We could alter our get function to use the json responseType, but we could also solve it in promises land: get en(function(response) return rse(response ).then(function(response) console. response ) Since rse takes a single argument and returns a transformed value, we can make a shortcut: get en(en(function(response) console. response ) see that in action here, check the console in devtools to see the result. In fact, we could make a getjson function really easily: function getjson(url) return get(url).then(rse getjson still returns a promise, one that fetches a url then parses the response as json. Queuing asynchronous actions you can also chain thens to run async actions in sequence. When you return something from a then callback, it's a bit magic.
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Onload function / This is called even on 404 etc / so check the status if (atus 200) / Resolve the promise with dissertation the response text resolve(sponse else / Otherwise reject with the status text / which will hopefully be a meaningful error reject(Error(atusText ;. Onerror function reject(Error Network Error ; / make the request nd now let's use it: get en(function(response) console. response, function(error) ror failed! error ) Click here to see that in action, check the console in devtools to see the result. Now we can make http requests without manually typing xmlhttpRequest, which is great, because the less I have to see the infuriating camel-casing of xmlhttpRequest, the happier my life will. Chaining then isn't the end of the story, you can chain thens together to transform values or run additional async actions one after another. Transforming values you can transform values simply by returning the new value: var promise new Promise(function(resolve, reject) resolve(1 en(function(val) console. Log(val / 1 return val 2; ).then(function(val) console. Log(val / 3 ) As a practical example, let's go back to: get en(function(response) console. response ) The response is json, but we're currently receiving it as plain text.
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If a promise has succeeded or failed and you later add a success/failure callback, the correct callback will be called, even though the event took place earlier. This is extremely useful for async success/failure, because you're less interested in the exact time something became available, and more interested in reacting to the outcome. Promise terminology domenic Denicola proof read the first draft of this article and graded me "F" for terminology. He put me in detention, forced me to copy out States and Fates 100 times, and wrote a worried letter to my parents. Despite that, i still get a lot of the terminology mixed up, but here are the basics: A promise can be: fulfilled - the action relating to the promise succeeded rejected - the action relating to the promise failed pending - hasn't fulfilled or rejected.
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Unfortunately, in the example above, it's possible that the events happened before we started listening for them, so we need to work around that using the "complete" property of images: var img1 g-1 function loaded / woo yey image loaded if (mplete) loaded else dEventListener. Also, this is loading one image, things get even writings more complex if we want to know when a set of images have loaded. Events aren't always the best way events are great for things that can happen multiple times on the same object—keyup, touchstart etc. With those events you don't really care about what happened before you attached the listener. But when it comes to async success/failure, ideally you want something like this: book / loaded / failed / and whenAllThesehaveloaded(img1, img2).callThis(function / all loaded / one or more failed this is what promises do, but with better naming. If html image elements had a "ready" method that returned a promise, we could do this: en(function / loaded, function / failed / and ady en(function / all loaded, function / one or more failed at their most basic, promises are a bit like event. It cannot succeed or fail twice, neither can it switch from success to failure or vice versa.
Fireworks explode, glittery paper rains from above, the guaranteed crowd goes wild. At this point you fall into one of these categories: people are cheering around you, but you're not sure what all the fuss is about. Maybe you're not even sure what a "promise". You'd shrug, but the weight of glittery paper is weighing down on your shoulders. If so, don't worry about it, it took me ages to work out why i should care about this stuff. You probably want to begin at the beginning. You punch the air!
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