JavaScript operating mechanism in detail: Event loop--cont.

transferred from: http://www.ruanyifeng.com/blog/2014/10/event-loop.htmlFive, timer

In addition to the events that place an asynchronous task, the task queue can also place timed events that specify how much time some code executes. This is called the timer function, which is the code that executes periodically.

The timer function is mainly by settimeout () and setinterval () These two functions to complete, their internal operation mechanism is exactly the same, the difference is that the former specified code is one-time execution, the latter is repeated execution. The following main discussion settimeout ().

SetTimeout () accepts two parameters, the first is a callback function, and the second is the number of milliseconds to defer execution.

Console.Log(1)settimeout (function< Span class= "token punctuation" > ({consolelog (2) ;} ,1000) ;console. Log (3)               

The result of the above code is 1,3,2, because settimeout () defers the second line to 1000 milliseconds after execution.

If you set the second parameter of settimeout () to 0, the callback function specified immediately (0 millisecond interval) is executed as soon as the current code finishes executing (execution stack emptying).

settimeout ( span class= "token keyword" >function ({console. Log (1) ; }0" ;console.log (2            

The result of the above code is always 2,1, because only after the second line is executed will the system execute the callback function in the task queue.

In summary, setTimeout (fn,0) means that a task is specified to execute at the earliest available idle time of the main thread, that is, to execute as early as possible. It adds an event at the end of the task queue, so it will not be executed until both the synchronization task and the task queue existing events are processed.

The HTML5 standard specifies the minimum value (minimum interval) of the second parameter of the settimeout (), which is not less than 4 milliseconds, and is automatically incremented if it is below this value. Prior to this, the older versions of the browser set the minimum interval to 10 milliseconds. In addition, the changes to those dom, especially those involving page re-rendering, are usually not executed immediately, but once every 16 milliseconds. The effect of using requestanimationframe () is better than settimeout ().

It is important to note that setTimeout () simply inserts the event into the "task queue" and must wait until the current code (execution stack) finishes executing, before the main thread executes the callback function it specifies. If the current code takes a long time, it may take a long time, so there is no guarantee that the callback function will be executed at settimeout ().

JavaScript operating mechanism in detail: Event loop--cont.