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#JSScarySide J AVA S C R I P T ’ S S C A R Y S I D E @JONATHANFMILLS

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Types

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Types NUMBER STRING BOOLEAN NULL UNDEFINED FUNCTION OBJECT SYMBOL

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typeof undefined // "undefined" typeof 0 // "number" typeof true // "boolean" typeof “PrDC” // "string"

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typeof null

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typeof null // object

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typeof NaN

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typeof NaN // number

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console.log(‘1’ + 1); // 11

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console.log(1 + 1);

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console.log ('Hello ' + 'World');

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console.log(‘1’ + 1); // 11

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console.log(‘1’ - 1); // 0

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console.log(42 + true);

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console.log(42 + true); // 43

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console.log([1,2,3].indexOf(3));

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console.log([1,2,3].indexOf(3)); // 2

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console.log(![1,2,3].indexOf(3));

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console.log(!![1,2,3].indexOf(3));

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console.log(!![1,2,3].indexOf(3)); // True

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Hoisting

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var myVariable = 10; console.log(myVariable);

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var myVariable = 10; console.log(myVariable); // 10

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console.log(myVariable); var myVariable = 10;

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console.log(myVariable); // undefined var myVariable = 10;

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console.log(myVariable);

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ReferenceError: myVariable is not defined at Object. (/Users/jonathanmills/Dropbox/Talks/scarySide/2- hoisting.js:3:13) at Module._compile (module.js:643:30) at Object.Module._extensions..js (module.js:654:10) at Module.load (module.js:556:32) at tryModuleLoad (module.js:499:12) at Function.Module._load (module.js:491:3) at Function.Module.runMain (module.js:684:10)

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console.log(myVariable); // undefined var myVariable = 10;

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console.log(myVariable); let myVariable = 10;

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console.log(myVariable); // myVariable is not defined let myVariable = 10;

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var myVariable = 10; function func(){ myVariable = 25; var myVariable; console.log(myVariable); } func(); console.log(myVariable);

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var myVariable = 10; function func(){ myVariable = 25; var myVariable; console.log(myVariable); // 25 } func(); console.log(myVariable); // 10

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var myVariable = 10; function func(){ myVariable = 25; console.log(myVariable); } func(); console.log(myVariable);

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var myVariable = 10; function func(){ myVariable = 25; console.log(myVariable); // 25 } func(); console.log(myVariable); // 25

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var myVariable = 10; function func(){ myVariable = 25; let myVariable; console.log(myVariable); } func(); console.log(myVariable);

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var myVariable = 10; function func(){ myVariable = 25; // myVariable is not defined let myVariable; console.log(myVariable); } func(); console.log(myVariable);

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Functions

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function greeter(name){console.log('Hello ' + name)} greeter('jon');

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greeter(‘jon'); function greeter(name){console.log('Hello ' + name)}

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greeter(‘jon’); var greeter = function(name){console.log(‘Hello ' + name)}

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greeter(‘jon’); // error var greeter = function(name){console.log(‘Hello ' + name)}

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function greeter(name){console.log('Hello ' + name)} greeter('jon');

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function getGreeter(){ var greeting = 'Hello' return { greeter: function(name){console.log(greeting + ' ' + name)} } } var g = getGreeter(); g.greeter(‘Jon');

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function getGreeter(){ var greeting = 'Hello' return { greeter: function(name){console.log(greeting + ' ' + name)} } } var g = getGreeter(); g.greeter(‘Jon’); // Hello Jon

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Closures

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function getGreeter(){ var greeting = 'Hello' return { greeter: function(name){console.log(greeting + ' ' + name)} } } var g = getGreeter(); g.greeter(‘Jon'); // Hello Jon

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function getGreeter(){ var greeting = 'Hello' return { greeter: function(name){console.log(greeting + ' ' + name)} } } var g = getGreeter(); var x = g.greeter; x('Jon');

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function getGreeter(){ var greeting = 'Hello' return { greeter: function(name){console.log(greeting + ' ' + name)} } } var g = getGreeter(); var x = g.greeter; x(‘Jon'); // Hello Jon

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This

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var greeter = { greeting: 'Hello', sayHi: function (name) { } } greeter.sayHi('Jon');

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var greeter = { greeting: 'Hello', sayHi: function (name) { document.write(this.greeting + ' ' + name) } } greeter.sayHi('Jon');

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var greeter = { greeting: 'Hello', sayHi: function (name) { document.write(this.greeting + ' ' + name) } } greeter.sayHi('Jon'); // Hello Jon

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var greeter = { greeting: 'Hello', sayHi: function (name) { document.write(this.greeting + ' ' + name) } } var coolGreeter = { greeting: ‘Whats up', } coolGreeter.sayHi = greeter.sayHi coolGreeter.sayHi(‘Jon');

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var greeter = { greeting: 'Hello', sayHi: function (name) { document.write(this.greeting + ' ' + name) } } var coolGreeter = { greeting: ‘Whats up', } coolGreeter.sayHi = greeter.sayHi coolGreeter.sayHi('Jon'); // Whats up Jon

