Blocking queue in Javascript

Question

I need to create a blocking queue in Javascript. There are pop() and shift() methods in Array.prototype, but the description says:

Return value: The last element from the array; undefined if the array is empty.

I need a method which does not return undefined, but waits until there is some element to return.

The purpose is, that my code is driven by multiple asynchronous operations which pushes elements to the queue and I need to process them.

Javascript is single-threaded so this type of pattern doesn't make sense. If the thread is blocked then nothing else runs. — ccarton
– ccarton, Commented Sep 28, 2016 at 12:20
@ccarton a "blocking queue" isn't necessarily the same as "a queue blocking the event loop". — robertklep
– robertklep, Commented Sep 28, 2016 at 12:43

Maxx · Accepted Answer · 2016-09-28 13:03:24Z

3

Simple implementation with push shift

function Queue() {
  this.listeners = [];
  this.queue = [];
}

Queue.prototype = {
  shift: function(cb) {
    this.queue.length > 0 ? cb(this.queue.shift()) : this.listeners.push(cb);
  },
  push: function(value) {
    if (this.listeners.length > 0) {
      this.listeners.shift()(value);
      return;
    }

    this.queue.push(value);
  }
}


var queue = new Queue();

// 'some value'
queue.shift(function(value) { console.log(value); });

setTimeout(function() { 
  queue.push('some value'); 
  queue.push('another value');

  // 'another value'
  queue.shift(function(value){ console.log(value); });
}, 3000);

edited Sep 28, 2016 at 13:03

answered Sep 28, 2016 at 12:31

Maxx

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4 Comments

tomasbedrich Over a year ago

So basically this is a buffer, right? You keep track of "pre-requested" items in this.listeners and "pushed, not requested yet" items in this.queue.

Maxx Over a year ago

something like that. i guess there are many libraries that do things like this better than my code, try to find something. This code is just for example

ccarton Over a year ago

This should really use push() and shift() so that it acts like a queue as its name implies rather than a stack. The listeners as well.

mwieczorek Over a year ago

I think the terms push and shift should be add and poll respectively

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