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> const lol = a + b + c – f * ( n + o ); NaN

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> NaN >= 0 false

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> NaN <= 0 false

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> typeof NaN number

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( a + b ) + c = a + ( b + c ) // mathematically true > 0.1 + 0.2 === 0.3 false > ( a + b ) + c == a + ( b + c ) ??

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> 0.1 + 0.2 0.30000000000000004

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> Math.round(0.4999999999999999722444243843710864894092082) 0 > Math.round(0.4999999999999999722444243843710864894092083) 1 I’ve got 99 problems but JavaScript ain’t 1.0000000000000009

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> [ 2, 10 ].sort() [ 10, 2 ]

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> ['10','10','10', '10', '10'].map( parseInt ) [ 10, NaN, 2, 3, 4 ]

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> typeof undefined // undefined > typeof true // boolean > typeof "hello" // string > typeof 1 // number > typeof {lol:true} // object > typeof [1,2,3] // object > typeof null // object

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> null == 0 // false > null > 0 // false > null < 0 // false > null >= 0 // true > null <= 0 // true > Number( null ) // 0

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> false > null > undefined > "" > 0 > NaN

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> wat.test("javascript"); // true > wat.test("wat r u doin"); // false > const wat = /a/g;

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// undefined function troll() { return { haha: "ha!" }; } troll(); // automatic semi-colon

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const arr = []; arr[1] = 1; arr[3] = 2; arr[10] = 3; arr.length // 11 arr[-1] = 4; arr.s = 5; arr.length // 11

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• trimStart() and trimEnd() • Optional Catch • Object.fromEntries() • flat() and flatMap()

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// String.prototype.trimStart() and trimEnd() const what = " \n \t what is love".trimStart(); const bacon = "bacon don't hurt me\n \n \n \t ".trimEnd(); const obj = { lyric: what + " " + bacon }; console.log( obj ); // { lyric: "what is love bacon don't hurt me" }

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// Optional Catch try { console.log( "Error objects?" ); throw new Error( "LOL" ); } catch { console.log( "We don't need no stinkin' Error objects!" ); } // Error objects? // We don't need no stinkin' Error objects!

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// Object.fromEntries() const props = [ [ "what", "is love" ], [ "bacon", "don't hurt me" ], [ "love", true ] ]; const obj = Object.fromEntries( props ); console.log( obj ); // { what: 'is love', bacon: "don't hurt me", love: true }

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// Array.prototype.flat() and .flatMap() const nestedArrays = [ "bacon", "eggs", [ "waffles" ], [ "coffee" ], [ "doughnuts", "milk" ] ]; const flattened = nestedArrays.flat(); console.log( flattened ); // [ 'bacon', 'eggs', 'waffles', 'coffee', 'doughnuts', 'milk' ]

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• Nullish coalescing ?? operator • Optional ?. chaining • Promise.allSettled() • Dynamic import()

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// Logical OR || operator works with any falsy value console.log( 0 || "zero value" ); // zero value console.log( "" || "empty string value" ); // empty string value console.log( false || "false value" ); // false value console.log( null || "null value" ); // null value console.log( undefined || "undefined value" ); // undefined value

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// Nullish (??) operator only works with null/undefined console.log( 0 ?? "zero value" ); // 0 console.log( "" ?? "empty string value" ); // "" console.log( false ?? "false value" ); // false console.log( null ?? "null value" ); // null value console.log( undefined ?? "undefined value" ); // undefined value

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// The old way of checking for potentially missing properties const customer = {}; let city; try { city = customer.address.city; } catch ( err ) { console.log( err ); // TypeError: Cannot read properties of undefined (reading 'city’) } console.log( city ); // undefined

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// The old way of checking for potentially missing properties const customer = {}; let city; if ( customer && customer.address && customer.address.city ) { city = customer.address.city; } console.log( city ); // undefined

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// Optional ?. Chaining const customer1 = {}; const customer2 = { address: { city: "Nashville" } }; const city1 = customer1?.address?.city; // undefined const city2 = customer2?.address?.city; // Nashville