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var greeter = { greeting: 'Hello', sayHi: function (name) { document.write(this.greeting + ' ' + name) } } var coolGreeter = { greeting: ‘Whats up', } var sayHi = greeter.sayHi sayHi('Jon');

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var greeter = { greeting: 'Hello', sayHi: function (name) { document.write(this.greeting + ' ' + name) } } var coolGreeter = { greeting: ‘Whats up', } var sayHi = greeter.sayHi sayHi('Jon'); // Undefined Jon

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var greeter = { greeting: 'Hello', sayHi: function (name) { document.write(this.greeting + ' ' + name) } } var coolGreeter = { greeting: ‘Whats up', } var sayHi = greeter.sayHi.bind(greeter) sayHi('Jon');

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var greeter = { greeting: 'Hello', sayHi: function (name) { document.write(this.greeting + ' ' + name) } } var coolGreeter = { greeting: ‘Whats up', } var sayHi = greeter.sayHi.bind(greeter) sayHi('Jon'); // Hello Jon

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var greeter = { greeting: 'Hello', sayHi: function (name) { document.write(this.greeting + ' ' + name) } } var coolGreeter = { greeting: ‘Whats up', } var sayHi = greeter.sayHi.bind(coolGreeter) sayHi('Jon');

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var greeter = { greeting: 'Hello', sayHi: function (name) { document.write(this.greeting + ' ' + name) } } var coolGreeter = { greeting: ‘Whats up', } var sayHi = greeter.sayHi.bind(coolGreeter) sayHi('Jon'); // Whats up Jon

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var greeter = { greeting: 'Hello', sayHi: function (name) { document.write(this.greeting + ' ' + name) } } var coolGreeter = { greeting: ‘Whats up', } var sayHi = greeter.sayHi sayHi.call(greeter,'Jon');

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var greeter = { greeting: 'Hello', sayHi: function (name) { document.write(this.greeting + ' ' + name) } } var coolGreeter = { greeting: ‘Whats up', } var sayHi = greeter.sayHi sayHi.call(greeter,'Jon'); // Hello Jon

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var greeter = { greeting: 'Hello', sayHi: function (name) { document.write(this.greeting + ' ' + name) } } var coolGreeter = { greeting: ‘Whats up', } var sayHi = greeter.sayHi.bind(coolGreeter) sayHi.call(greeter,'Jon');

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var greeter = { greeting: 'Hello', sayHi: function (name) { document.write(this.greeting + ' ' + name) } } var coolGreeter = { greeting: ‘Whats up', } var sayHi = greeter.sayHi.bind(coolGreeter) sayHi.call(greeter,'Jon'); // Whats up Jon

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function Greeter(greeting){ this.greeting = greeting; this.greet = function(name){return this.greeting + ' ' + name} this.greetAsync = function(name, cb){ setTimeout(function(){cb(this.greeting + ' ' + name)},0) } } var g = new Greeter('hello'); g.greetAsync('Jon', function(greeting){console.log(greeting)})

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function Greeter(greeting){ this.greeting = greeting; this.greet = function(name){return this.greeting + ' ' + name} this.greetAsync = function(name, cb){ setTimeout(function(){cb(this.greeting + ' ' + name)},0) } } var g = new Greeter('hello'); g.greetAsync('Jon', function(greeting){console.log(greeting)}) // Undefined Jon

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function Greeter(greeting){ this.greeting = greeting; this.greet = function(name){return this.greeting + ' ' + name} this.greetAsync = function(name, cb){ setTimeout(function(){cb(this.greeting + ' ' + name)},0) } } var g = new Greeter('hello'); g.greetAsync('Jon', function(greeting){console.log(greeting)}) // Undefined Jon

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function Greeter(greeting){ this.greeting = greeting; this.greet = function(name){return this.greeting + ' ' + name} this.greetAsync = function(name, cb){ setTimeout(function(){cb(this.greeting + ' ' + name)}.bind(this),0) } } var g = new Greeter('hello'); g.greetAsync('Jon', function(greeting){console.log(greeting)}) // Hello Jon

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Arrow Functions

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function Greeter(greeting){ this.greeting = greeting; this.greet = function(name){return this.greeting + ' ' + name} this.greetAsync = function(name, cb){ setTimeout(()=>{cb(this.greeting + ' ' + name)},0) } } var g = new Greeter('hello'); g.greetAsync('Jon', function(greeting){console.log(greeting)}) // Hello Jon

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Prototypes

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function Greeter(greeting){ this.greeting = greeting; } Greeter.prototype.greet = function(name){console.log(this.greeting + ' ' + name)}; var greeter = new Greeter('Hello'); var coolGreeter = new Greeter('Whats Up');

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Greeter.Prototype greet: function(){} coolGreeter greeting: Whats up greeter greeting: Hello

