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I need to run a javascript function each 10 seconds.
I understand the syntax must work like follow but I am not getting any success:
function funcName() {
alert("test");
}
var func = funcName();
var run = setInterval("func",10000)
But this isn't working. Any help?
A lot of other answers are focusing on a pattern that does work, but their explanations aren't really very thorough as to why your current code doesn't work.
Your code, for reference:
function funcName() {
alert("test");
}
var func = funcName();
var run = setInterval("func",10000)
Let's break this up into chunks. Your function funcName is fine. Note that when you call funcName (in other words, you run it) you will be alerting "test". But notice that funcName() -- the parentheses mean to "call" or "run" the function -- doesn't actually return a value. When a function doesn't have a return value, it defaults to a value known as undefined.
When you call a function, you append its argument list to the end in parentheses. When you don't have any arguments to pass the function, you just add empty parentheses, like funcName(). But when you want to refer to the function itself, and not call it, you don't need the parentheses because the parentheses indicate to run it.
So, when you say:
var func = funcName();
You are actually declaring a variable func that has a value of funcName(). But notice the parentheses. funcName() is actually the return value of funcName. As I said above, since funcName doesn't actually return any value, it defaults to undefined. So, in other words, your variable func actually will have the value undefined.
Then you have this line:
var run = setInterval("func",10000)
The function setInterval takes two arguments. The first is the function to be ran every so often, and the second is the number of milliseconds between each time the function is ran.
However, the first argument really should be a function, not a string. If it is a string, then the JavaScript engine will use eval on that string instead. So, in other words, your setInterval is running the following JavaScript code:
func
// 10 seconds later....
func
// and so on
However, func is just a variable (with the value undefined, but that's sort of irrelevant). So every ten seconds, the JS engine evaluates the variable func and returns undefined. But this doesn't really do anything. I mean, it technically is being evaluated every 10 seconds, but you're not going to see any effects from that.
The solution is to give setInterval a function to run instead of a string. So, in this case:
var run = setInterval(funcName, 10000);
Notice that I didn't give it func. This is because func is not a function in your code; it's the value undefined, because you assigned it funcName(). Like I said above, funcName() will call the function funcName and return the return value of the function. Since funcName doesn't return anything, this defaults to undefined. I know I've said that several times now, but it really is a very important concept: when you see funcName(), you should think "the return value of funcName". When you want to refer to a function itself, like a separate entity, you should leave off the parentheses so you don't call it: funcName.
So, another solution for your code would be:
var func = funcName;
var run = setInterval(func, 10000);
However, that's a bit redundant: why use func instead of funcName?
Or you can stay as true as possible to the original code by modifying two bits:
var func = funcName;
var run = setInterval("func()", 10000);
In this case, the JS engine will evaluate func() every ten seconds. In other words, it will alert "test" every ten seconds. However, as the famous phrase goes, eval is evil, so you should try to avoid it whenever possible.
Another twist on this code is to use an anonymous function. In other words, a function that doesn't have a name -- you just drop it in the code because you don't care what it's called.
setInterval(function () {
alert("test");
}, 10000);
In this case, since I don't care what the function is called, I just leave a generic, unnamed (anonymous) function there.
Change setInterval("func",10000) to either setInterval(funcName, 10000) or setInterval("funcName()",10000). The former is the recommended method.
That's because you should pass a function, not a string:
function funcName() {
alert("test");
}
setInterval(funcName, 10000);
Your code has two problems:
var func = funcName(); calls the function immediately and assigns the return value.
Just "func" is invalid even if you use the bad and deprecated eval-like syntax of setInterval. It would be setInterval("func()", 10000) to call the function eval-like.
Try this:
function funcName() {
alert("test");
}
var run = setInterval(funcName, 10000)
Btw this didn't work
setInterval(function () {
alert("test");
}, 10000);
I had to give a name (whatever you like) to the anonymous function:
setInterval(function alertNotification () {
alert("test");
}, 10000);
I know there is an Answer for this But!! All The Answers covered with only one setTimeout in the loop this Question Looks relevant to me How do I add a delay in a JavaScript loop?
