# Promises - The Unsung Heroes ofJavaScript

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PROMISES THE UNSUNG HEROES OF ASYNC IN JAVASCRIPT DEAN RADCLIFFE • OK GROW! • @DEANIUSDEV

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A SPORTING BET "I BET YOU \$20 THE REDS WON'T BEAT THE CUBS THIS SEASON..."

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TWO PROMISES ➢ Begin Season then Cubs Lose then Season Ends ↩ I Pay You ➢ Begin Season then Season Ends ↩ You Pay Me

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TWO PROMISES ➢ Begin Season then Cubs Lose // ^ What if we made the bet after a loss ? then Season Ends ↩ I Pay You ➢ Begin Season then Season Ends ↩ You Pay Me

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PROMISES: EVENTS, AT ANY TIME A Promise represents a point in time which is: ▸ ASAP, if something has already happened ▸ ASAP the moment it happens

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PROMISES: VALUES, AT ANY TIME A Promise provides access to a value whether: ▸ It is already known ▸ It is being computed in this turn of the Event Loop ▸ It may be un-knowable (maybe use a TimeOut)

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PROMISES: ONE OR MORE CONDITIONS & ACTIONS A Promise can represent an entire chain of events, with branching possibilities. var firstStep = new Promise() var allSteps = firstStep .then(isReady => isReady || makeReady) .then(lastStep) Promise.await(allSteps)

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OUR BET - MAKE IT SO ➢ let ourBet = BeginSeason() .then(Promise.race(CubsLoseToReds(), SeasonEnds())) .then(cubsLose => cubsLose ? iPayYou : youPayMe) //.then(iPayYou, youPayMe) ▸ CubsLoseToReds and SeasonEnds return Promises ▸ CubsLose should return truthy (such as the score), ▸ SeasonEnds should return falsy, or throw or reject

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UNSUNG HEROES OF ASYNC ▸ Standard, explicit contract of behavior ▸ A promise can resolve or reject ▸ Its state may change 1x from pending to fulfilled ▸ p.then(youPayMe, iPayYou) ▸ Or: p.then((value)=>{}) .catch((err)=>{})

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BACK ON THE CHAIN GANG Each step in a .then chain: RETURNS A CUMULATIVE PROMISE FOR ALL BEFORE ➢ let chain = promise1 .then(promise2) .then(action1)

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BACK ON THE CHAIN GANG Each step in a .then chain may: RETURN A VALUE / RETURN A PROMISE FOR A NEW VALUE ➢ BeginSeasonLossless .then(CubsLoseToReds) // <-- a Promise .then(iPayYou)

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BACK ON THE CHAIN GANG Each step in a .then chain may: RAISE AN EXCEPTION / RETURN REJECTED PROMISE ➢ BeginSeasonLossless .then(Promise.race(CubsLoseToReds,TimeOutAtSeasonEnd)) .then(iPayYou) .catch(youPayMe)

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EXCEPTIONS ▸ An uncaught exception in a .then is OK! ▸ A catch at the end can handle errors for the whole chain

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COMIC RELIEF CAT PICTURE

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QUESTION: HOW DO THESE EVENTS BEHAVE ? If you add an event listener after an event has happened, does it make a sound ? document.addEventListener("DOMContentLoaded") // vs. \$(document).ready // vs. \$(document).on("ready")

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QUESTION: HOW DO THESE EVENTS BEHAVE ? If you add an event listener after an event has happened, does it make a sound ? document.addEventListener("DOMContentLoaded") // vs. \$(document).ready // <<-- only this one - like a Promise // vs. \$(document).on("ready")

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WHAT'S WRONG WITH CALLBACKS ? Implicit, not formalized standard of behavior: 1. Function that takes 2 arguments ▸ first argument is an error ▸ second argument is the result ▸ Never pass both ▸ error should be instanceof Error

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WHAT'S WRONG WITH CALLBACKS ? Implicit, not formalized standard of behavior (cont.) 1. Must never execute on the same tick of the event loop 2. Must be passed as last argument to function 3. Return value is ignored 4. Must not throw / must pass resulting errors 5. Must never be called more than once

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WHAT'S WRONG WITH CALLBACKS ? ▸ Burdensome to check & propogate errors correctly function doAsyncStuff (cb) { try { doStuff1(function(err, value) { if (err) return cb(err); try { doStuff2(value, function(err, value2) { if (err) return cb(err); cb(null, value2); }); } catch (e) { cb(e); } }) } catch (e) { cb(e); } }

