Category Archive Get promise value react


Get promise value react

By using our site, you acknowledge that you have read and understand our Cookie PolicyPrivacy Policyand our Terms of Service. The dark mode beta is finally here. Change your preferences any time. Stack Overflow for Teams is a private, secure spot for you and your coworkers to find and share information. The example below is copied verbatim from their docs with their comment included:.

I'm not clear how this works. If I can call. It is not a Number. So what do they mean by "its value will be the result of promiseA incremented by 1"?

Am I supposed to access that as promiseB. I'm missing something.

get promise value react

Accessing the value of promiseB is done in the same way we accessed the result of promiseA. You can now write:. Now there is no promiseB, because we've unwrapped the result from promiseA using awaitand you can work with it directly.

However, await can only be used inside an async function. So to zoom out slightly, the above would have to be contained like so:. The then call returns promiseB immediately.

When promiseA is resolved, it will pass the result to promiseA's success handler. This states that promiseB is a promise but will be resolved immediately after promiseA is resolved. Another way of looking at this means that promiseA.

This means that the value that promiseA resolved to is the value that promiseB will receive as its successCallback value:.

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However, there is a way to access the promise's value directly after it has been resolved by using the following unsupported internal node. RTCPeerConnection w.

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This example I find self-explanatory. Notice how await waits for the result and so you miss the Promise being returned.

Fetch API Introduction

The line with await would normally return a promise. To confirm that the asynchronous function has returned, you can log connection for example, and then you're ready to use the variable.

One of course wouldn't want to count on the asynchronous function being resolved yet for any code in the script outside the asynchronous function. Here the repository. I assign it to the variable account within the then statement. Learn more. How to access the value of a promise? Ask Question. Asked 5 years ago. Active 3 months ago. Viewed k times. I asked a related question: Why does Promise not have a get function?

Does this answer your question?It also provides several other useful promise-related concepts, such as joining multiple promises and mapping and reducing collections of promises. If you've never heard about promises before, read this first.

A Deferred represents a computation or unit of work that may not have completed yet. Typically but not alwaysthat computation will be something that executes asynchronously and completes at some point in the future.

While a deferred represents the computation itself, a Promise represents the result of that computation. Thus, each deferred has a promise that acts as a placeholder for its actual result. A deferred represents an operation whose resolution is pending. It has separate promise and resolver parts. See Promise for more information. Returns the promise of the deferred, which you can hand out to others while keeping the authority to modify its state to yourself.

Resolves the promise returned by promise. Rejects the promise returned by promisesignalling that the deferred's computation failed. Triggers progress notifications, to indicate to consumers that the computation is making progress toward its result. A promise represents an eventual outcome, which is either fulfillment success and an associated value, or rejection failure and an associated reason.

Once in the fulfilled or rejected state, a promise becomes immutable. Neither its state nor its result or error can be modified. Transforms a promise's value by applying a function to the promise's fulfillment or rejection value. Returns a new promise for the transformed result. The then method registers new fulfilled, rejection and progress handlers with a promise all parameters are optional :. A promise makes the following guarantees about handlers registered in the same call to then :.

Since the purpose of done is consumption rather than transformation, done always returns null. Similar asynchronous code with doSomething that returns a promise can be written:. A cancellable promise provides a mechanism for consumers to notify the creator of the promise that they are not longer interested in the result of an operation. The cancel method notifies the creator of the promise that there is no further interest in the results of the operation. Once a promise is settled either fulfilled or rejectedcalling cancel on a promise has no effect.

The promise constructor receives a resolver function and an optional canceller function which both will be called with 3 arguments:. If the resolver or canceller throw an exception, the promise will be rejected with that thrown exception as the rejection reason. The resolver function will be called immediately, the canceller function only once all consumers called the cancel method of the promise. It's recommended to use resolve for creating resolved promises.

It's recommended to use reject for creating rejected promises. All functions working on promise collections like allracesome etc.By using our site, you acknowledge that you have read and understand our Cookie PolicyPrivacy Policyand our Terms of Service. Software Engineering Stack Exchange is a question and answer site for professionals, academics, and students working within the systems development life cycle.

It only takes a minute to sign up. Can the promise be made to return the data found in the genres?

