Control structures: Generators

generators.js

var sleep = require('co-sleep');
var co = require('co');

co(function *() {
  var sum = 0,
      stop = 10;

  while (sum < stop) {
    // Arbitrary 250ms async method to simulate async process
    // In real usage it could just be a normal async event that 
    // returns a Promise.
    yield sleep(250);
    sum++;

    // Print out the sum thus far to show progress
    console.log(sum);
  }

  console.log("Done");
});

promises-only.js

// From: http://blog.victorquinn.com/javascript-promise-while-loop

// And below is a sample usage of this promiseWhile function
var sum = 0,
    stop = 10;

promiseWhile(function() {
    // Condition for stopping
    return sum < stop;
}, function() {
    // Action to run, should return a promise
    return new Promise(function(resolve, reject) {
        // Arbitrary 250ms async method to simulate async process
        // In real usage it could just be a normal async event that 
        // returns a Promise.
        setTimeout(function() {
            sum++;
            // Print out the sum thus far to show progress
            console.log(sum);
            resolve();
        }, 250);
    });
}).then(function() {
    // Notice we can chain it because it's a Promise, 
    // this will run after completion of the promiseWhile Promise!
    console.log("Done");
});

Quick comparison between promises and promises with generators. Note how generators make JavaScript control structures available again in asynchronous code. We can use while instead of having to use a promiseWhile function or library.

The promise example is a real example from a highly search engine ranking blog post on the subject. The generator version is a port I wrote. To keep it sort of fair in terms of code length, I kept any comments related to functionality and only stripped comments related to explaining the non-native control structure. I.e. comments you arguably no longer need.