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// Promise.allSettled() const p1 = new Promise( ( resolve, reject ) => { setTimeout( () => { resolve( "p1 resolved" ); }, 250 ); } ); const p2 = new Promise( ( resolve, reject ) => { setTimeout( () => { resolve( "p2 resolved" ); }, 450 ); } ); const p3 = new Promise( ( resolve, reject ) => { setTimeout( () => { reject( "p3 rejected" ); }, 650 ); } );

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// Promise.allSettled() const results = await Promise.allSettled( [ p1, p2, p3 ] ); console.log( results ); // [ // { status: 'fulfilled', value: 'p1 resolved' }, // { status: 'fulfilled', value: 'p2 resolved' }, // { status: 'rejected', reason: 'p3 rejected' } // ]

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// Dynamic import() const example = await import( "./importExample.js" ); console.log( example ); // [Module: null prototype] { // default: { whatIsLove: [Function: whatIsLove] } // }

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• String.prototype.replaceAll() • Promise.any() • Logical assignment operators ||= &&= ??= • Numeric separator

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// Old string replace() const qs = "q=what+is+love"; const nope = qs.replace( "+", " " ); // "q=what is+love”

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// Old string replace() const qs = "q=what+is+love"; const nope = qs.replace( "+", " " ); // "q=what is+love” const yep = qs.replace( /\+/g, " " ); // "q=what is love"

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// Old string replace() const qs = "q=what+is+love"; const nope = qs.replace( "+", " " ); // "q=what is+love” const yep = qs.replace( /\+/g, " " ); // "q=what is love" const lol = qs.split( "+" ).join( " " ); // "q=what is love"

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// String.prototype.replaceAll() const n00b = "Hey, I can leet speak, too, yo"; const l33t = n00b.replaceAll( "e", "3" ) .replaceAll( "o", "0" ) .replaceAll( ",", "" ) .toLowerCase(); console.log( l33t );

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// String.prototype.replaceAll() const n00b = "Hey, I can leet speak, too, yo"; const l33t = n00b.replaceAll( "e", "3" ) .replaceAll( "o", "0" ) .replaceAll( ",", "" ) .toLowerCase(); console.log( l33t ); // h3y i can l33t sp3ak t00 y0

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// Promise.any() example const p1 = new Promise( res => { setTimeout( () => { res( "knock, knock" ); }, 200 ); } ); const p2 = new Promise( res => { setTimeout( () => { res( "who's there?" ); }, 400 ); } ); const interruptingCow = Promise.reject( "MOO!!" ); const lol = await Promise.any( [ p1, p2, interruptingCow ] ); console.log( lol );

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// Promise.any() example const p1 = new Promise( res => { setTimeout( () => { res( "knock, knock" ); }, 200 ); } ); const p2 = new Promise( res => { setTimeout( () => { res( "who's there?" ); }, 400 ); } ); const interruptingCow = Promise.reject( "MOO!!" ); const lol = await Promise.any( [ p1, p2, interruptingCow ] ); console.log( lol ); // knock, knock

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// Logical assignment operators // OR only assigns if x is a falsy value let x = false; x ||= "yes"; x ||= "what is";

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// Logical assignment operators // OR only assigns if x is a falsy value let x = false; x ||= "yes"; x ||= "what is"; // yes

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// Logical assignment operators // OR only assigns if x is a falsy value let x = false; x ||= "yes"; x ||= "what is"; // yes // AND assigns if y is not a falsy value let y = "gimme-all-yer"; y &&= "i love";

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// Logical assignment operators // OR only assigns if x is a falsy value let x = false; x ||= "yes"; x ||= "what is"; // yes // AND assigns if y is not a falsy value let y = "gimme-all-yer"; y &&= "i love"; // i love

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// Logical assignment operators // OR only assigns if x is a falsy value let x = false; x ||= "yes"; x ||= "what is"; // yes // AND assigns if y is not a falsy value let y = "gimme-all-yer"; y &&= "i love"; // i love // ??= Nullish only assigns if z is null or undefined let z = "bacon"; z ??= y;

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// Logical assignment operators // OR only assigns if x is a falsy value let x = false; x ||= "yes"; x ||= "what is"; // yes // AND assigns if y is not a falsy value let y = "gimme-all-yer"; y &&= "i love"; // i love // ??= Nullish only assigns if z is null or undefined let z = "bacon"; z ??= y; // bacon

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// Numeric separators example const numbersMakeMeSad = 10204005020405;

