First of all rollback is something that I do not care about.

I would like to be able to lock a sequence of async functions/promises/tasks (let's call it a "transaction") with a name/id (or array of names), so that they happen in sequence, and so that any other "transaction" with the same name(s) that are run by another part of the system are delayed from starting until the running transaction using the same name(s) has completed. So it basically is queueing the sequences of async tasks, or "transaction"s.

Here is some example code of the situation:

function a()
{
  // do stuff
  return new Promise(/*...*/);
}


function b()
{
  // do stuff
  return new Promise(/*...*/);
}



function c()
{
  // do stuff
  return a.then(() => b());
}

Now at any time the system could call a, b, or c, and when it does I don't want c and b running at the same time, but obvious c depends on b.

I've been looking for a package on npm to help with this but I haven't found anything, I wonder if anyone can suggest something that I might have missed that would help with this?

First of all rollback is something that I do not care about.

I would like to be able to lock a sequence of async functions/promises/tasks (let's call it a "transaction") with a name/id (or array of names), so that they happen in sequence, and so that any other "transaction" with the same name(s) that are run by another part of the system are delayed from starting until the running transaction using the same name(s) has completed. So it basically is queueing the sequences of async tasks, or "transaction"s.

Here is some example code of the situation:

function a()
{
  // do stuff
  return new Promise(/*...*/);
}


function b()
{
  // do stuff
  return new Promise(/*...*/);
}



function c()
{
  // do stuff
  return a.then(() => b());
}

Now at any time the system could call a, b, or c, and when it does I don't want c and b running at the same time, but obvious c depends on b.

I've been looking for a package on npm to help with this but I haven't found anything, I wonder if anyone can suggest something that I might have missed that would help with this?

• javascript

• 1 I don't really see what the question is. If you call c().then(...), does that not do exactly what you want? Also, I think the body of c() is supposed to be return a().then(() => b()), not return a.then(() => b()) – Patrick Roberts Commented Jul 3, 2017 at 18:27
• Why not just using an shared-scope variable as a flag to know if there are already a B or C function executing? – Kulvar Commented Jul 4, 2017 at 12:02
• If there are conditions upon calling a function, sounds like what you need is a delegator. Since b returns a promise, I'm guessing what you want is for a call to b to wait until c is no longer being processed. You'd then have the delegator wait for a flag before calling b. – Dacheng Commented Jul 6, 2017 at 20:01
• Is this for one of those web pages that sometimes don't load and get stuck. – danny117 Commented Jul 6, 2017 at 21:23

6 Answers 6

I think gulp tasks can help you out of the box. This guarantees that c always run after b and so b after a

const gulp = require('gulp');
gulp.task('a', done => {
  // do stuff
  console.log('a');
  done();
});

gulp.task('b', ['a'], done => {
  // do stuff
  console.log('b');
  done();
});

gulp.task('c', ['b'], done => {
  // do more stuff
  console.log('c');
  done();
});

gulp.start('c'); // Logs a, b, c

Try it!

You could write your own little transaction manager.

const transactions = {};

function doTransaction(name, promiseFunc) {
  transactions[name] = (transactions[name] || Promise.resolve()).then(promiseFunc);
}

Use async/await and have babel transpile it. Async Babel Docs

function a()
{
  // do stuff
  return new Promise(/*...*/);
}


async function b()
{
  const aData = await a();
  // do stuff
  return new Promise(/*...*/);
}



async function c()
{
  const bData = await b();
  // do stuff
  return bData;
}

You can go for https://github.com/Reactive-Extensions/RxJS

They have many functions to handle single/multiple/dependent/parallel async calls.

function a()
{
  // do stuff
  return new Promise(/*...*/);
}

function b()
{
  // do stuff
  return new Promise(/*...*/);
}

function c()
{
  // do stuff
  return new Value;
}

a().then(function(data_a) {
  // you can make use of return value (which is data_a) here or as an argument for function b or even function c
  b().then(function(data_b) {
    // you can make use of return value (which is data_b) here or as an argument for function c
    c().then(function(data_c) {
      // do your coding here
    });
  });
});

you can check this link for reference : https://spring.io/understanding/javascript-promises

Ok, here's my take. You use a wrapper for your function b which returns and object with 2 methods: doCall and wait. The wrapper should be called only once.

doCall will call your function and trace its completion for the wait() function.

wait() will wait for the completion and always resolve when doCall() finishes

Now for the code, also on CodePen (see developer console):

function wrapPromiseFn(fn) {
  var prev = null;
  var doCall = function() {
    var retValue;

    prev = new Promise(function(resolve, reject) {
      retValue = fn();
      retValue.then(function(result) {
        resolve(true);
      });
      retValue.catch(function(result) {
        resolve(true);
      });
    });

    return retValue;
  };

  var wait = function() {
    return prev ? prev : Promise.resolve(false);
  };

  return {
    doCall: doCall,
    wait: wait
  };
}

function a() {
  return Promise.resolve(42);
}

function b() {
  //return Promise.reject(new Date());
  return Promise.resolve(new Date().getTime());
}

var wrappedB = wrapPromiseFn(b);

function doStuff() {
  return wrappedB.wait().then(function(didWait) {
    return a().then(function(_a) {
      return wrappedB.doCall().then(function(_b) {
        console.log("didWait, a, b: ", didWait, _a, _b);
      });
    });
  });
}

//run it twice to prove it did wait
doStuff().then(doStuff)

It proves the concept, of course it would need some polish to pass arguments from doCall to the wrapped function.