U N L I M I T E D P O W E R E S 6 G E N E R A T O R S B R I A N H O LT T W I T T E R : H O LT B T G I T H U B : B T H O LT

E S 6 ? • ECMAScript 6. The latest version of JavaScript. • More correctly known as ES2015. • Includes features such as arrow functions, symbols, classes, enhanced object literal syntax, template strings, and …

G E N E R AT O R S • Generators are a new construct to JavaScript that enables new patterns of asynchronous programming that looks synchronous. • Can greatly reduce the complexity of some problems. • Can be confusing due to their versatility in how they handle input and output

C A N I U S E I T N O W ? • Yes! • An engineer formerly of Facebook and currently of Meteor named Benjamin Newman emulated generator behavior using ES5, giving us the opportunity to use it now! • Babel uses Regenerator under the hood to give generator support. If you’re not using Babel now, you really should. Babel is a JS preprocessor that allows you to process ES6 files into ES5 compatible files. • iojs works out of the box with no preprocessing.

L O L O K , W H AT I S A G E N E R AT O R ? • A generator is a function that returns a generator object. This object is an iterable; that is, you can iterate over it. • Generators can generate output, process input, or both; however I would suggest where possible to either process input or generate output. It simplifies things a lot. • Let’s look at a few examples.

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O K AY, B I G D E A L . N O W S H O W M E S O M E T H I N G C O O L • co is the name of a library put out by TJ Holowaychuk (same guy as Stylus, Mocha, Express, Koa, and half the other things you use.) • co allows you to simulate ES7’s async/await functionality today using generators. async/await is going to change the way we do a lot of things with JavaScript.

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Y I E L D * • I heard you like generators so now you can call generator functions from your generator functions. • yield* is a keyword that calls another generator and adds all its content to the current generator.

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G E N E R AT O R S A S O B S E R V E R S • Think of a function you can call a bunch of times with different parameters and preserve the state between calls. • Can be done other ways but a large point of generators to simplify code and make it look more synchronous despite being asynchronous.

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L O L O K , N O W D O S O M E T H I N G C O O L • Ideal for situations where you can receive input multiple times at asynchronous intervals. • Conducive to reactive (ie RxJS, Bacon, etc) programming. • Awesome for WebSockets / polling

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G E N E R AT O R S A S C O - R O U T I N E S • As I was saying earlier, I prefer to take generators and compose them into either data producing or data consuming, not both. • Seeing a generator both produce and consume takes the advantage of generators away for me: simplifying asynchronous code.

. T H R O W ( ) , . R E T U R N ( ) , T RY, C AT C H , A N D F I N A L LY • Calling `.return()` (instead of `.next()`) executes a return at where ever your generator is paused. This can be useful if you need to terminate your generator. • Calling `.throw()` is similar: it throws an error there and allows you to deal with it using a `catch` statement. • `finally` statements are interesting because if you `.return()` you can clean up your generator by putting code into a finally block. • `.return()` doesn’t work out-of-the-box with iojs.

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B U T WA I T ! I WA S N ’ T D O N E !

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T H R O W W O R K S A B O U T T H E S A M E B U T W I L L J U S T H I T T H E ` C AT C H ` B L O C K I N S T E A D

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T L ; D R • Generators make async easy by making your async code look synchronous. • Generators can accept input, give output, or both. • Generators can be used to model things like observables, iterables, csp, lazily evaluated infinite arrays, and much more fun stuff. • Use generators. They give you power.

B R I A N H O LT T W I T T E R : H O LT B T G I T H U B : B T H O LT G I T H U B . C O M / B T H O LT / G E N E R AT O R - E X A M P L E S U N L I M I T E D P O W E R : E S 6 G E N E R AT O R S