But in my Scenario I Have two setTimeout in the Script, How can this be implemented correctly with timing !! The Program works correctly but the timing what I want is not correct !!!
function clickDate(i)
{
setTimeout((function(){
alert("4");
})(),2000);
}
function clickButton(i)
{
setTimeout((function(){
alert("5");
})(),4000);
}
function doEverything(i)
{
clickDate(i);
clickButton(i);
}
for(var i = 0; i < 4; i++)
{
doEverything(i);
}
You're immediately calling the function when you pass it to setTImeout. Remove the extra parenthesis.
function clickDate(i)
{
setTimeout(function(){
alert("4");
},2000);
}
function clickButton(i)
{
setTimeout(function(){
alert("5");
},4000);
}
function doEverything(i)
{
clickDate(i);
clickButton(i);
}
for(var i = 0; i < 4; i++)
{
doEverything(i);
}
EDIT
It's a little unclear what exactly it is you want your code to do seeing as you're passing i into your function I assume you want to use it somehow. Currently you're creating timeouts that will all launch at once. You'll need to stagger the delay times if you want them to launch in sequence. The code below logs a "4" every 2 seconds and a "5" every "4" seconds by multiplying the delay time by i+1.
// Currently this code displays a 4 every 2 seconds and a 5 every 4 seconds
function clickDate(i)
{
setTimeout(function(){
console.log("4");
},2000 * (i+1));
}
function clickButton(i)
{
setTimeout(function(){
console.log("5");
},4000 * (i+1));
}
function doEverything(i)
{
clickDate(i);
clickButton(i);
}
for(var i = 0; i < 4; i++)
{
doEverything(i);
}
Hello I think you havent read documentation about javascript.
It's asynchronous and it will not wait for the event and continue the process. I will give the answer but I highly recommend to read about Javascript it's good for you only here you will get timing problem because your both the function will be called at the same time. Let me give you the example.
function clickDate(i,callback)
{
setTimeout(function(){
alert("4");
callback();//this will call anonymous function in doEverything
},2000);
}
function clickButton(i)
{
setTimeout(function(){
alert("5");
},4000);
}
function doEverything(i)
{
console.log("In loop index is " , i);
clickDate(i,function(){
clickButton(i);
});
//look closely here I have passed the function in changeData and will call that funtion from clickDate
console.log("In loop terminating index is " , i);
}
for(var i = 0; i < 4; i++)
{
doEverything(i);
}
So here console log will make you clear about asynchronous
functionality. You will see that for loop terminates as it continues
it's work and easily completed in 2 seconds so before your first alert
for loop will complete it's iteration.
Hopefully this will help.
you are calling the callback immediately by adding () to the end of function .
you need to pass the callback with timeout and it will be call for you
setTimeout(function(){
alert('hello');
} , 3000);
function functionName() {
setTimeout(function(){ //Your Code }, 3000);
}
Try this one.
Your approach to mocking asynchronous behavior in JavaScript with setTimeout is a relatively common practice. However, providing each function with its own invocation of setTimeout is an anti-pattern that is working against you simply due to the asynchronous nature of JavaScript itself. setTimeout may seem like it's forcing JS to behave in a synchronous way, thus producing the 4 4 4 4 then 5 5 you are seeing on alert with iteration of the for loop. In reality, JS is still behaving asynchronously, but because you've invoked multiple setTimeout instances with callbacks that are defined as anonymous functions and scoped within their own respective function as an enclosure; you are encapsulating control of JS async behavior away from yourself which is forcing the setTimeout's to run in a strictly synchronous manner.
