The “producing code” takes whatever time it needs to produce the promised result, and the “promise” makes… The promise object returned by the new Promise constructor has these internal properties:
state — initially… ” promise.… We can attach handlers to settled promises
If a promise is pending, .then/catch/finally handlers… Promises are more flexible.
) and returns a new promise.… The new promise resolves when all listed promises are resolved, and the array of their results becomes… In case of an error, other promises are ignored
If one promise rejects, Promise.all immediately… If all of the given promises are rejected, then the returned promise is rejected with AggregateError… Summary.There are 6 static methods of Promise class:
Promise.all(promises) – waits for all promises
Promises provide a couple of recipes to do that.
In this chapter we cover promise chaining.… ), which in turn creates a new promise (resolved with 2 value).… get the same result – the result of that promise.… It will be treated the same way as a promise.… Promise.
Promise chains are great at error handling.… When a promise rejects, the control jumps to the closest rejection handler.… Implicit try…catch.The code of a promise executor and promise handlers has an "invisible try..catch… The same thing is possible for promises.… A similar thing happens with unhandled promise rejections.
, and wraps non-promises in it.… resumes it with the promise result.… The idea is that a third-party object may not be a promise, but promise-compatible: if it supports .then… Error handling.If a promise resolves normally, then await promise returns the result.… The await keyword before a promise makes JavaScript wait until that promise settles, and then:
If it
Promise handlers .then/.catch/.finally are always asynchronous.… That’s strange, because the promise is definitely done from the beginning.… Promise handlers always go through this internal queue.… How can we make code finished appear after promise done?… Summary.Promise handling is always asynchronous, as all promise actions pass through the internal “promise
It’s the conversion of a function that accepts a callback into a function that returns a promise.… But promises are more convenient, so it makes sense to promisify them.… It calls it providing its own callback that translates to promise resolve/reject.… Now loadScriptPromise fits well in promise-based code.… If we like promises more than callbacks (and soon we’ll see more reasons for that), then we will use
The next() method should return a promise (to be fulfilled with the next value).… This method must return the object with next() method returning a promise (2).… The next() method doesn’t have to be async, it may be a regular method returning a promise, but async… method to provide iterator
Symbol.iterator
Symbol.asyncIterator
next() return value is
any value
Promise… For async generators, the generator.next() method is asynchronous, it returns promises.
As we know, fetch returns a promise.… And JavaScript generally has no concept of “aborting” a promise.… When a fetch is aborted, its promise rejects with an error AbortError, so we should handle it, e.g. in
The browser starts the request right away and returns a promise that the calling code should use to get… First, the promise, returned by fetch, resolves with an object of the built-in Response class as soon… The promise rejects if the fetch was unable to make HTTP-request, e.g. network problems, or there’s no… Response provides multiple promise-based methods to access the body in various formats:
response.text… Or, the same without await, using pure promises
They are usually created by promises: an execution of .then/catch/finally handler becomes a microtask… Microtasks are used “under the cover” of await as well, as it’s another form of promise handling.… code shows first, because it’s a regular synchronous call.
promise shows second, because .then passes… Also promise handlers go through the microtask queue.
The import() expression.The import(module) expression loads the module and returns a promise that resolves
We can also use async/await with the help of a promise-based wrapper, like https://github.com/jakearchibald… Let’s use a thin promise wrapper https://github.com/jakearchibald/idb further in this chapter.… An uncaught error becomes an “unhandled promise rejection” event on window object.… For a promise wrapper and async/await the situation is the same.… The basic usage can be described with a few phrases:
Get a promise wrapper like idb.
We use browser methods in examples here
To demonstrate the use of callbacks, promises… One of the best ways is to use “promises”, described in the next chapter.
For instance, test if a built-in Promise object exists (it doesn’t in really old browsers):
returns the class for a custom element with the given name,
customElements.whenDefined(name) – returns a promise
Built-in objects: Internal slots.Many built-in objects, for example Map, Set, Date, Promise and others