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Using JavaScript Generators in Firefox

Warning: This technique works only in Firefox, not IE, Chrome, Safari, etc.

Generators can be used to simplify asynchronous code in Firefox by opting in to using JavaScript version 1.7 or later. You can opt in in HTML as follows:

<script type="text/javascript;version=1.7" src="myScript.js"></script>

Then your myScript.js file might look like this:

// Need to stash the generator in a global variable.
var generator;

// Simple event listener function to pass the received event to the generator.
function grabEvent(event) {

// When we're all done we can close the generator, but that must happen outside
// of the generator so we use a timeout.
function closeGenerator() {
  setTimeout(function() {
  }, 0);

// Our main steps
function databaseOperation() {"MyTestDatabase").onsuccess = grabEvent;
  var event = yield;

  var db =;

  if (db.version != "1.0") {
    db.setVersion("1.0").onsuccess = grabEvent;
    event = yield;

    var transaction = event.transaction;

    transaction.oncomplete = grabEvent;

  db.transaction(["stuff"]).objectStore("stuff").get("foo").onsuccess = grabEvent;
  event = yield;

  alert("Got result: " +;

  // We're all done.

  // Always have an extra yield at the end or you will see StopIteration
  // exceptions.

generator = databaseOperation();;

Document Tags and Contributors

 Contributors to this page: jswisher, chrisdavidmills, grendel
 Last updated by: chrisdavidmills,