As an alternative approach to dealing with callback's when using setTimeout, first create a method that provides the timing delay you want to occur. Example:
// timer gives us an empty named "placeholder" we can use later
// to reference the setTimeout instance. This is important because
// remember, JS is async. As such, setTimeout can still have methods
// conditionally set to work against it.
let timer
// "delayHandler", defined below, is a function expression which when
// invoked, sets the setTimeout function to the empty "timer" variable
// we defined previously. Setting the callback function as the function
// expression used to encapsulate setTimeout provides extendable control
// for us later on however we may need it. The "n" argument isn't necessary,
// but does provide a nice way in which to set the delay time programmatically
// rather than hard-coding the delay in ms directly in the setTimeout function
// itself.
const delayHandler = n => timer = setTimeout(delayHandler, n)
Then, define the methods intended as handlers for events. As a side-note, to help keep your JS code from getting messy quickly, wrap your event handler methods within one primary parent function. One (old school) way to do this would be to utilize the JS Revealing Module Pattern. Example:
const ParentFunc = step => {
// "Private" function expression for click button event handler.
// Takes only one argument, "step", a.k.a the index
// value provided later in our for loop. Since we want "clickButton"
// to act as the callback to "clickDate", we add the "delayHandler"
// method we created previously in this function expression.
// Doing so ensures that during the for loop, "clickDate" is invoked
// when after, internally, the "clickButton" method is fired as a
// callback. This process of "Bubbling" up from our callback to the parent
// function ensures the desired timing invocation of our "delayHandler"
// method. It's important to note that even though we are getting lost
// in a bit of "callback hell" here, because we globally referenced
// "delayHandler" to the empty "timer" variable we still have control
// over its conditional async behavior.
const clickButton = step => {
console.log(step)
delayHandler(8000)
}
// "Private" function expression for click date event handler
// that takes two arguments. The first is "step", a.k.a the index
// value provided later in our for loop. The second is "cb", a.k.a
// a reference to the function expression we defined above as the
// button click event handler method.
const clickDate = (step, cb) => {
console.log(step)
cb(delayHandler(8000))
}
// Return our "Private" methods as the default public return value
// of "ParentFunc"
return clickDate(step, clickButton(step))
}
Finally, create the for loop. Within the loop, invoke "ParentFunc". This starts the setTimeout instance and will run each time the loop is run. Example:
for(let i = 0; i < 4; i++) {
// Within the for loop, wrap "ParentFunc" in the conditional logic desired
// to stop the setTimeOut function from running further. i.e. if "i" is
// greater than or equal to 2. The time in ms the setTimeOut was set to run
// for will no longer hold true so long as the conditional we want defined
// ever returns true. To stop the setTimeOut method correctly, use the
// "clearTimeout" method; passing in "timer", a.k.a our variable reference
// to the setTimeOut instance, as the single argument needed to do so.
// Thus utilizing JavaScript's inherit async behavior in a "pseudo"
// synchronous way.
if(i >= 2) clearTimeout(timer)
ParentFunc(i)
}
As a final note, though instantiating setTimeOut is common practice in mocking asynchronous behavior, when dealing with initial invocation/execution and all subsequent lifecycle timing of the methods intended to act as the event handlers, defer to utilizing Promises. Using Promises to resolve your event handlers ensures the timing of method(s) execution is relative to the scenario in which they are invoked and is not restricted to the rigid timing behavior defined in something like the "setTimeOut" approach.
In JavaScript, the setTimeout function requires code to be enclosed in a function.
Examples of invalid timeouts:
window.setTimeout(console.log('PING'),3000)
function ping(){console.log('PING')};window.setTimeout(ping(),3000)
Example of valid timeouts:
window.setTimeout(function(){console.log('PING')},3000)
function ping(){console.log('PING')};window.setTimeout(function(){ping()},3000)
Now my question: why? I understand why normal code might need to be enclosed in a function, but why is it ALWAYS necessary to enclose code in function(){}?
It doesn't always require an anonymous function. You can also pass a function reference as the first argument, for example, let's assume you have a function called log defined. You can validly write:
function log()
{
console.log( 'PING' );
}
window.setTimeout( log, 200 );
Notice that we don't pass the parentheses with the first argument here.
However, you're not able to pass parameters directly to log() in this instance, so it's necessary to wrap the function call inside an anonymous function.
The code is required to be enclosed in a function because, the setTimeout function does not execute individual lines of code. It takes two, arguments - the first argument is a callback function, and the second argument is the time in milliseconds. The callback function will be called by the setTimeout function internally after the specified amount of time passes.
In the example you gave
window.setTimeout(function(){console.log('PING')},3000)
you pass an anonymous function which would be called after 3000 milliseconds or 3 seconds.
Basically because setTimeout is an asynchronous operation and you need to give what to do next once the timeout is done (i.e. you give a callback function).
Unless JavaScript runtime could block the UI thread and resume execution once the setTimeout ends, you need to provide some executable code to perform an action once the asynchronous operation has been completed:
setTimeout(function() {
// Do stuff after 1 second
}, 1000);
In the other hand, what would be the point of giving a function return value as argument? Check this wrong code to be more illustrative:
// You do some action even BEFORE the timeout operation has been completed?
setTimeout(doStuff(), 1000);
Further reading
Callbacks in Wikipedia
It doesn't need to be enclosed in function(){} but it does need to be a parameter of type Function.
In the case of window.setTimeout(console.log('PING'),3000), what would happen is that console.log() would immediately be executed and the return value (which is undefined) would be passed to the setTimeout function. This code isn't passing a function as a parameter, it's passing the return value of a function as a parameter. Essentially, it's just a shorter way of writing this:
var retVal = console.log('PING'); // retVal === undefined
window.setTimeout(retVal,3000);
That's not special to setTimeout. console.log without () is a function, but console.log() means to invoke that function.
There are other methods to pass a function in to setTimeout, but the anonymous function is typically the cleanest.
Technically, this would work, too:
window.setTimeout(console.log,3000)
but it would not allow you to specify a parameter, making it rather useless here. This could be avoided by binding parameters:
window.setTimeout(console.log.bind(null,'PING'),3000)
In this case, bind is a function which is being invoked (as you can see by the fact it has parameters supplied), so just as before bind is immediately executed. However, bind is a function whose return value is itself a function, and it's that returned function that is passed to setTimeout and called three seconds later. This technique (which you will see used with bind, call, and apply) is called partial application which really just means that you transform a function taking some parameters into a function taking fewer parameters (in this case, zero parameters) by specifying the parameters now but executing the function later.
Because setTimeout is a function that takes two argument - a function, and a number of seconds before that function executes.
https://developer.mozilla.org/en-US/docs/Web/API/WindowTimers/setTimeout
Parameters:
func: A function to be executed after the timer expires.
code: An
optional syntax allows you to include a string instead of a function,
which is compiled and executed when the timer expires. This syntax is
not recommended for the same reasons that make using eval() a security
risk.
delay: Optional The time, in milliseconds (thousandths of a
second), the timer should wait before the specified function or code
is executed. If this parameter is omitted, a value of 0 is used. Note
that the actual delay may be longer; see Reasons for delays longer
than specified below.
param1, ..., paramN Optional Additional
parameters which are passed through to the function specified by func
once the timer expires.
Apparently YOU can just use code, as per the second argument above, but I have never seen it done.
Simply put...
why does
setTimeout('playNote('+currentaudio.id+', '+noteTime+')', delay);
work perfectly, calling the function after the the specified delay, but
setTimeout(playNote(currentaudio.id,noteTime), delay);
calls the function playNote all at the same time?
(these setTimeout()s are in a for loop)
or, if my explanation is too hard to read, what is the difference between the two functions?
The first form that you list works, since it will evaluate a string at the end of delay. Using eval() is generally not a good idea, so you should avoid this.
The second method doesn't work, since you immediately execute a function object with the function call operator (). What ends up happening is that playNote is executed immediately if you use the form playNote(...), so nothing will happen at the end of the delay.
Instead, you have to pass an anonymous function to setTimeout, so the correct form is:
setTimeout(function() { playNote(currentaudio.id,noteTime) }, delay);
Note that you are passing setTimeout an entire function expression, so it will hold on to the anonymous function and only execute it at the end of the delay.