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// Numeric separators example const numbersMakeMeSad = 10204005020405; const notSoBad = 10_204_005_020_405;

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// Numeric separators example const numbersMakeMeSad = 10204005020405; const notSoBad = 10_204_005_020_405; console.log( numbersMakeMeSad === notSoBad ); // true

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• Array.prototype.at() & String .at() • Top-level await • Error cause • New static, private class members

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// String and Array at() const arr = [ "what", "is", "love", "bacon", "don't", "hurt", "me" ]; console.log( arr.at( 3 ) );

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// String and Array at() const arr = [ "what", "is", "love", "bacon", "don't", "hurt", "me" ]; console.log( arr.at( 3 ) ); // bacon

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// String and Array at() const arr = [ "what", "is", "love", "bacon", "don't", "hurt", "me" ]; console.log( arr.at( 3 ) ); // bacon console.log( arr.at( -1 ) );

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// String and Array at() const arr = [ "what", "is", "love", "bacon", "don't", "hurt", "me" ]; console.log( arr.at( 3 ) ); // bacon console.log( arr.at( -1 ) ); // me

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// String and Array at() const arr = [ "what", "is", "love", "bacon", "don't", "hurt", "me" ]; console.log( arr.at( 3 ) ); // bacon console.log( arr.at( -1 ) ); // me const msg = "I love rock n' roll"; console.log( msg.at( 4 ) );

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// String and Array at() const arr = [ "what", "is", "love", "bacon", "don't", "hurt", "me" ]; console.log( arr.at( 3 ) ); // bacon console.log( arr.at( -1 ) ); // me const msg = "I love rock n' roll"; console.log( msg.at( 4 ) ); // v

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// Top-level await async function sleep( ms ) { const msg = `I slept for ${ ms } milliseconds`; return new Promise( resolve => setTimeout( () => resolve( msg ), ms ) ); } const result = await sleep( 500 ); console.log( result ); console.log( "finished" );

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// Top-level await async function sleep( ms ) { const msg = `I slept for ${ ms } milliseconds`; return new Promise( resolve => setTimeout( () => resolve( msg ), ms ) ); } const result = await sleep( 500 ); console.log( result ); console.log( "finished" ); // I slept for 500 milliseconds // finished

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// Error _with_ a cause function breakin() { throw new Error( "Gonna stop you cold" ); } function breakin2ElectricBoogaloo() { try { breakin(); } catch ( err ) { throw new Error( "Whacked", { cause: err } ); } }

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// Error _with_ a cause function shutUpAndDance() { try { breakin2ElectricBoogaloo(); } catch ( err ) { console.log( err ); } } shutUpAndDance();

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// Error _with_ a cause shutUpAndDance(); // Error: Whacked // at breakin2ElectricBoogaloo ... // [cause]: Error: Gonna stop you cold // at breakin ... // }

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// Private and static class members class Of86GlenbrookNorthHigh { #reason; static car = "Ferrari"; static #miles = 26000; constructor( reason ) { this.#reason = reason; } ... }

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// Private and static class members class Of86GlenbrookNorthHigh { #reason; static car = "Ferrari"; static #miles = 26000; static #getMiles() { Of86GlenbrookNorthHigh.#miles += 123; return Of86GlenbrookNorthHigh.#miles; } }

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// Private and static class members class Of86GlenbrookNorthHigh { #reason; static car = "Ferrari"; static #miles = 26000; static #getMiles() { … } #carStatus() { const miles = Of86GlenbrookNorthHigh.#getMiles(); return `The ${ Of86GlenbrookNorthHigh.car } has ${ miles } miles`; } }

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// Private and static class members class Of86GlenbrookNorthHigh { #carStatus() { … } status() { console.log( `Ferris can't go to school, he's ${ this.#reason }` ); console.log( this.#carStatus() ); } }

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// Private and static class members const myClass = new Of86GlenbrookNorthHigh( "sick" ); myClass.status(); // Ferris can't go to school, he's sick // The Ferrari has 26123 miles

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// Private and static class members const myClass = new Of86GlenbrookNorthHigh( "sick" ); myClass.status(); myClass.#carStatus(); // SyntaxError console.log( myClass.#car ); // SyntaxError

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