Ajax Girl

It is so great that John is blogging concepts from his upcoming book before it goes out. What a treat for us, and hopefully great marketing for his book!

In How JavaScript Timers Work, John takes us through the actual mechanics of setInterval and setTimeout:

This enables us to grok the difference between:

These two pieces of code may appear to be functionally equivalent, at first glance, but they are not. Notably the setTimeout code will always have at least a 10ms delay after the previous callback execution (it may end up being more, but never less) whereas the setInterval will attempt to execute a callback every 10ms regardless of when the last callback was executed.

There’s a lot that we’ve learned here, let’s recap:

• JavaScript engines only have a single thread, forcing asynchronous events to queue waiting for execution.
• setTimeout and setInterval are fundamentally different in how they execute asynchronous code.
• If a timer is blocked from immediately executing it will be delayed until the next possible point of execution (which will be longer than the desired delay).
• Intervals may execute back-to-back with no delay if they take long enough to execute (longer than the specified delay).

Source: Ajaxian
Original Article: http://feeds.feedburner.com/~r/ajaxian/~3/240845499/how-javascript-timers-work