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Functional version of a promise that allows you to resolve it outside of the callback in the promise constructor
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/** | |
* Creates an intermediate value that can be resolved later. | |
* @return {Object} Eventual | |
* @return {Function} Eventual.then Acts like Promise.then for the eventual value | |
* @return {Function} Eventual.resolve Acts like Promise.resolve for the eventual | |
* @return {Function} Eventual.reject Acts like Promise.reject for the eventual | |
*/ | |
const eventual = () => ( | |
_eventual => ({ | |
/** | |
* Forward all arguments to the promise contained inside of eventual | |
* @param {Function(Function resolve, Function reject))} | |
* @return {Promise} | |
*/ | |
then: (...args) => _eventual.then(({ promise }) => promise.then(...args)), | |
/** | |
* Resolve the promise once the call stack is empty | |
* @param {Value} val Value to resolve to | |
*/ | |
resolve: val => { | |
_eventual.then(({ resolve }) => resolve(val)); | |
}, | |
/** | |
* Reject the promise once the call stack is empty | |
* @param {Value} val Value to reject with | |
*/ | |
reject: val => { | |
_eventual.then(({ reject }) => reject(val)); | |
}, | |
}) | |
)( | |
/* | |
* Outer promise is only here to resolve the methods | |
* in the inner promise callback | |
*/ | |
new Promise(handler => { | |
/* | |
* The `setTimeout(..., 0)` is so that the inner promise | |
* calls the outer resolver only once the call stack | |
* is empty and `promise` has been assigned the rvalue | |
* returned by the inner promise's constructor. | |
*/ | |
const promise = new Promise((resolve, reject) => { | |
setTimeout( | |
() => handler({ promise, resolve, reject }), | |
0, | |
); | |
}); | |
}) | |
); | |
/** | |
* Creation is easy enough | |
*/ | |
const thing = eventual(); | |
/** | |
* We can safely assume that the value is ready when cubed | |
*/ | |
thing.then(x => x ** 2).then(console.log); | |
thing.then(x => x ** 3).then(console.log); | |
thing.then(x => x ** 4).then(console.log); | |
/** | |
* Some undetermined amount of time later | |
*/ | |
thing.resolve(3); | |
/** | |
* Console output: | |
* 9 | |
* 27 | |
* 81 | |
*/ |
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