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Understanding Iterators & Iterables in JavaScript

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Hi! I’m Jenn! Senior Frontend Engineer @gurlcode @ Rent the Runway

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! @gurlcode

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... Spread Operator @gurlcode

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[ ] @gurlcode

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[ 1, 2, 3 ] @gurlcode

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...[ 1, 2, 3 ] @gurlcode

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...[ 1, 2, 3 ] // 1, 2, 3 @gurlcode

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..."abc" // "a", "b", "c" @gurlcode

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function* generator( ) { yield 1; yield 2; yield 3; } …generator( ) // 1, 2, 3 @gurlcode

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@gurlcode

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Generators 101 @gurlcode

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function* count() { yield 1; yield; yield 3; } @gurlcode

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function* count() { yield 1; yield; yield 3; } var generator = count(); Generator {}; @gurlcode

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function* count() { yield 1; yield; yield 3; } var generator = count(); generator.next(); { value: 1, done: false } Generator {};

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function* count() { yield 1; yield; yield 3; } var generator = count(); generator.next(); generator.next(); generator.next(); generator.next(); { value: 1, done: false } { value: undefined, done: false } { value: 3, done: false } { value: undefined, done: true } Generator {}; @gurlcode

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function* count() { yield 1; yield; yield 3; } var generator = count(); generator.next(); generator.next(); generator.next(); generator.next(); { value: 1, done: false } { value: undefined, done: false } { value: 3, done: false } { value: undefined, done: true } Generator {}; @gurlcode

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function* count() { yield 1; yield; yield 3; } var generator = count(); generator.next(); generator.next(); generator.next(); generator.next(); { value: 1, done: false } { value: undefined, done: false } { value: 3, done: false } { value: undefined, done: true } Generator {}; @gurlcode

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function* add() { const num = yield; yield 2 + num; yield 4 + num; } var generator = add(); generator.next(); generator.next(2); generator.next(); generator.next(); { value: undefined, done: false } { value: 4, done: false } { value: 6, done: false } { value: undefined, done: true } Generator {}; @gurlcode

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function* add() { const num = yield; yield 2 + num; yield 4 + num; } var generator = add(); generator.next(); generator.next(2); generator.next(); generator.next(); { value: undefined, done: false } { value: 4, done: false } { value: 6, done: false } { value: undefined, done: true } Generator {}; @gurlcode

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function* add() { const num = yield; yield 2 + num; yield 4 + num; } var generator = add(); generator.next(); generator.next(2); generator.next(); generator.next(); { value: undefined, done: false } { value: 4, done: false } { value: 6, done: false } { value: undefined, done: true } Generator {}; @gurlcode

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function* add() { const num = yield; yield 2 + num; yield 4 + num; } var generator = add(); generator.next(); generator.next(2); generator.next(); generator.next(); { value: undefined, done: false } { value: 4, done: false } { value: 6, done: false } { value: undefined, done: true } Generator {}; @gurlcode

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yield* @gurlcode

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function* outer() { yield 1; yield* inner(); yield 2; } function* inner() { yield “a”; yield “b”; } var generator = outer(); Generator {}; @gurlcode

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function* outer() { yield 1; yield* inner(); yield 2; } function* inner() { yield “a”; yield “b”; } var generator = outer(); Generator {}; @gurlcode

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function* outer() { yield 1; yield* inner(); yield 2; } function* inner() { yield “a”; yield “b”; } var generator = outer(); Generator {}; @gurlcode

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function* outer() { yield 1; yield* inner(); yield 2; } function* inner() { yield “a”; yield “b”; } var generator = outer(); Generator {}; @gurlcode

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function* outer() { yield 1; yield* inner(); yield 2; } function* inner() { yield “a”; yield “b”; } generator.next(); generator.next(); generator.next(); generator.next(); generator.next(); { value: 1, done: false } { value: “a”, done: false } { value: “b”, done: false } { value: 2, done: false } { value: undefined, done: true }

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function* outer() { yield 1; yield* inner(); yield 2; } function* inner() { yield “a”; yield “b”; } generator.next(); generator.next(); generator.next(); generator.next(); generator.next(); { value: 1, done: false } { value: “a”, done: false } { value: “b”, done: false } { value: 2, done: false } { value: undefined, done: true }

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function* outer() { yield 1; yield* inner(); yield 2; } function* inner() { yield “a”; yield “b”; } generator.next(); generator.next(); generator.next(); generator.next(); generator.next(); { value: 1, done: false } { value: “a”, done: false } { value: “b”, done: false } { value: 2, done: false } { value: undefined, done: true }

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function* outer() { yield 1; yield* inner(); yield 2; } function* inner() { yield “a”; yield “b”; } generator.next(); generator.next(); generator.next(); generator.next(); generator.next(); { value: 1, done: false } { value: “a”, done: false } { value: “b”, done: false } { value: 2, done: false } { value: undefined, done: true }

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function* generator( ) { yield 1; yield 2; yield 3; } ...generator() // 1, 2, 3 @gurlcode

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Arrays Strings Maps Sets Generators @gurlcode

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Iterable a data structure that provides an interface for iteration @gurlcode

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Iterable Protocol

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Symbol.iterator @gurlcode

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Symbol( "key" ) @gurlcode

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Array.prototype[ Symbol.iterator ] @gurlcode

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String.prototype[ Symbol.iterator ] @gurlcode

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function* generator( ) { } generator[ Symbol.iterator ] @gurlcode

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Iterator any object which implements the Iterator protocol @gurlcode

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Iterator Protocol @gurlcode

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function fakeIterator( ) { return { next: function( ) { } }; } @gurlcode

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function fakeIterator( ) { return { next: function( ) { // return object // with a value // & a done key } }; } @gurlcode

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const iterator = fakeIterator( ); const { value, done } = iterator.next( ); @gurlcode

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const iterator = fakeIterator( ); const { value, done } = iterator.next( ); @gurlcode

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Let’s create our own! @gurlcode

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No content

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Async Iterator @gurlcode

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Symbol.asyncIterator @gurlcode

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... Spread Operator @gurlcode

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{ } @gurlcode

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...{ } @gurlcode

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Thank you! @gurlcode