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WHAT'S RIGHT WITH PROMISES ? BEAUTIFUL TO CHECK ERRORS, EXCEPTION OR ASYNC! // Promises FTW !! function doAsyncStuff () { return doStuff1() .then(doStuff2) .catch(function(err){ /*...*/}) }

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THE EVENT LOOP (AKA TASK QUEUE)

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THE EVENT LOOP (AKA TASK QUEUE)

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THE 2 WAYS OF CODING JS

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PROMISES AND REACTIVITY Keep an upperCase var in sync with a variable called lowerCase let lowerCase = new ReactiveVar let upperCase = new ReactiveVar

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PROMISES AND REACTIVITY Keep an upperCase var in sync with a variable called lowerCase let lowerCase = new ReactiveVar let upperCase = new ReactiveVar Tracker.autorun(() => { let x = lowerCase.get() upperCase.set(x.toUpperCase()) })

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PROMISES AND REACTIVITY Keep an upperCase var in sync with a variable called lowerCase lowerCase.set(x).then((x) => { upperCase.set(x.toUpperCase()) })

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PATTERN RETURNING A PROMISE FROM A METEOR METHOD

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RETURNING A PROMISE FROM A METEOR METHOD Return a Promise from a Method Return an Eventual Value from a Method

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RETURNING A PROMISE FROM A METEOR METHOD Return a Promise from a Method Return an Eventual Value from a Method Eventually return a value from a Method

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RETURNING A PROMISE FROM A METHOD, 1 Meteor.methods({ futureTime() { return Promise.await( ); } })

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RETURNING A PROMISE FROM A METHOD, 2 Meteor.methods({ futureTime() { return Promise.await(new Promise((resolve) => { //some callback code which should call 'resolve' })); } })

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RETURNING A PROMISE FROM A METHOD, 3 Meteor.methods({ futureTime() { return Promise.await(new Promise((resolve) => { setTimeout(() => { console.log("its the future now") resolve(new Date()) }, 5000) })); } })

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RETURNING A PROMISE FROM A METHOD, 4 Meteor.methods({ futureTime() { let actions = new Promise((resolve) => setTimeout(()=> resolve(new Date), 5000)) .then(logAndReturn) .catch(logAndReturn); return Promise.await(actions); } })

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ASYNC SHEEP

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CALLING A METHOD Meteor.promise("futureTime") .then(log)

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CALLING A METHOD, NOW AND LATER ➢ Promise.resolve(new Date) .then(log) .then(() => Meteor.promise("futureTime")) .then(log) --Fri Sep 18 2015 22:11:26 GMT-0500 (CDT) ↩ Promise {[[PromiseStatus]]: "pending"} --Fri Sep 18 2015 22:11:31 GMT-0500 (CDT)

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CALLING A METHOD, FUNNILY Promise.resolve(new Date) .then(log) .then(() => "futureTime") .then(Meteor.promise) .then(log) --Fri Sep 18 2015 22:11:46 GMT-0500 (CDT) ↩ Promise {[[PromiseStatus]]: "pending", --Fri Sep 18 2015 22:11:51 GMT-0500 (CDT)

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var ComicReliefCatPicture; // TODO fill in

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PROMISES AND REACTIVITY okgrow:promise A promise's resolution can invalidate a computation ? Awwwwwww yeah !

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DEFINE A HELPER AROUND A PROMISE <template name="example1Demo"> <input id="arg1" value="foo"/> <input id="arg2" value="bar"/> <textarea> {{meteorPromise "addWithSleep" "arg1" "arg2"}} </textarea> </template>

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RETURN A PROMISE FROM A HELPER // {{meteorPromise "addWithSleep" "arg1" "arg2"}} Template.example1Demo.helpers({ meteorPromise: function (methodName, part1, part2) { var template = Template.instance(), value1 = template[part1].get(), value2 = template[part2].get(); // want to - but cant do this ! return Meteor.callPromise(methodName, value1, value2); } });

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RETURN A PROMISE FROM A HELPER // {{meteorPromise "addWithSleep" "arg1" "arg2"}} Template.example1Demo.helpers({ meteorPromise: ReactivePromise(theHelperFunction, {pending: "loading..."}) }); var theHelperFunction = function (methodName, part1, part2) { var template = Template.instance(), value1 = template[part1].get(), value2 = template[part2].get(); return Meteor.callPromise(methodName, value1, value2); }

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TMTOWTDI CALLBACKS, PROMISES, FIBERS, REACTIVITY, AND ES6 GENERATORS ARE ALL WAYS TO BE ASYNC! (YOU CAN ALWAYS CONVERT)

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QUESTIONS ??

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PROMISES Я AWESOME

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PROMISES Я AWESOME AND SO ARE YOU!

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