => { if (fns.length === 0) { return args[0]; } const last = fns[fns.length - 1]; const rest = fns.slice(0, -1); return rest.reduceRight((memo, fn) => { return fn(memo); }, last(...args)); };

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const compose = (...fns) => (...args) => { if (fns.length === 0) { return args[0]; } const last = fns[fns.length - 1]; const rest = fns.slice(0, -1); return rest.reduceRight((memo, fn) => { return fn(memo); }, last(...args)); };

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const compose = (...fns) => (...args) => { if (fns.length === 0) { return args[0]; } const last = fns[fns.length - 1]; const rest = fns.slice(0, -1); return rest.reduceRight((memo, fn) => { return fn(memo); }, last(...args)); };

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const compose = (...fns) => (...args) => { if (fns.length === 0) { return args[0]; } const last = fns[fns.length - 1]; const rest = fns.slice(0, -1); return rest.reduceRight((memo, fn) => { return fn(memo); }, last(...args)); };

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RECURSION SOLVE A PROBLEM IN TERMS OF ITSELF

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FACTORIAL

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

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

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

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

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factorial(4);

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factorial(4); 4 * factorial(3);

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

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

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

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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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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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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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STEPS Find the recurrence
 (n × n-1 × n-2 × … 1)
 Find the base case
 (n < 2)

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PERFORMANCE RECURSION

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

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

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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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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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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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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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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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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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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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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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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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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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IN ES2015! TAIL CALL OPTIMIZATION

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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RECAP PREDICTABLE SAFE TRANSPARENT MODULAR

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RESOURCES

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

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ES6/7/LATER babeljs.io

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

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

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