You can also pass setTimeout a reference, since a reference isn't executed immediately, but then you can't pass arguments:
setTimeout(playNote, delay);
Note:
For repeated events you can use setInterval() and you can set setInterval() to a variable and use the variable to stop the interval with clearInterval().
You say you use setTimeout() in a for loop. In many situations, it is better to use setTimeout() in a recursive function. This is because in a for loop, the variables used in the setTimeout() will not be the variables as they were when setTimeout() began, but the variables as they are after the delay when the function is fired.
Just use a recursive function to sidestep this entire problem.
Using recursion to deal with variable delay times:
// Set original delay
var delay = 500;
// Call the function for the first time, to begin the recursion.
playNote(xxx, yyy);
// The recursive function
function playNote(theId, theTime)
{
// Do whatever has to be done
// ...
// Have the function call itself again after a delay, if necessary
// you can modify the arguments that you use here. As an
// example I add 20 to theTime each time. You can also modify
// the delay. I add 1/2 a second to the delay each time as an example.
// You can use a condition to continue or stop the recursion
delay += 500;
if (condition)
{ setTimeout(function() { playNote(theID, theTime + 20) }, delay); }
}
Try this.
setTimeout(function() { playNote(currentaudio.id,noteTime) }, delay);
Don't use string-timeouts. It's effective an eval, which is a Bad Thing. It works because it's converting currentaudio.id and noteTime to the string representations of themselves and hiding it in the code. This only works as long as those values have toString()s that generate JavaScript literal syntax that will recreate the value, which is true for Number but not for much else.
setTimeout(playNote(currentaudio.id, noteTime), delay);
that's a function call. playNote is called immediately and the returned result of the function (probably undefined) is passed to setTimeout(), not what you want.
As other answers mention, you can use an inline function expression with a closure to reference currentaudio and noteTime:
setTimeout(function() {
playNote(currentaudio.id, noteTime);
}, delay);
However, if you're in a loop and currentaudio or noteTime is different each time around the loop, you've got the Closure Loop Problem: the same variable will be referenced in every timeout, so when they're called you'll get the same value each time, the value that was left in the variable when the loop finished earlier.
You can work around this with another closure, taking a copy of the variable's value for each iteration of the loop:
setTimeout(function() {
return function(currentaudio, noteTime) {
playNote(currentaudio.id, noteTime);
};
}(currentaudio, noteTime), delay);
but this is getting a bit ugly now. Better is Function#bind, which will partially-apply a function for you:
setTimeout(playNote.bind(window, currentaudio.id, noteTime), delay);
(window is for setting the value of this inside the function, which is a feature of bind() you don't need here.)
However this is an ECMAScript Fifth Edition feature which not all browsers support yet. So if you want to use it you have to first hack in support, eg.:
// Make ECMA262-5 Function#bind work on older browsers
//
if (!('bind' in Function.prototype)) {
Function.prototype.bind= function(owner) {
var that= this;
if (arguments.length<=1) {
return function() {
return that.apply(owner, arguments);
};
} else {
var args= Array.prototype.slice.call(arguments, 1);
return function() {
return that.apply(owner, arguments.length===0? args : args.concat(Array.prototype.slice.call(arguments)));
};
}
};
}
I literally created an account on this site to comment on Peter Ajtai's answer (currently highest voted), only to discover that you require 50 rep (whatever that is) to comment, so I'll do it as an answer since it's probably worth pointing out a couple things.
In his answer, he states the following:
You can also pass setTimeout a reference, since a reference isn't executed immediately, but then you can't pass arguments:
setTimeout(playNote, delay);
This isn't true. After giving setTimeout a function reference and delay amount, any additional arguments are parsed as arguments for the referenced function. The below would be better than wrapping a function call in a function.
setTimeout(playNote, delay, currentaudio.id, noteTime)
Always consult the docs.
That said, as Peter points out, a recursive function would be a good idea if you want to vary the delay between each playNote(), or consider using setInterval() if you want there to be the same delay between each playNote().
