When Function Finshed Run It Again Python
The result loop
JavaScript has a runtime model based on an event loop, which is responsible for executing the lawmaking, collecting and processing events, and executing queued sub-tasks. This model is quite different from models in other languages like C and Coffee.
Runtime concepts
The following sections explain a theoretical model. Modern JavaScript engines implement and heavily optimize the described semantics.
Visual representation
Stack
Office calls course a stack of frames.
function foo ( b ) { let a = 10 return a + b + eleven } office bar ( x ) { let y = three return foo (x * y) } const baz = bar ( 7 ) // assigns 42 to baz Order of operations:
- When calling
bar, a first frame is created containing references tobar'southward arguments and local variables. - When
barcallsfoo, a 2nd frame is created and pushed on top of the first 1, containing references tofoo'due south arguments and local variables. - When
fooreturns, the tiptop frame element is popped out of the stack (leaving onlybar's call frame). - When
barreturns, the stack is empty.
Note that the arguments and local variables may continue to exist, equally they are stored outside the stack — and then they can exist accessed by any nested functions long later on their outer function has returned.
Heap
Objects are allocated in a heap which is just a name to announce a large (generally unstructured) region of memory.
Queue
A JavaScript runtime uses a message queue, which is a list of messages to be processed. Each bulletin has an associated office that gets called to handle the message.
At some point during the outcome loop, the runtime starts handling the messages on the queue, starting with the oldest ane. To practice and so, the message is removed from the queue and its corresponding office is chosen with the message every bit an input parameter. Equally always, calling a function creates a new stack frame for that function'southward utilize.
The processing of functions continues until the stack is once once more empty. So, the result loop volition procedure the next message in the queue (if at that place is i).
Upshot loop
The upshot loop got its name because of how it's commonly implemented, which usually resembles:
while (queue. waitForMessage ( ) ) { queue. processNextMessage ( ) } queue.waitForMessage() waits synchronously for a message to arrive (if one is not already available and waiting to be handled).
"Run-to-completion"
Each bulletin is candy completely before whatsoever other message is processed.
This offers some nice properties when reasoning about your programme, including the fact that whenever a part runs, it cannot be preempted and will run entirely before whatever other code runs (and tin change data the role manipulates). This differs from C, for case, where if a function runs in a thread, information technology may be stopped at any indicate by the runtime system to run some other code in another thread.
A downside of this model is that if a message takes too long to consummate, the spider web awarding is unable to process user interactions like click or curl. The browser mitigates this with the "a script is taking as well long to run" dialog. A practiced practice to follow is to make message processing brusk and if possible cut down one message into several messages.
Calculation letters
In web browsers, messages are added anytime an result occurs and there is an result listener attached to information technology. If there is no listener, the event is lost. So a click on an element with a click event handler will add a message—besides with any other event.
The function setTimeout is chosen with 2 arguments: a message to add to the queue, and a time value (optional; defaults to 0). The time value represents the (minimum) delay later on which the message will exist pushed into the queue. If there is no other message in the queue, and the stack is empty, the bulletin is candy right after the filibuster. Withal, if there are messages, the setTimeout message will take to wait for other letters to be processed. For this reason, the second argument indicates a minimum fourth dimension—not a guaranteed time.
Here is an example that demonstrates this concept (setTimeout does not run immediately after its timer expires):
const seconds = new Engagement ( ) . getSeconds ( ) ; setTimeout ( function ( ) { // prints out "2", meaning that the callback is not called immediately afterward 500 milliseconds. console. log ( ` Ran later ${ new Date ( ) . getSeconds ( ) - seconds} seconds ` ) ; } , 500 ) while ( true ) { if ( new Date ( ) . getSeconds ( ) - seconds >= 2 ) { console. log ( "Good, looped for 2 seconds" ) break ; } } Zilch delays
Cipher filibuster doesn't mean the phone call back will fire-off after zero milliseconds. Calling setTimeout with a delay of 0 (zero) milliseconds doesn't execute the callback function after the given interval.
The execution depends on the number of waiting tasks in the queue. In the case beneath, the message "this is only a message" will exist written to the panel before the message in the callback gets processed, because the delay is the minimum time required for the runtime to process the request (not a guaranteed fourth dimension).
The setTimeout needs to wait for all the code for queued messages to consummate even though yous specified a particular time limit for your setTimeout.
( office ( ) { console. log ( 'this is the start' ) ; setTimeout ( function cb ( ) { console. log ( 'Callback one: this is a msg from remember' ) ; } ) ; // has a default fourth dimension value of 0 console. log ( 'this is but a message' ) ; setTimeout ( function cb1 ( ) { console. log ( 'Callback 2: this is a msg from phone call dorsum' ) ; } , 0 ) ; console. log ( 'this is the end' ) ; } ) ( ) ; // "this is the start" // "this is simply a message" // "this is the end" // "Callback 1: this is a msg from telephone call back" // "Callback ii: this is a msg from call back" Several runtimes communicating together
A web worker or a cross-origin iframe has its own stack, heap, and message queue. Two singled-out runtimes can but communicate through sending letters via the postMessage method. This method adds a message to the other runtime if the latter listens to message events.
Never blocking
A very interesting property of the consequence loop model is that JavaScript, unlike a lot of other languages, never blocks. Handling I/O is typically performed via events and callbacks, and so when the awarding is waiting for an IndexedDB query to render or an XHR asking to return, information technology tin notwithstanding process other things like user input.
Legacy exceptions exist similar alert or synchronous XHR, but it is considered good practice to avert them. Beware: exceptions to the exception do exist (only are normally implementation bugs, rather than annihilation else).
Run across too
Source: https://developer.mozilla.org/en-US/docs/Web/JavaScript/EventLoop
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