Demonstration of how to RELEASE ZALGO

release-zalgo.js

/**
 * Demonstration of how to RELEASE ZALGO
 * @author Bastian Masanek (2017)
 */

let cache;

function releaseZalgo(cb) {
  if (cache) {
    // This is synchronous and execucted immediately.
    // YOU JUST RELEASED ZALGO!
    return cb(cache);

    // Correct approach:
    // return process.nextTick(() => cb(cache));
  }

  // Any async operation here.
  setTimeout(() => {
    cache = 'All fine.';
    cb(cache);
  }, 1000);
}

function zalgoFactory() {
  let listeners = [];

  releaseZalgo(data => {
    listeners.forEach(listener => listener(data));
  });

  return {
    onData: listener => listeners.push(listener)
  };
}

let a = zalgoFactory();
a.onData(data => {
  console.log('data from a:', data);

  let b = zalgoFactory();
  // This will never be called because releaseZalgo() already executed
  // BEFORE .onData() was even defined.
  b.onData(data => {
    console.log('data from b:', data);
  });

  let c = zalgoFactory();
  // This will never be called because releaseZalgo() already executed
  // BEFORE .onData() was even defined.
  c.onData(data => {
    console.log('data from c:', data);
  });
});

/*
Example output:

$ node zalgo.js
data from a: All fine

Nothing else will happen after the first asynchronous call.

===============================================================================
 RULE OF THUMB:
-------------------------------------------------------------------------------
  Use Promises. You can't release Zalgo when using Promises:

    function releaseZalgo() {
      if (cache) {
        return Promise.resolve(cache);
      }
      return new Promise((resolve, reject) => {
        ...
      });
    }

  If you have (or want) to use callbacks, choose whether your function is
  synchronous or asynchronous -- and stick with it. 
  Never mix those concepts, because it can lead to unpredictable, weird and 
  thus hard to track down errors in your application.
  
  If you find Zalgo, you can fight it using process.nextTick().
  
  References:
  - Designing APIs for Asynchrony
    http://blog.izs.me/post/59142742143/designing-apis-for-asynchrony
  - Don't release Zalgo!
    https://github.com/oren/oren.github.io/blob/master/posts/zalgo.md
===============================================================================
*/