This document discusses the concept of promises in JavaScript, explaining their role in managing asynchronous tasks by representing future values and events. It contrasts promises with traditional callbacks, highlighting the clarity and error handling benefits promises provide. Additionally, it illustrates various coding patterns and use cases for promises, including event handling and integration within frameworks like Meteor.

1. PROMISESTHE UNSUNG HEROES OF ASYNC IN
JAVASCRIPT
DEAN RADCLIFFE • OK GROW! •
@DEANIUSDEV

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

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

4. 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

5. 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

6. 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)

7. 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)

8. 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

9. 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)=>{})

10. 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)

11. 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)

12. 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)

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

14. COMIC RELIEF
CAT PICTURE

15. 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")

16. 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")

17. PROVIDED ITS LOADED, 1
➢ DomContentLoadedPromise.then(function () {
console.log('Im ready')
});
↩ Promise {[[PromiseStatus]]: "resolved"}
-----
Im ready

18. PROVIDED ITS LOADED, 2
➢ DomContentLoadedPromise.then(function () {
console.log('Im ready')
});
↩ Promise {[[PromiseStatus]]: "pending"}
-----

19. PROVIDED ITS LOADED, 2..
➢ DomContentLoadedPromise.then(function () {
console.log('Im ready')
});
↩ Promise {[[PromiseStatus]]: "pending"}
-----
...

20. PROVIDED ITS LOADED, 3
➢ DomContentLoadedPromise.then(function () {
console.log('Im ready')
});
↩ Promise {[[PromiseStatus]]: "pending"}
-----
...
_____
Im ready

21. 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

22. 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

23. 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);
}
}

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

25. THE EVENT LOOP
(AKA TASK
QUEUE)

26. THE EVENT LOOP (AKA TASK QUEUE)

27. THE 2 WAYS OF
CODING JS

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

29. 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())
})

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

31. PATTERN
RETURNING A PROMISE FROM
A METEOR METHOD

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

33. 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

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

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

36. 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)
}));
}
})

37. 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);
}
})

38. ASYNC SHEEP

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

40. 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)

41. 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)

42. var ComicReliefCatPicture; // TODO fill in

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

44. 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>

45. 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);
}
});

46. 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);
}

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

48. QUESTIONS ??

49. PROMISES Я AWESOME

50. PROMISES Я AWESOME
AND SO ARE YOU!

51. DEAN RADCLIFFE, OK GROW!
@DEANIUSDEV