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function Greeter(greeting){ this.greeting = greeting; } Greeter.prototype.greet = function(name){console.log(this.greeting + ' ' + name)}; var greeter = new Greeter('Hello'); var coolGreeter = new Greeter('Whats Up');

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function Greeter(greeting){ this.greeting = greeting; } Greeter.prototype.greet = function(name){console.log(this.greeting + ' ' + name)}; var greeter = new Greeter('Hello'); var coolGreeter = new Greeter('Whats Up'); greeter.greet(‘jon’); // Hello Jon coolGreeter.greet(‘jon’); // Whats up Jon

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Greeter.Prototype greet: function(){} coolGreeter greeting: Whats up greeter greeting: Hello

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function Greeter(greeting){ this.greeting = greeting; } Greeter.prototype.greet = function(name){console.log(this.greeting + ' ' + name)}; var greeter = new Greeter('Hello'); var coolGreeter = new Greeter('Whats Up'); greeter.greet(‘jon’); // Hello Jon coolGreeter.greet(‘jon’); // Whats up Jon Greeter.prototype.greet = 2; greeter.greet(‘jon’);

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function Greeter(greeting){ this.greeting = greeting; } Greeter.prototype.greet = function(name){console.log(this.greeting + ' ' + name)}; var greeter = new Greeter('Hello'); var coolGreeter = new Greeter('Whats Up'); greeter.greet(‘jon’); // Hello Jon coolGreeter.greet(‘jon’); // Whats up Jon Greeter.prototype.greet = 2; greeter.greet(‘jon’); // greeter.greet is not a function

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Event Loop

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EVENT LOOP CALL STACK EXTERNAL API CALLBACK QUEUE function demo() { console.log('Start') setTimeout(() => { console.log('Callback'); }, 100); let sum = 1 + 1; console.log(sum); console.log('End'); }

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function demo() { console.log('Start') setTimeout(() => { console.log('Callback'); }, 100); let sum = 1 + 1; console.log(sum); console.log('End'); } EVENT LOOP demo() log('Start') CALL STACK EXTERNAL API CALLBACK QUEUE

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function demo() { console.log('Start') setTimeout(() => { console.log('Callback'); }, 100); let sum = 1 + 1; console.log(sum); console.log('End'); } EVENT LOOP demo() CALL STACK EXTERNAL API CALLBACK QUEUE setTimeout(...)

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function demo() { console.log('Start') setTimeout(() => { console.log('Callback'); }, 100); let sum = 1 + 1; console.log(sum); console.log('End'); } EVENT LOOP demo() CALL STACK EXTERNAL API 100ms timer CALLBACK QUEUE

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function demo() { console.log('Start') setTimeout(() => { console.log('Callback'); }, 100); let sum = 1 + 1; console.log(sum); console.log('End'); } EVENT LOOP demo() CALL STACK EXTERNAL API 100ms timer log(2) CALLBACK QUEUE

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function demo() { console.log('Start') setTimeout(() => { console.log('Callback'); }, 100); let sum = 1 + 1; console.log(sum); console.log('End'); } EVENT LOOP demo() CALL STACK EXTERNAL API log('Callback') CALLBACK QUEUE

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function demo() { console.log('Start') setTimeout(() => { console.log('Callback'); }, 100); let sum = 1 + 1; console.log(sum); console.log('End'); } EVENT LOOP demo() CALL STACK EXTERNAL API log ('Callback') CALLBACK QUEUE

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function demo() { console.log('Start') setTimeout(() => { console.log('Callback'); }, 100); let sum = 1 + 1; console.log(sum); console.log('End'); } EVENT LOOP demo() CALL STACK EXTERNAL API CALLBACK QUEUE log ('Callback') log('End')

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function demo() { console.log('Start') setTimeout(() => { console.log('Callback'); }, 100); let sum = 1 + 1; console.log(sum); console.log('End'); } EVENT LOOP log('Callback') CALL STACK EXTERNAL API CALLBACK QUEUE

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function demo() { console.log('Start') setTimeout(() => { console.log('Callback'); }, 100); let sum = 1 + 1; console.log(sum); console.log('End'); } EVENT LOOP CALL STACK EXTERNAL API CONSOLE OUTPUT Start 2 End Callback

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function demo() { console.log('Start') setTimeout(() => { console.log('Callback'); }, 0); let sum = 1 + 1; console.log(sum); console.log('End'); } EVENT LOOP CALL STACK EXTERNAL API CONSOLE OUTPUT Start 2 End Callback

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function demo() { console.log('Start') setTimeout(() => { console.log('Callback'); }, 100); let result = blockingCode(); console.log(result); console.log('End'); } EVENT LOOP demo() CALL STACK EXTERNAL API 100ms timer blockingCode() CALLBACK QUEUE

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EVENT LOOP demo() CALL STACK EXTERNAL API blockingCode() CALLBACK QUEUE this blocks callback execution log ('Callback') function demo() { console.log('Start') setTimeout(() => { console.log('Callback'); }, 100); let result = blockingCode(); console.log(result); console.log('End'); }