4/3/2023 0 Comments Actionally awaiting a call![]() ![]() When the Promise state moves to Resolved, execution of the calling async function can continue. If you await a Promise, execution of your function will pause while the Promise state is Pending. Whenever you await an async function you’re actually awaiting the Promise it returns: await myFunctionAsync() /************ same thing as *************/ var myPromise = myFunctionAsync() await myPromise Most of the time though you’ll be awaiting a Promise returned by a function. Most of the time you don’t actually need to create your own Promise - if you write code inside an async function your return value will be wrapped in a Promise automatically - but occasionally the need may arise. In the same way you can await an async function, you can await a Promise directly. Resolved (async operation completed successfully).awaiting PromisesĪ Promise represents an asynchronous operation. If you forget either of these points it’s no big deal: the IDE should remind you when you try to use await. All async functions have to return a Promise. Second, getTemperatureNowAsync() returns a Promise. You can only use await inside an async function. First getTemperatureNowAsync()is proceeded by the async keyword. This is part of the reason async and await are so useful - you don’t have to switch styles when working with sync vs async code. This looks really similar to synchronous code in fact the only real difference is the inclusion of the word await. This means that on line 3 (and beyond) we can use the results of that API call. It won’t resume execution until the API responds. When I await my third party API call on line 2, getTemperatureNowAsync() stops executing. I can await that async function like this: Any function which returns a Promise is awaitable! If I wanted to be extra careful though I could check to see that getWeatherAsync() returns a Promise. The package developers have been really helpful and included Async on the end of the method name which tells me this is an async function. The method I care about is client.getWeatherAsync(). The third party API I’m relying on has put out a package to help me call it correctly. The main takeaway is that when your app needs to wait for some result to be available this is something we want to do asynchronously wherever possible. Option B avoids blocking which is super important for real applications you don’t want one user’s request to prevent others from being fulfilled or to prevent other background operations from running. ![]() During that time it would not be able to handle getTime() requests and the clock would stop ticking as a result. This is because the Node.js server would be busy waiting for the third-party API. It’s actually pretty hard to block like this in JavaScript, but if you pulled it off you’d notice that the clock in the UI would freeze for 10 seconds every hour. ![]() This is where your application is stuck waiting for something without actually doing any work itself. Eventually server will see the third-party API has responded, at that point parse response and return temp to UI.Periodically check to see if the third-party API has responded, if not, go back to doing other stuff.For example, when getTime() requests come in it fulfils those right away Server works on whatever else it has to do.Handle the ~10 getTime() requests that have backed up since server started waiting. ![]() Get response and return temperature to UI.Neglect other duties to diligently wait for third-party API response.Ask the third-party API for the weather.Your server has two options when fulfilling the getTemperature() request: Extracts temp from that weather response and then returns it.Īssume the third-party API is really slow -it takes 10 seconds or so to fulfil each request. getTemperature() - Calls on a third-party API for the weather.getTime() -Gets the current time using new Date().getTime().The Node.js server is the thing we care about here. ![]()
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |