JavaScript Promises
Syntax
- new Promise( /* executor function: */ function(resolve, reject) { })
- promise.then(onFulfilled[, onRejected])
- promise.catch(onRejected)
- Promise.resolve(resolution)
- Promise.reject(reason)
- Promise.all(iterable)
- Promise.race(iterable)
Remarks
Promises are part of the ECMAScript 2015 specification and browser support is limited, with 88% of browsers worldwide supporting it as of July 2017. The following table gives an overview of the earliest browser versions that provide support for promises.
| Chrome | Edge | Firefox | Internet Explorer | Opera | Opera Mini | Safari | iOS Safari |
|---|---|---|---|---|---|---|---|
| 32 | 12 | 27 | x | 19 | x | 7.1 | 8 |
In environments which do not support them, Promise can be polyfilled. Third-party libraries may also provide extended functionalities, such as automated "promisification" of callback functions or additional methods like progress—also known as notify.
The Promises/A+ standard website provides a list of 1.0 and 1.1 compliant implementations. Promise callbacks based on the A+ standard are always executed asynchronously as microtasks in the event loop.
