Javascript completable future?

Question

I'm looking at both PromiseJS and ES6 for something similar to a CompletableFuture, but couldn't find any.

Any suggestions how this can be achieved in javascript? Thanks

Note: Promise.resolve() isn't suitable, unless you can create the Promise object first and complete it later.

Update

As all answers pointed out, the correct way is to use Promise's constructor:

new Promise((resolve, reject) => { resolve(data); })

Yes, it is possible to create `Promise` object, then resolve at future point. Can you include `javascript` tried at Question? Describe expected result? — guest271314, Jun 24 '16 at 13:53
Please show us the code that you would like to write and we can tell you how to do it correctly. As it stands, it really isn't a very specific question. — Bergi, Jun 24 '16 at 14:44
Here's how you make a deferred object using ES6 standard promises (if that's what you're trying to do): http://stackoverflow.com/questions/37651780/why-does-the-promise-constructor-need-an-executor/37673534#37673534 — jfriend00, Jun 24 '16 at 15:03

score 4 · Accepted Answer · answered Jun 24 '16 at 14:00

Note: Promise.resolve() isn't suitable, unless you can create the Promise object first and complete it later.

You can use Promise constructor, pass onFulfilled, onRejected arguments outside scope of executor function

var complete, err, now = new Date().getTime();

var promise = new Promise((onFulfilled, onRejected) => {
  complete = onFulfilled; err = onRejected;
});

promise.then((data) => {
  console.log(data)
});

setTimeout(() => {
  complete("promise resolved " 
           + (new Date().getTime() - now) 
           + " in the future")
}, Math.random() * 3500)

score 0 · Answer 2 · edited May 23 '17 at 12:08

0

There is no deferred-like API in JavaScript for good reason. You don't have a CompletableFuture with a .complete method, instead you just use the new Promise constructor. You already linked some documentation containing examples so I'm not going to repeat them here.

edited May 23 '17 at 12:08

Community

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answered Jun 24 '16 at 14:43

Bergi

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score 0 · Answer 3 · answered Feb 06 '20 at 05:57

Just meet the same need, this is how I do this in TypeScript

export function createLazyPromise<T = any>(): Promise<T> & { reject: () => void, resolve: (v: T) => void } {
  let onResolve, onReject;
  const promise = new Promise<T>(((resolve, reject) => {
    onResolve = resolve;
    onReject = reject
  }));
  promise['resolve'] = onResolve;
  promise['reject'] = onReject;
  return promise as any
}

Usee like this

test('test shell', async () => {
  // resolve later
  const promise = createLazyPromise();
  const result = shell.exec(
    'ps aux | head -3',
    {
      silent: true,
    },
    (code, stdout, stderr) => {
      promise.resolve({code, stdout, stderr});
    }
  );

  console.log(result.pid);

  console.log(await promise);
});

Javascript completable future?

3 Answers3