JavaScript Promises

JavaScript Promises
Pulak (@pulakb)
http://pulakonline.com/

Discussions
• Callbacks
• JavaScript Promises
– jQuery
– ‘q’ library
– AngularJs
– Node.js

• What is a Callback function?
• How Callback function work?
• ‘Callback hell’

What is a Callback function?
• A callback function (say, Y) is a function that is passed to another function
(say, X) as a parameter, and Y gets executed inside X.
• JavaScript uses callback functions to handle asynchronous control flow.
1
2
3
$( "#dataRow" ).on( "click", function() {
alert(‘hello’);
});

How Callback function work?
function writeCode(callback) {
//do something
callback();
//…..
}
function introduceBugs() {
//…. Make bugs
}
writeCode(introduceBugs)

fs = require('fs');
fs.readFile ('f1.txt','utf8',function(err,data) {
if (err) {
return console.log(err);
}
console.log(data);
});

fs = require('fs');
fs.readFile('f1.txt','utf8',function(err,data){
if (err) {
}
console.log(data);
});

fs = require('fs');
fs.readFile('f1.txt','utf8',
function(err,data){
if (err) {
}
console.log(data);
}
);
Equivalent
formatting

‘Callback hell’
• Code complexity
• Modularity
• Maintainability
• Tightly coupled interface

‘Callback hell’
When working with callbacks, nesting of functions can make reading
and understanding the code very difficult

Promises
• What is Promise?
• What is Deferred?
• Deferred & Promise
• What are the use cases of Promise?
• What does Promises guarantee?
• Why promises are awesome?

What is Promise?
• A promise is an object that represents the return value or the
thrown exception that the function may eventually provide.
• In other words, a promise represents a value that is not yet known.
A promise is an asynchronous value.
• The core idea behind promises is that a promise represents the
result of an asynchronous operation.
• A Promise has 3 possible states
– Pending
– Fulfilled
– Rejected

Deferred & Promise
• A deferred (object) represents a work that is not yet finished.
• A promise (property) is a placeholder for a result which is initially
unknown.
• Every deferred has a promise which functions as a proxy for the future
result.
• From a semantic perspective this means that instead of calling a function
( callback ), we are able to return a value ( promise ).

What are the use cases of Promise?
• An AJAX request and callbacks(a single request, parallel/chained requests)
• An asynchronous loading of assets and actions to perform
• Animation
• Modal User Interaction

What does Promise guarantee?
promiseForResult.then(onFulfilled, onRejected);
• Only one of onFulfilled or onRejected will be called.
• onFulfilled will be called with a single fulfillment value (⇔ return value).
• onRejected will be called with a single rejection reason (⇔ thrown
exception).
• If the promise is already settled, the handlers will still be called once you
attach them.
• The handlers will always be called asynchronously.

What does Promise guarantee?
• At ﬁrst, a Promise is in a pending state. Eventually, it’s either resolved or
rejected.
• Once a Promise is resolved or rejected, it’ll remain in that state forever,
and its callbacks will never ﬁre again
• Promises can be chained

Why promises are awesome
• Cleaner method signatures
• Uniform return/error semantics
• Easy composition
• Easy sequential/parallel join
• Always async
• Exception-style error bubbling

Case 1: Simple Functional Transform
var author = getAuthors();
var authorName = author.name;
// becomes
var authorPromise = getAuthors().then(function (author) {
return author.name;
});

Case 2: Reacting with an Exception
var author = getAuthors();
if (author === null)
throw new Error("null author!");
becomes
var authorPromise = getAuthors().then(function (author) {
if (author === null)
throw new Error("null author!");
return author;
});

Case 3: Handling an Exception
try {
updateAuthor(data);
} catch (ex) {
console.log("There was an error:", ex);
}
// becomes
var updatePromise = updateAuthor(data).then(undefined, function (ex) {
console.log("There was an error:", ex);
});

Case 4: Rethrowing an Exception
try {
updateAuthor(data);
} catch (ex) {
throw new Error("Updating author failed. Details: " + ex.message);
}
// becomes
var updatePromise = updateAuthor(data).then(undefined, function (ex) {
throw new Error("Updating author failed. Details: " + ex.message);
});

Async Case: Waiting
var name = promptForNewAuthorName();
updateAuthor({ name: name });
refreshScreen();
// becomes
promptForNewAuthorName()
.then(function (name) {
return updateAuthor({ name: name });
})
.then(refreshScreen);

jQuery - q differences
• https://github.com/kriskowal/q/wiki/Coming-from-jQuery
jQuery Q Notes
then then
Q's then, and in fact all of its methods, have
different exception-handling behavior, as described
above.
done then
then does not support multiple handlers; use
multiple calls to then to attach them.
fail catch
catch does not support multiple handlers; use
multiple calls to catch to attach them.
deferred.promise(method) deferred.promise(property)
You *must* get the promise part of the deferred;
the deferred does not have the promise API.

AngularJS
– Limitations of Promise in Angular
In Angular's $Q implementation, If you don't fire off $scope.$apply(),
after resolving, the resolved values will never propagate to your 'then'
functions. Sometimes I want to use promises that aren't tied to my
Angular $digest cycle.

URLs
Credit goes to all the people for sharing their thoughts:
• http://wiki.commonjs.org/wiki/Promises/A
• http://promisesaplus.com/
• http://promisesaplus.com/differences-from-promises-a
• http://api.jquery.com/category/deferred-object/
• http://sitr.us/2012/07/31/promise-pipelines-in-javascript.html
• http://stackoverflow.com/questions/12160785/jquery-deferred-promise-design-patterns-and-use-cases
• http://stackoverflow.com/questions/6801283/what-are-the-differences-between-deferred-promise-and-future-in-javascript
• http://stackoverflow.com/questions/5436327/jquery-deferreds-and-promises-then-vs-done
• http://blog.mediumequalsmessage.com/promise-deferred-objects-in-javascript-pt1-theory-and-semantics
• https://gist.github.com/domenic/3889970
• http://domenic.me/2012/10/14/youre-missing-the-point-of-promises/
• https://github.com/kriskowal/q
• https://github.com/kriskowal/uncommonjs/blob/master/promises/specification.md
• http://james.padolsey.com/jquery/#v=2.0.3&fn=jQuery.Deferred
• http://www.dwmkerr.com/promises-in-angularjs-the-definitive-guide/
• http://spion.github.io/posts/why-i-am-switching-to-promises.html
• http://blog.ometer.com/2011/07/24/callbacks-synchronous-and-asynchronous/

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