Generally, if you don't know if a value is a promise or not, Promise.resolve(value) it instead and work with the return value as a promise. has a then method), the returned promise will "follow" that thenable, adopting its eventual state otherwise, the returned promise will be fulfilled with the value. Returns a new Promise object that is resolved with the given value. Returns a new Promise object that is rejected with the given reason. If it rejects, it is rejected with the reason from the first promise that was rejected. If the returned promise fulfills, it is fulfilled with the value of the first promise in the iterable that fulfilled. Wait until any of the promises is fulfilled or rejected. Takes an iterable of Promise objects and, as soon as one of the promises in the iterable fulfills, returns a single promise that fulfills with the value from that promise. Returns a Promise that fulfills after all of the given promises is either fulfilled or rejected, with an array of objects that each describe the outcome of each promise. Wait until all promises have settled (each may fulfill or reject). If it rejects, it is rejected with the reason from the first promise in the iterable that was rejected. If the returned promise fulfills, it is fulfilled with an aggregating array of the values from the fulfilled promises, in the same order as defined in the iterable of multiple promises. Wait for all promises to be fulfilled, or for any to be rejected. Parallel execution in JavaScript can only be achieved through worker threads. Note that JavaScript is single-threaded by nature, so at a given instant, only one task will be executing, although control can shift between different promises, making execution of the promises appear concurrent. The subclass must also have a resolve static method that can be called like Promise.resolve() to resolve values to promises. To do so, the subclass's constructor must implement the same signature as the Promise() constructor - accepting a single executor function that can be called with the resolve and reject callbacks as parameters. They all support subclassing, which means they can be called on subclasses of Promise, and the result will be a promise of the subclass type. In other words, fulfills when any of the promises fulfills rejects when any of the promises rejects.Īll these methods take an iterable of promises ( thenables, to be exact) and return a new promise. Settles when any of the promises settles. Promise.any()įulfills when any of the promises fulfills rejects when all of the promises reject. Promise.allSettled()įulfills when all promises settle. The Promise class offers four static methods to facilitate async task concurrency: Promise.all()įulfills when all of the promises fulfill rejects when any of the promises rejects. Colloquially, "resolved" promises are often equivalent to "fulfilled" promises, but as illustrated in "States and fates", resolved promises can be pending or rejected as well. The States and fates document from the original Promise proposal contains more details about promise terminology. You will also hear the term resolved used with promises - this means that the promise is settled or "locked-in" to match the eventual state of another promise, and further resolving or rejecting it has no effect. If the promise has already been fulfilled or rejected when a corresponding handler is attached, the handler will be called, so there is no race condition between an asynchronous operation completing and its handlers being attached.Ī promise is said to be settled if it is either fulfilled or rejected, but not pending. When either of these options occur, the associated handlers queued up by a promise's then method are called. The eventual state of a pending promise can either be fulfilled with a value or rejected with a reason (error). rejected: meaning that the operation failed.fulfilled: meaning that the operation was completed successfully.pending: initial state, neither fulfilled nor rejected.This lets asynchronous methods return values like synchronous methods: instead of immediately returning the final value, the asynchronous method returns a promise to supply the value at some point in the future. It allows you to associate handlers with an asynchronous action's eventual success value or failure reason. ()Ī Promise is a proxy for a value not necessarily known when the promise is created.Object.prototype._lookupSetter_() Deprecated.Object.prototype._lookupGetter_() Deprecated.Object.prototype._defineSetter_() Deprecated.
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