promises promises
An introduction to Javascript Promises
WHAT?
What does that mean?
-
a way to refactor your callbacks so that you don't get so much nesting
-
a cleaner way to do error handling
You start off with
var user = login('vee');
var tweets = getTweetsFor(user);
display(tweets);
But That Blocks, so...
login('vee', function(user) {
getTweetsFor(user, function(tweets) {
display(tweets);
}
}
And what if something goes wrong?
try {
login('vee', function (user) {
try {
TwitterAPI.getTweetsFor(user, function() {
try {
display(tweets);
} catch (e) {
display("fail");
}
} );
} catch(e) {
display("fail");
}
});
} catch (e) {
display("fail");
}
THe problem
Enter Promises...
login('vee').
then(TwitterAPI.getTweetsFor ).
then(display,
function(error) {
display("fail");
});
Moar about promises
-
Thenables - do this, then do this, then do this...
- Callback aggregation
- Chaining <3
- Reduced nesting
- Badass error handling...
Error handling in async code
Try/catch on async code is like playing catch with a robot that randomly throws a baseball directly to your forehead.
— Luke Arduini (@luk) February 19, 2013
error handling 101
try {
(function() {
throw "river"
}());
} catch (e) {
console.log('cry me a ....' + e);
}
Async error handling fail
try {
setTimeout(function() {
throw "river";
}, 1000);
} catch (e) {
console.log('cry me a ....' + e); // or not
}
error handlers in promises
- Promises take a success callback and an error callback
- If a promise is fulfilled then the success is handler is called
- If a promise is rejected then the error handler is called
- If there isn't an error handler, then the error bubbles up the promise chain
Promise error handling win!
login('vee').
then(function (user) {
TwitterAPI.getTweetsFor(user);
}).
then(function (tweets) {
display(tweets);
}, function(error) {
display("fail");
});
But nobody's perfect...
- Hiding the async can be confusing - MAGIC
- jQuery implemented it badly, so some implementations don't interop
- Its not a standard yet