The document discusses common patterns for asynchronous JavaScript including callbacks, promises, and generators. It provides examples of using callbacks for event handling and avoiding conflicts. It then introduces promises as a better solution than nested callbacks and shows how to chain and wait for multiple promises. Finally, it presents generators as a cleaner way to write asynchronous code that looks synchronous using yield and async/await syntax.

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Asynchronous Javascript
Common and useful patterns
by Steven Rémot

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Table of contents
1. Callbacks
2. Promises
3. Generators

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Table of contents
1. Callbacks
2. Promises
3. Generators

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Callbacks - Usage
function GameController() {
var dirChangedCb = function () {};
this.onDirectionChanged = function (callback) {
dirChangedCb = callback;
};
document.addEventListener('keydown', function (event) {
dirChangedCb(determineEventDirection(event));
});
}
var gc = new GameController();
gc.onDirectionChanged(function (direction) {
hero.move(direction);
});

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Callbacks - Avoiding conflicts
function GameController() {
var dirChangedCb = function () {};
this.onDirectionChanged = function (callback) {
dirChangedCb = callback;
};
document.addEventListener('keydown', function (event) {
dirChangedCb(determineEventDirection(event));
});
}
var gc = new GameController();
gc.onDirectionChanged(function (direction) {
hero.move(direction);
});
gc.onDirectionChanged(function (direction) {
enemy.reactTo(direction);
});
Only one callback is saved

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Callbacks - Avoiding conflicts
function GameController() {
var dirChangedCbs = [];
this.onDirectionChanged = function (callback) {
dirChangedCbs.push(callback);
};
document.addEventListener('keydown', function (event) {
for (var i = 0; i < dirChangedCbs.length; i++) {
dirChangedCbs[i](determineEventDirection(event));
}
});
}
var gc = new GameController();
gc.onDirectionChanged(function (direction) {
hero.move(direction);
});
gc.onDirectionChanged(function (direction) {
enemy.reactTo(direction);
});

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Callbacks - Nested operations
function loadHero(loadingFinishedCallback) {
http.get('/characters/hero.json', function (heroData) {
loadingFinishedCallback({ data: heroData });
});
}

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Callbacks - Nested operations
function loadHero(loadingFinishedCallback) {
http.get('/characters/hero.json', function (heroData) {
http.get(heroData.modelUrl, function (model) {
loadingFinishedCallback({
data: heroData,
model: model
});
});
});
}

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Callbacks - Nested operations
function loadHero(loadingFinishedCallback) {
http.get('/characters/hero.json', function (heroData) {
http.get(heroData.modelUrl, function (model) {
http.get(model.materialUrl, function (material) {
loadingFinishedCallback({
data: heroData,
model: model,
material: material
});
});
});
});
}

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Callbacks - Nested operations
function loadHero(loadingFinishedCallback) {
http.get('/characters/hero.json', function (heroData) {
loadGraphics(heroData.modelUrl, function (graphics) {
loadingFinishedCallback({
data: heroData,
model: graphics.model,
material: graphics.material
});
});
});
}

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Callbacks - Limits
● Nested callbacks are not handy
● Even worse with error catching
● Waiting for multiple callbacks is hard
Callbacks are bad for async operations

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Table of contents
1. Callbacks
2. Promises
3. Generators

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Promises - Basics
function httpGet(url) {
return new Promise(function (resolve) {
http.get(url, function (data) {
resolve(data);
});
});
}

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Promises - Basics
function httpGet(url) {
return new Promise(function (resolve) {
http.get(url, resolve);
});
}

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Promises - Basics
function httpGet(url) {
return new Promise(function (resolve, reject) {
http.get(url, resolve, reject);
});
}

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Promises - Chaining
function loadHero() {
return httpGet('/characters/hero.json')
.then(function (heroData) {
return {
data: heroData
};
});
}
loadHero().then(function (hero) {
console.log(hero.data);
});

