有关HTML5 IndexedDB异步API的疑问

Question

While reading the HTML5 IndexedDB Specification I had some doubts about its asynchronous request model. When looking at the request api example , the open method is used to start an async request.

var request = indexedDB.open('AddressBook', 'Address Book');
request.onsuccess = function(evt) {...};
request.onerror = function(evt) {...};

At the time this request is started, there are no event handlers defined yet.

• Isn't this a race condition?
• What happens when the open method succeeds before the javascript interpreter executes the assignment to onsuccess ?
• Or is the request only really started once both callbacks are registered?

In my opinion an api like the following would be much more logical:

db.open('AddressBook', 'Address Book', {
    onsuccess: function(e) { ... },
    onerror  : function(e) { ... }
});

Answer 1

There will be no race condition because JavaScript engine will finish executing actual scope (function) and then fire any callback or event handler. Read following comment on Mozilla Hacks.