JavaScript Promises - Thinking Sync in an Async World

by Kerrick Long

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JavaScript Promises Thinking Sync in an Async World then

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Kerrick Long Things I make and do Where to ﬁnd me online twitter.com/KerrickLong github.com/Kerrick Lead Front-end Developer! at Second Street KerrickLong.com www. meetup.com/STLEmber

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try {! console.log(getLatestGrade(getStudent(name)));! }! catch (error) {! logError(error);! }! finally {! logOut();! }!

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try {! console.log(getLatestGrade(getStudent(name)));! }! catch (error) {! logError(error);! }! finally {! logOut();! }! XMLHttpRequest

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getStudent(name, function(student) {! getLatestGrade(student, function(grade) {! console.log(grade);! logOut();! });! });!

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try {! console.log(getLatestGrade(getStudent(name)));! }! catch (error) {! logError(error);! }! finally {! logOut();! }! Awesome

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getStudent(name)! .then(getLatestGrade)! .then(console.log)! .catch(logError)! .then(logOut)! ;! Promises

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Promise Basics

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Promise Basics Consuming Promises

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Promise Basics Consuming Promises Creating Promises

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Promise Basics Consuming Promises Creating Promises Advanced Techniques

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Promise Basics

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Getting Promises

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RSVP.js Q.js • Bluebird

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RSVP.js Q.js • Bluebird Promises/A+

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Promise Guarantees

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Always Async

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Always Async Returns a Promise

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Always Async Returns a Promise Handled Once

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Always Async Returns a Promise Handled Once Then’able

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Promise States

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Promise States Pending Fulﬁlled Rejected

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Promise States Pending Fulﬁlled Rejected } Settled

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Promise States Pending Fulﬁlled Rejected

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Promise States Pending Fulﬁlled Rejected Value Reason

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Promise States Pending Fulﬁlled Rejected Return Value Thrown Exception

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Consuming Promises

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Promise Prototype Methods

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getJSON('/comments')! .then(onFulfilled, onRejected) Promise.prototype.then

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getJSON('/comments')! .then(function(comments) {! if (comments) return 'Good'! else throw new Error('Bad')! }, function(reason) {! // handle getJSON errors! }) Promise.prototype.then

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getJSON('/comments')! .then(fulfillOne, rejectOne)! .then(fulfillTwo)! .then(null, rejectTwo) Promise.prototype.then

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getJSON('/comments')! .then(fulfillOne, rejectOne)! .then(fulfillTwo)! .then(null, rejectTwo) Promise.prototype.then THROW

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getJSON('/comments')! .then(fulfillOne, rejectOne)! .then(fulfillTwo)! .then(null, rejectTwo) Promise.prototype.then THROW

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getJSON onFulﬁlled onRejected getJSON onFulﬁlled onRejected

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getJSON('/comments') // waiting...! .then(extractFirstId)! .then(getCommentById) // waiting...! .then(showComment) Promise.prototype.then

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var promise = getJSON('/comments');! somethingElse();! promise.then(onFulfilled, onRejected); Promise.prototype.then

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getJSON('/comments')! .then(null, onRejected) Promise.prototype.catch

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getJSON('/comments')! .catch(onRejected) Promise.prototype.catch

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Promise Static Methods

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Promise.cast('Hi you!')! .then(onFulfilled, onRejected) Promise.cast

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Promise.cast(seededRandom(42))! .then(onFulfilled, onRejected) Promise.cast

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Promise.cast($.ajax(config))! .then(onFulfilled, onRejected) Promise.cast

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$.ajax(config)! .then(onFulfilled, onRejected)

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$.ajax(config)! .then(onFulfilled, onRejected) Lies, all LIES!

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Promise.cast($.ajax(config))! .then(onFulfilled, onRejected) Promise.cast

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var promises = [ getJSON('/a'),! getJSON('/b'), getJSON('/c') ]! ! Promise.all(promises)! .then(allFulfilled, firstRejected) Promise.all

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var promises = [ getJSON('/a'),! 'Hi you!', 42, getJSON('/c') ]! ! Promise.all(promises)! .then(allFulfilled, firstRejected) Promise.all

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var promises = [ saveTo(server1),! saveTo(server2), saveTo(server3) ]! ! Promise.race(promises)! .then(firstFulfilled, firstRejected) Promise.race

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Creating Promises

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new Promise()

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function() {! var name = prompt('Your name?')! if (!name)! throw new Error('Rude user!')! else! return name! } new Promise()

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new Promise(function(fulfill, reject) {! var name = prompt('Your name?')! if (!name)! throw new Error('Rude user!')! else! return name! }) new Promise()

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new Promise(function(fulfill, reject) {! var name = prompt('Your name?')! if (!name)! reject(new Error('Rude user!'))! else! fulfill(name)! }) new Promise()

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Shortcuts

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new Promise(function(fulfill, reject) {! fulfill(something)! }) Promise.resolve()

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new Promise(function(fulfill, reject) {! fulfill(something)! })! Promise.resolve(something) Promise.resolve()

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new Promise(function(fulfill, reject) {! reject(something)! }) Promise.reject()

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new Promise(function(fulfill, reject) {! reject(something)! })! Promise.reject(something) Promise.reject()

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Advanced Techniques

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this.get('name') // 'Kerrick'! ! getJSON('/comments')! .then(function(comments) {! this.get('name') // throws! }) this and bind()

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this.get('name') // 'Kerrick'! var self = this! getJSON('/comments')! .then(function(comments) {! self.get('name') // 'Kerrick'! }) this and bind()

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this.get('name') // 'Kerrick'! ! getJSON('/comments')! .then(function(comments) {! this.get('name') // 'Kerrick'! }.bind(this)) this and bind()

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getJSON('/comments')! .then(function(comments) {! throw new Error('Hello?')! }) Absorbed Rejections

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getJSON('/comments')! .then(function(comments) {! throw new Error('Hello?')! })! .catch(console.error.bind(console)) Absorbed Rejections

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getStudent(name) // ! .then(getLatestGrade)! .then(log)! .catch(logError)! .then(logOut) Promise-aware Functions

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log(getLatestGrade(getStudent(name)))! .catch(logError)! .then(logOut) Promise-aware Functions

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function getLatestGrade(promise) {! return Promise.cast(promise)! .then(function(value) {! // getLatestGrade Logic! })! } Promise-aware Functions