How does setTimeout work in Node JS

Question

I guess that once it s executed it s on the queue  but in the queue is there any assurance it will invoke exactly after X milliseconds  Or will other heavy tasks higher on the queue delay it

User · Answer

setTimeout is a kind of Thread  it holds a operation for a given time and execute   setTimeout function time in mills     in here the first argument should be a function type  as an example if you want to print your name after 3 seconds  your code should be something like below   setTimeout function   console log  your name    3000     Key point to remember is  what ever you want to do by using the setTimeout method  do it inside a function  If you want to call some other method by parsing some parameters  your code should look like below   setTimeout function   yourOtherMethod parameter    3000

User · Answer

The semantics of setTimeout are roughly the same as in a web browser  the timeout arg is a minimum number of ms to wait before executing  not a guarantee   Furthermore  passing 0  a non-number  or a negative number  will cause it to wait a minimum number of ms   In Node  this is 1ms  but in browsers it can be as much as 50ms   The reason for this is that there is no preemption of JavaScript by JavaScript   Consider this example   setTimeout function        console log  boo      100  var end   Date now     5000 while  Date now    lt  end    console log  imma let you finish but blocking the event loop is the best bug of all TIME     The flow here is    schedule the timeout for 100ms  busywait for 5000ms  return to the event loop   check for pending timers and execute    If this was not the case  then you could have one bit of JavaScript  interrupt  another   We d have to set up mutexes and semaphors and such  to prevent code like this from being extremely hard to reason about   var a   100  setTimeout function        a   0     0   var b   a     100 or 0    The single-threadedness of Node s JavaScript execution makes it much simpler to work with than most other styles of concurrency   Of course  the trade-off is that it s possible for a badly-behaved part of the program to block the whole thing with an infinite loop   Is this a better demon to battle than the complexity of preemption   That depends

User · Answer

setTimeout callback t  is used to run callback after at least t millisecond  The actual delay depends on many external factors like OS timer granularity and system load   So  there is a possibility that it will be called slightly after the set time  but will never be called before      A timer can t span more than 24 8 days

User · Answer

The idea of non-blocking is that the loop iterations are quick  So to iterate for each tick should take short enough a time that the setTimeout will be accurate to within reasonable precision  off by maybe  lt 100 ms or so    In theory though you re right  If I write an application and block the tick  then setTimeouts will be delayed  So to answer you re question  who can assure setTimeouts execute on time  You  by writing non-blocking code  can control the degree of accuracy up to almost any reasonable degree of accuracy   As long as javascript is  single-threaded  in terms of code execution  excluding web-workers and the like   that will always happen  The single-threaded nature is a huge simplification in most cases  but requires the non-blocking idiom to be successful   Try this code out either in your browser or in node  and you ll see that there is no guarantee of accuracy  on the contrary  the setTimeout will be very late   var start   Date now        expecting something close to 500 setTimeout function    console log Date now   - start      500       fiddle with the number of iterations depending on how quick your machine is for var i 0  i lt 5000000    i      Unless the interpreter optimises the loop away  which it doesn t on chrome   you ll get something in the thousands  Remove the loop and you ll see it s 500 on the nose

User · Answer

The only way to ensure code is executed is to place your setTimeout logic in a different process   Use the child process module to spawn a new node js program that does your logic and pass data to that process through some kind of a stream  maybe tcp    This way even if some long blocking code is running in your main process your child process has already started itself and placed a setTimeout in a new process and a new thread and will thus run when you expect it to   Further complication are at a hardware level where you have more threads running then processes and thus context switching will cause  very minor  delays from your expected timing  This should be neglible and if it matters you need to seriously consider what your trying to do  why you need such accuracy and what kind of real time alternative hardware is available to do the job instead   In general using child processes and running multiple node applications as separate processes together with a load balancer or shared data storage  like redis  is important for scaling your code

[javascript] How does setTimeout work in Node.JS?

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