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Promises - Chaining
function loadHero() {
return httpGet('/characters/hero.json')
.then(function (heroData) {
return httpGet(heroData.modelUrl).then(function (model) {
return {
data: heroData,
model: model
};
});
});
}
loadHero().then(function (hero) {
console.log(hero.data);
console.log(hero.model);
});

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Promises - Chaining
function loadHero() {
var hero = {};
return httpGet('/characters/hero.json')
.then(function (heroData) {
hero.data = heroData;
return httpGet(heroData.modelUrl);
})
.then(function (model) {
hero.model = model;
return hero;
});
}
loadHero().then(function (hero) {
console.log(hero.data);
console.log(hero.model);
});

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Promises - Chaining
function loadHero() {
var hero = {};
return httpGet('/characters/hero.json')
.then(function (heroData) {
hero.data = heroData;
return httpGet(heroData.modelUrl);
})
.then(function (model) {
hero.model = model;
return httpGet(model.materialUrl);
})
.then(function (material) {
hero.material = material;
return hero;
});
}

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Promises - Chaining
function loadHero() {
var hero = {};
return httpGet('/characters/hero.json')
.then(function (heroData) {
hero.data = heroData;
return httpGet(heroData.modelUrl);
})
.then(function (model) {
hero.model = model;
return httpGet(model.materialUrl);
})
.then(function (material) {
hero.material = material;
return hero;
})
.catch(function (error) {
console.log(error);
return null;
});

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Promises - Waiting all promises
function loadObjects() {
return Promise.all([
loadHero(),
loadEnemy(),
loadDoor(),
loadWall(),
// ...
]);
}
loadObjects().then(function (objects) {
var hero = objects[0];
var enemy = objects[1];
// ...
});

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Promises - How to get them
● In Ecmascript 6 standard library
● Available through polyfills:
https://github.com/jakearchibald/es6-promise
● Alternative APIs:
https://github.com/kriskowal/q
https://github.com/petkaantonov/bluebird

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Promises
Nice, but…
● Verbose
● Still writing in an asynchronous way
We can do better!

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Table of contents
1. Callbacks
2. Promises
3. Generators

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Generator - Basics
function* counter() {
for (var i = 0; i < 10; i++) {
yield i;
}
}
var count = counter();
console.log(count.next()); // 0
console.log(count.next()); // 1
// ...

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Generator - Basics
function* loadHero()
{
var hero = {};
hero.data = yield httpGet('/characters/hero.json');
hero.model = yield httpGet(hero.data.modelUrl);
hero.material = yield httpGet(hero.model.materialUrl);
return hero;
}
spawn(loadHero).then(function (hero) {
console.log(hero); // { data: <...>, model: <...>, ... }
});

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Generator - Basics
function* loadHero()
{
var hero = {};
try {
hero.data = yield httpGet('/characters/hero.json');
hero.model = yield httpGet(hero.data.modelUrl);
hero.material = yield httpGet(hero.model.materialUrl);
} catch (error) {
console.log(error);
hero = null;
}
return hero;
}
spawn(loadHero).then(function (hero) {
console.log(hero); // { data: <...>, model: <...>, ... }
});

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Generator - Basics
async function loadHero()
{
var hero = {};
try {
hero.data = await httpGet('/characters/hero.json');
hero.model = await httpGet(hero.data.modelUrl);
hero.material = await httpGet(hero.model.materialUrl);
} catch (error) {
console.log(error);
hero = null;
}
return hero;
}
loadHero().then(function () {
console.log(hero); // { data: <...>, model: <...>, ... }
});

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Generator - Browser support
● Generator are ES6
● Async functions are ES7
● Use them today with transpilers:
https://github.com/google/traceur-compiler
https://babeljs.io/

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Thanks for listening!
References:
https://developer.mozilla.org/fr/docs/Web/JavaScript/Reference/Objets_globaux/Promise
https://www.promisejs.org/
https://developer.mozilla.org/fr/docs/Web/JavaScript/Reference/Instructions/function*
https://github.com/lukehoban/ecmascript-asyncawait

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