Question

Javascript how settimeout will work if settimeout will set with 0

Answer 1

This works because you're doing co-operative multi-tasking.

A browser has to do a number of things pretty much all at once, and just one of those is execute JavaScript. But one of the things JavaScript is very often used for is to ask the browser to build a display element. This is often assumed to be done synchronously (particularly as JavaScript is not executed in parallel) but there is no guarantee this is the case and JavaScript does not have a well-defined mechanism for waiting.

The solution is to "pause" the JavaScript execution to let the rendering threads catch up. And this is the effect that setTimeout() with a timeout of 0 does. It is like a thread/process yield in C. Although it seems to say "run this immediately" it actually gives the browser a chance to finish doing some non-JavaScript things that have been waiting to finish before attending to this new piece of JavaScript.

(In actuality, setTimeout() re-queues the new JavaScript at the end of the execution queue. See the comments for links to a longer explanation.)

IE6 just happens to be more prone to this error, but I have seen it occur on older versions of Mozilla and in Firefox.

Answer 2

Explanation:

The setTimeout() function is commonly used if you wish to have your function called once after the specified delay. The setInterval() function is commonly used to set a delay for functions that are executed again and again, such as animations.