Javascript queue a function after another completes

Question

I have a long running function A(). And my function B(){} can be called anytime.

Note function B doesn't have to be called after A. But if it is called and if A is running, then A must be finished. And then run B.

I thought about promise.then(), but B may not necessarily be called after A. So it is not much useful in this situation.

How to 'queue' B to ensure the logic?

What makes you think that B will start executing before A finishes? What is your environment? You may want to read something like this: http://stackoverflow.com/questions/2734025/is-javascript-guaranteed-to-be-single-threaded — Vladimir M, May 10 '17 at 20:13
are A and B synchronous or asynchronous? If they are both synchronous you don't need to do anything, A will halt the execution of B until it's finished. — Brian Glaz, May 10 '17 at 20:14
Set a flag. Call A. Use a .then. If the flag is set, place B in a queue array, else call B. In the .then clear the flag, then call any functions in the queue array. — Travis J, May 10 '17 at 20:15

score 3 · Accepted Answer · answered May 10 '17 at 21:57

Don't use flags! They are completely unnecessary.

Also don't do anything special in A() or B(). Just write them as you would normally to perform their duties.

Just implement a queue in the form of a dynamically updated promise chain.

var q_ = Promise.resolve();

function queue(fn) {
    q_ = q_.then(fn);
    return q_;
}

Now you can queue A and B, or any other function, as follows :

queue(A);
queue(B);
queue(someOtherFunction);

Or, if you need to pass parameters :

queue(A.bind(null, 'a', 'b', 'c'));
queue(B.bind(null, 'x', 'y', 'z'));
queue(someOtherFunction.bind(null, 1, 2, 3));

As a bonus,

A() and B() (and other functions) remain available to be called directly (unqueued).
you needn't worry whether functions passed to queue() are synchronous or asynchronous. It will work with either.

DEMO

score 1 · Answer 2 · answered May 10 '17 at 20:15

1

Do something like this: Define a variable var inQueue = 0; When A starts, set a variable inQueue = 1, when it finishes, set it to inQueue = 0. Now place a check like

if(!inQueue) B();

This will ensure B won't interrupt A.

answered May 10 '17 at 20:15

gaganshera

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score 1 · Answer 3 · answered May 10 '17 at 20:17

Use a variable that is true when A is running and is set to false at the end of A. Check that within B. If it is running, have it wait a second and call B again.

var isARunning

A() {
  isARunning = true
  //do the things
  isARunning = false
}

B() {
  if (isARunning) {
   setTimeout(() => { B() }, 1000);
  }
  else {
    // do more things
  }
}

Javascript queue a function after another completes

3 Answers3