Timers

Timers are used to schedule events in the future. They are used when one seeks to delay the execution of some block of code until a specified number of milliseconds have passed, to schedule periodic execution of a particular function, and so on.

JavaScript provides two asynchronous timers: setInterval() and setTimeout(). It is assumed that the reader is fully aware of how to set (and cancel) these timers, so very little time will be spent discussing the syntax. We'll instead focus more on gotchas and less well-known details about timeouts and intervals.

The key takeaway will be this: when using timers, one should make no assumptions about the amount of actual time that will expire before the callback registered for this timer fires, or about the ordering of callbacks. Node timers are not interrupts. Timers simply promise to execute as close as possible to the specified time (though never before), beholden, as with every other event source, to event loop scheduling.

At least one thing you may not know about timers-we are all familiar with the standard arguments to setTimeout: a callback function and timeout interval. Did you know that many additional arguments are passed to the callback function? setTimeout(callback, time, [passArg1, passArg2…])