How does Facebook manage to kill javascript when going to a new page?

Question

Try this.

Type this code into your browser console on the homepage of Facebook (your Facebook feed):

setInterval(function(){console.log('blah')}, 100)

Then navigate to the "Messages" or "Events" section of the website. Notice that the whole page is not reloaded and the new html content is inserted dynamically with javascript. However, somehow Facebook cancels the setInterval. This also happens with setTimeout.

How is this happening?

perhaps they are clearing all intervals on the window object when you actively open an item such as messages to avoid timers reloading fresh content? — cillierscharl, Nov 17 '12 at 01:08
@pst http://stackoverflow.com/questions/958433/how-can-i-clearinterval-for-all-setinterval second answer. — cillierscharl, Nov 17 '12 at 01:09
ooooh that is what they are doing! Typing `setTimeout` on Facebook shows their own function. — user730569, Nov 17 '12 at 01:14
seems they're clearing timeouts `onLeave` what that means in facebook context and their minfied js code, who knows. — cillierscharl, Nov 17 '12 at 01:17
@user730569 If the approach is determined through inspection, remember to self-answer for future reference. — , Nov 17 '12 at 01:20
@pst I figured f0x would want to answer b/c he really figured it out. — user730569, Nov 17 '12 at 01:23
Nah don't stress; mine was just a guess, you found that they're rolling their own timeout function so go ahead ;p — cillierscharl, Nov 17 '12 at 01:25

score 2 · Answer 1 · answered Nov 17 '12 at 01:41

Turns out that Facebook is overriding the native setTimeout and setInterval functions with their own. You can see this by typing setTimeout into your browser console and it will return a function different from the typical native code function. Facebook therefore has access to all setTimeouts on the page, even ones by third party code, and can therefore clear them all when you access a new page (or whenever they want, really).

How does Facebook manage to kill javascript when going to a new page?

1 Answers1