Also worth noting that if you want to parse the i of your for loop into a setTimeout(), you need to wrap it in a function, as detailed here.
It may help to understand when javascript executes code, and when it waits to execute something:
let foo2 = function foo(bar=baz()){ console.log(bar); return bar()}
The first thing javascript executes is the function constructor, and creates a function object. You can use either the function keyword syntax or the => syntax, and you get similar (but not identical) results.
The function just created is then assigned to the variable foo2
At this point nothing else has been run: no other functions called (neither baz nor bar, no values looked up, etc. However, the syntax has been checked inside the function.
If you were to pass foo or foo2 to setTimeout then after the timeout, it would call the function, the same as if you did foo(). (notice that no args are passed to foo. This is because setTimeout doesn't by default pass arguments, although it can, but those arguments get evaluated before the timeout expires, not when it expires.)
After foo is called, default arguments are evaluated. Since we called foo without passing arguments, the default for bar is evaluated. (This would not have happened if we passed an argument)
While evaluating the default argument for bar, first javascript looks for a variable named baz. If it finds one, it then tries to call it as a function. If that works, it saves the return value to bar.
Now the main body of the function is evaluated:
Javascript looks up the variable bar and then calls console.log with the result. This does not call bar. However, if it was instead called as bar(), then bar would run first, and then the return value of bar() would be passed to console.log instead. Notice that javascript gets the values of the arguments to a function it is calling before it calls the function, and even before it looks up the function to see if it exists and is indeed a function.
Javascript again looks up bar, and then it tries to call it as a function. If that works, the value is returned as the result of foo()
So, function bodies and default arguments are not called immediately, but everything else is. Similarly, if you do a function call (i.e. ()), then that function is executed immediately as well. However, you aren't required to call a function. Leaving off the parentheses will allow you to pass that function around and call it later. The downside of that, though, is that you can't specify the arguments you want the function to be called with. Also, javascript does everything inside the function parentheses before it calls the function or looks up the variable the function is stored in.
Because the second one you're telling it to call the playNote function first and then pass the return value from it to setTimeout.
I have a function called "tryMe" and I'm calling it without the parenthesis not precisely this but the idea is like what you would do here:
setTimeout(tryMe,200);
how can I pass parameters that I need?
I am using a jquery plugin that enables me to call a function but I have to call it withou the parenthesis or it executes itself upon loading.
setTimeout(function() { tryMe(parm1, parm2); }, 200);
A more robust offering, to ensure that the values of parm1, parm2 don't change before the timeout fires (per #lincolnk's comment):
setTimeout(function() {
var p1 = parm1;
var p2 = parm2;
tryMe(p1, p2);
}, 200);
#patrick dw, you're right, has to be evaluated before.
You can wrap tryMe in a closure.
For example:
var f = function(){tryMe('some parameter');};
setTimeout(f, 200);
Here, we create a function object, f, which calls tryMe with the desired parameter(s). Then we pass f to setTimeout. When the timeout expires, f will be called, which will in turn call tryMe with the desired parameters.
A word of warning if you wish to pass in parameters that may change before the timeout gets called (for example, if you are setting several timeouts in a for loop): you will want to bind those variables like so:
var f = function(someParamter){return function(){tryMe(someParameter);};};
setTimeout(f(someParameter), 200);
The reason simply doing something like
setTimeout(tryMe('some parameter'), 200); //Does not work.
doesn't work is because you are passing the result of evaluating tryMe instead of the function object tryMe itself.
You are not calling the function, the browser does - after 200 milliseconds. You are merely providing it with the function that needs being called. Firefox actually allows specifying additional parameters in the setTimeout call:
setTimeout(tryMe, 200, "first parameter", "second parameter");
However, as far as I know no other browsers support this feature so you should really use a closure function as explained by other answers already. Here you create a temporary function with the sole purpose of calling your original function with the right parameters.
setTimeout(function() {tryMe("first parameter", "second parameter")}, 200);