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Although promise implementations follow a standardized behaviour, their overall APIs differ. Do something within the callback, perhaps async, then call resolve if everything worked, otherwise call reject. The benefit of Error objects is they capture a stack trace, making debugging tools more helpful.
Both are optional, so you can add a callback for the success or failure case only. To bring browsers that lack a complete promises implementation up to spec compliance, or add promises to other browsers and Node. Thankfully you can cast them to standard promises, which is worth doing as soon as possible: Since it has a then method, Promise.
However, sometimes deferreds pass multiple arguments to their callbacks, for example: Say we want to: Start a spinner to indicate loading Fetch some JSON for a story, which gives us the title, and urls for each chapter Add title to the page Fetch each chapter Add the story to the page Stop the spinner … but also tell the user if something went wrong along the way.
Transforming values You can transform values simply by returning the new value: We could alter our get function to use the JSON responseTypebut we could also solve it in promises land: In fact, we could make a getJSON function really easily: Queuing asynchronous actions You can also chain thens to run async actions in sequence.
If you return a value, the next then is called with that value. This is when promises really start to stand out from simple callback patterns. You could even make a shortcut method to get chapters: Error handling As we saw earlier, then takes two arguments, one for success, one for failure or fulfill and reject, in promises-speak: Note that the two code examples above do not behave the same, the latter is equivalent to: With then func1, func2func1 or func2 will be called, never both.
Instead it moves onto the catch callback. As a result, "Failed to show chapter" will be added to the page if any of the previous actions failed. The above becomes a non-blocking async version of: To do this, just rethrow the error.
We’ve just released initiativeblog.com with the biggest set of features we ever shipped David (the other one) presented on his blog the demo we built with Michel Rousseau: The Sponza demo.
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