Using Resolve later

⬅️ Execute Program JavaScript Concurrency

Using Resolve later

• We can store the resolve function in a variable. This allows us to “capture” the resolve function, then use it outside of the new Promise constructor.

let savedResolve;
 
const promise = new Promise(resolve => { savedResolve = resolve; });
 
/* The `savedResolve` variable now contains the function that we got from the
 * `new Promise` constructor. Calling it will fulfill the promise, even
 * though it now looks like "savedResolve" and "promise" are unrelated. */
savedResolve(3);
promise;
// Async Result:
{fulfilled: 3}
 

• The net effect of this is: we can split promises into two pieces. We might pass the resolve function to one part of the system, then pass the promise itself to a totally different part. When the “reading” side tries to look inside the promise with a then, it will have to wait until the “writing” side has called resolve.

let savedResolve;
const promise = new Promise(resolve => { savedResolve = resolve; })
  .then(n => n * 2);
setTimeout(() => savedResolve(3), 1000);
promise;
 
// Async Result:
{fulfilled: 6}

• we can only resolve the promise once