Introduction to ES6 Promises – The Four Functions You Need To Avoid Callback Hell

Is there a better way to achieve the spec description? It sounds like you aren't understanding how promises are used. You return a promise. Then, later when your code resolves the promise, it resolves it with a result and that result is passed to the. Unlike using. It's also much nicer to look atsince it's consistent with the rest of JavaScript, than. Await is supported in all current browsers and node. Sign up to join this community. The best answers are voted up and rise to the top.

Home Questions Tags Users Unanswered. How do I make a JavaScript promise return something other than a promise? Ask Question. Asked 4 years, 11 months ago. Active 4 months ago. Viewed k times. When it resolves, it returns an array. If given an array of genres, ['comedy', 'drama', 'action'] Here is a skeleton method with a promise: MovieLibrary.

Jesse C. Slicer 5, 2 2 gold badges 28 28 silver badges 42 42 bronze badges. Promises don't "return" values, they pass them to a callback which you supply with. The spec simply sounds confused to me.Apart from being new and shiny, Promises are a great way to clean up your code, reduce dependencies on external libraries, and prepare yourself for async and await in ES7.

That said, promises can take a lot of work to understand. They feel completely different to the callbacks which we often use to accomplish the same thing, and there are a few surprises in their design which often cause beginners hours of debugging pain. Up until promises arrived, developers in JavaScript land had been using callbacks. Our code will look something like this:. Looks awful, right? It gets so bad, in fact, that people have come up with a name for it — callback hell.

And these callback pyramids appear everywhere — in handling HTTP requests, database manipulation, animation, inter-process communication, and all manner of other places. There is one exception, though:. Perhaps the easiest way to grok promises is to work with what you already know and contrast them with callbacks. There are four major differences:. Callbacks are just blocks of code which can be run in response to events such as as timers going off or messages being received from the server.

Any function can be a callback, and every callback is a function. Promises are objects which store information about whether or not those events have happened yet, and if they have, what their outcome is. Callbacks are defined independently of the functions they are called from — they are passed in as arguments.

These functions then store the callback, and call it when the event actually happens. Promises are created inside of asynchronous functions those which might not return a response until laterand then returned.

When an event happens, the asynchronous function will update the promise to notify the outside world. Callbacks are generally called with information on whether an operation succeeded or failed, and must be able to handle both scenarios.

get promise value react

Promises can only represent one event — they are either successful once, or failed once. ES6 Promises are instances of the Promise built-in, and are created by calling new Promise with a single function as an argument.

For example:. Running new Promise will immediately call the function passed in as an argument. In order to do so, the function you pass to the constructor can take two arguments, which are themselves callable functions — resolve and reject. Calling resolve value will mark the promise as resolved and cause any success handlers will be run. Call reject errorwill cause any failure handler to be run.

You should not call both. The above example uses the setTimeout function, which takes a callback — but we want to return a promise instead. If an isAnimationSupported step function is available, we could implement this with reject :. Finally, it is important to note that if an exception is thrown within the passed-in function, the promise will automatically be marked as rejected, with the exception object being stored as the rejected value, just as if it had been passed as the argument to reject.

Now you understand how to create a promise. Depending on which arguments you supply, you can handle success, failure, or both:.JavaScript is single threaded, meaning that two bits of script cannot run at the same time; they have to run one after another.

In browsers, JavaScript shares a thread with a load of other stuff that differs from browser to browser. But typically JavaScript is in the same queue as painting, updating styles, and handling user actions such as highlighting text and interacting with form controls. Activity in one of these things delays the others. As a human being, you're multithreaded.

You can type with multiple fingers, you can drive and hold a conversation at the same time.

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The only blocking function we have to deal with is sneezing, where all current activity must be suspended for the duration of the sneeze. That's pretty annoying, especially when you're driving and trying to hold a conversation.

You don't want to write code that's sneezy. This isn't sneezy at all. We get the image, add a couple of listeners, then JavaScript can stop executing until one of those listeners is called. 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:.

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This doesn't catch images that errored before we got a chance to listen for them; unfortunately the DOM doesn't give us a way to do that. Also, this is loading one image.

