Write Better JavaScript with Promises

http://davidwalsh.name/write-javascript-promises

by clockwork_189 • 12 years ago

44 8 12 years ago

aroman• 12 years ago

This was a helpful and informative writeup, but it hasn't really convinced me that I should use promises.

I currently use use callbacks and caolan/async on node.js, using in-place callbacks where simple and async methods where things get complex. It's been working very well so far, and it avoids my biggest issue with promises, which is that it seems I'd need all the modules I want to use to support promises, which they surely don't, and mixing two styles seems worse than picking a poison.

Maybe someone can show me why my reasoning is flawed, though.

rlp• 12 years ago

I'm not a huge fan of promises on their own. I think they are clearer than straight callbacks, but it gets confusing as you do more complex things. However, they really start to shine when combined with generators. Being able to yield a promise makes code very clean and clear.

And you can always use something like bluebird's promisify if you're combining promises with existing callback code.

dmitrig01• 12 years ago

There are several promise libraries (the most promising among them seems to be Bluebird, though Q has some popularity as well), which can easily convert callback-style functions into promises. For example:

    var b = require('bluebird'), request = b.promisify(require('request'));
    request(...).then(...)

These libraries also have utilities for controlling callback flow -- e.g calling several promises in parallel, which is done in the article with jQuery's $.when, waiting until a certain number finish, etc

iamwil• 12 years ago

I hadn't heard of bluebird at all. Does it work for any library you require?

When I used promises for real in a section of my code processing data:

1) I found it hard to use mostly because of all the work I had to do to "lift" non promise code up into promises, before I could use them.

2) When chaining them, it wasn't clear how you would handle a "fan-out", where a piece of data generated an array, which you then had to process with callbacks.

3) Unless I inserted a fail() after every then(), it was hard to tell where exactly in the chain something failed, and the stack trace wasn't very helpful.

I suspect maybe I'm doing something wrong--that even though you can chain promises sequentially, you generally want to make functions using promises, that you can then chain? I still haven't seen well written promise code. Anyone got examples other than toy examples?

davexunit• 12 years ago

You've probably heard the talk around the water cooler about how promises are the future.

Just wait until the JavaScript developers learn about continuations!

andyzweb• 12 years ago

BiwaScheme has call/cc

Write Better JavaScript with Promises

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