Things get even more complex if we want to know when a set of images have loaded. Events are great for things that can happen multiple times on the same object— keyuptouchstart etc. With those events you don't really care about what happened before you attached the listener. 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:. Domenic Denicola proof read the first draft of this article and graded me "F" for terminology.

get promise value react

He put me in detention, forced me to copy out States and Fates 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:. The spec also uses the term thenable to describe an object that is promise-like, in that it has a then method.

If you're a jQuery user, they have something similar called Deferreds. Although promise implementations follow a standardized behaviour, their overall APIs differ.The Promise. If the value is a promise, that promise is returned; if the value is a thenable i.

get promise value react

This function flattens nested layers of promise-like objects e. A Promise that is resolved with the given value, or the promise passed as value, if the value was a promise object. The static Promise.

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Last modified: Jan 22,by MDN contributors. Related Topics. Standard built-in objects Promise Methods Promise. Learn the best of web development Get the latest and greatest from MDN delivered straight to your inbox.

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Opera Full support Safari Full support 8. WebView Android Full support 4. Chrome Android Full support Firefox Android Full support Opera Android Full support Safari iOS Full support 8. Samsung Internet Android Full support 2.This post is part of the series 30 Days of React. In this series, we're starting from the very basics and walk through everything you need to know to get started with React. If you've ever wanted to learn React, this is the place to start!

Today, we're going to look at what we need to know to understand Promises from a high-level, so we can build our applications using this incredibly useful concept. Yesterday we installed the fetch library into our create-react-app project we started on day Today we'll pick up from yesterday discussing the concept and the art of Promises.

As defined by the Mozilla, a Promise object is used for handling asynchronous computations which has some important guarantees that are difficult to handle with the callback method the more old-school method of handling asynchronous code.

A Promise object is simply a wrapper around a value that may or may not be known when the object is instantiated and provides a method for handling the value after it is known also known as resolved or is unavailable for a failure reason we'll refer to this as rejected. Using a Promise object gives us the opportunity to associate functionality for an asynchronous operation's eventual success or failure for whatever reason. It also allows us to treat these complex scenarios by using synchronous-like code.

For instance, consider the following synchronous code where we print out the current time in the JavaScript console:. This is pretty straight-forward and works as the new Date object represents the time the browser knows about. Now consider that we're using a different clock on some other remote machine.

For instance, if we're making a Happy New Years clock, it would be great to be able to synchronize the user's browser with everyone elses using a single time value for everyone so no-one misses the ball dropping ceremony.

Suppose we have a method that handles getting the current time for the clock called getCurrentTime that fetches the current time from a remote server. We'll represent this now with a setTimeout that returns the time like it's making a request to a slow API :.


Our console. Traditionally, we can update the code using a callback to get called when the time is available:. What if there is an error with the rest? How do we catch the error and define a retry or error state? Now, what if we want to make a request based upon the first request's value?

As a short example, let's reuse the getCurrentTime function inside again as though it were a second method, but allows us to avoid adding another complex-looking function :. Dealing with asynchronousity in this way can get complex quickly. In addition, we could be fetching values from a previous function call, what if we only want to get one Using promises, on the other hand helps us avoid a lot of this complexity although is not a silver bullet solution.

The previous code, which could be called spaghetti code can be turned into a neater, more synchronous-looking version:.

JavaScript Promises: An introduction

To catch the value on success, we'll use the then function available on the Promise instance object. The then function is called with whatever the return value is of the promise itself.

For instance, in the example above, the getCurrentTime function resolves with the currentTime value on successful completion and calls the then function on the return value which is another promise and so on and so forth. To catch an error that occurs anywhere in the promise chain, we can use the catch method. We're using a promise chain in the above example to create a chain of actions to be called one after another. A promise chain sounds complex, but it's fundamentally simple.

Essentially, we can "synchronize" a call to multiple asynchronous operations in succession. Each call to then is called with the previous then function's return value. For instance, if we wanted to manipulate the value of the getCurrentTime call, we can add a link in the chain, like so:.


A pending promise can only ever lead to either a fulfilled state or a rejected state once and only oncewhich can avoid some pretty complex error scenarios. This means that we can only ever return a promise once. If we want to rerun a function that uses promises, we need to create a new one.

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Mooguzilkree Posted on10:12 pm - Oct 2, 2012

Wacker, Ihr Gedanke ist sehr gut