I want to call function with arguement periodically.
I tried setTimeout("fnName()",timeinseconds); and it is working.
But when I add an arguement it won't work. eg: setTimeout("fnName('arg')",timeinseconds);
You can add an anonymous function:
setTimeout(function() { fnName("Arg"); }, 1000);
Use an anonymous function, like this:
setTimeout(function() { fnName('arg'); }, time);
In general, never pass a string to setTimeout() or setInterval() if you can avoid it, there are other side-effects besides being bad practice...e.g. the scope you're in when it runs.
Just as a side-note, if you didn't need an argument, it's just:
setTimeout(fnName, time);
setTimeout accepts an expression or a function name or an anonymous function but NO () operator.
() will start executing the function immediately and results in setTimeout accept an invalid parameter.
Related
How come I can say:
var myFunction = function() {
setTimeout(myFunction, 1000);
}
myFunction();
Why does the function call in the setTimeout not require parentheses, but the last line does?
Nutshell
myFunction references the function
myFunction() calls the function
More Words
setTimeout expects a function reference* as an argument.
There are circumstances where setTimeout(myFunction(), 1000) might make sense, like if myFunction() returns a function, e.g.
function myFunction() {
return function() {
alert("ohai")
}
}
// Or
const myFunction = () => () => alert("ohai")
So:
setTimeout(myFunction(), 1000);
setTimeout gets the return value of myFunction
myFunction returns a function (that calls alert)
meaning there will be an alert every second.
See also Why function statement requires a name?
* Or a string to be evaluated, but a reference is preferred.
myFunction is a function
myFunction() calls the function and yields whatever value the function returns.
The purpose of setTimeout is running code after some time elapses. You need to pass just the function to it (so setTimeout can itself call the function when appropriate) because if you called the function (with the parenthesis) before passing it to setTimeout it would execute now instead of after 1 second.
When you use the parenthesis, it's saying 'call this function now'. So if you say setTimeout(myFunction(),1000);, it will use the return value of the function as the callback for the timeout. If the return value for the function is not itself a function, you'll get an error because it will try to execute something that isn't executable after the timeout (a string, a number, undefined, etc).
In line 2, the function myFunction is not called, but passed as an argument to the setTimeout function, whereas in line 4 myFunction is called; to call a function, you always have to use parentheses, even if there are no arguments.
I think this example would make it clearer if i may,
function callback() {
console.log('this function runs on page loads.');
}
setTimeout(callback(), 2000);
Here callback() function will run immediately after page loads and won't wait 2 seconds.
function callback() {
console.log('this function runs after page loads.');
}
setTimeout(callback, 2000);
Here callback() function will run after 2 seconds.
How come I can say:
var myFunction = function() {
setTimeout(myFunction, 1000);
}
myFunction();
Why does the function call in the setTimeout not require parentheses, but the last line does?
Nutshell
myFunction references the function
myFunction() calls the function
More Words
setTimeout expects a function reference* as an argument.
There are circumstances where setTimeout(myFunction(), 1000) might make sense, like if myFunction() returns a function, e.g.
function myFunction() {
return function() {
alert("ohai")
}
}
// Or
const myFunction = () => () => alert("ohai")
So:
setTimeout(myFunction(), 1000);
setTimeout gets the return value of myFunction
myFunction returns a function (that calls alert)
meaning there will be an alert every second.
See also Why function statement requires a name?
* Or a string to be evaluated, but a reference is preferred.
myFunction is a function
myFunction() calls the function and yields whatever value the function returns.
The purpose of setTimeout is running code after some time elapses. You need to pass just the function to it (so setTimeout can itself call the function when appropriate) because if you called the function (with the parenthesis) before passing it to setTimeout it would execute now instead of after 1 second.
When you use the parenthesis, it's saying 'call this function now'. So if you say setTimeout(myFunction(),1000);, it will use the return value of the function as the callback for the timeout. If the return value for the function is not itself a function, you'll get an error because it will try to execute something that isn't executable after the timeout (a string, a number, undefined, etc).
In line 2, the function myFunction is not called, but passed as an argument to the setTimeout function, whereas in line 4 myFunction is called; to call a function, you always have to use parentheses, even if there are no arguments.
I think this example would make it clearer if i may,
function callback() {
console.log('this function runs on page loads.');
}
setTimeout(callback(), 2000);
Here callback() function will run immediately after page loads and won't wait 2 seconds.
function callback() {
console.log('this function runs after page loads.');
}
setTimeout(callback, 2000);
Here callback() function will run after 2 seconds.
How come I can say:
var myFunction = function() {
setTimeout(myFunction, 1000);
}
myFunction();
Why does the function call in the setTimeout not require parentheses, but the last line does?
Nutshell
myFunction references the function
myFunction() calls the function
More Words
setTimeout expects a function reference* as an argument.
There are circumstances where setTimeout(myFunction(), 1000) might make sense, like if myFunction() returns a function, e.g.
function myFunction() {
return function() {
alert("ohai")
}
}
// Or
const myFunction = () => () => alert("ohai")
So:
setTimeout(myFunction(), 1000);
setTimeout gets the return value of myFunction
myFunction returns a function (that calls alert)
meaning there will be an alert every second.
See also Why function statement requires a name?
* Or a string to be evaluated, but a reference is preferred.
myFunction is a function
myFunction() calls the function and yields whatever value the function returns.
The purpose of setTimeout is running code after some time elapses. You need to pass just the function to it (so setTimeout can itself call the function when appropriate) because if you called the function (with the parenthesis) before passing it to setTimeout it would execute now instead of after 1 second.
When you use the parenthesis, it's saying 'call this function now'. So if you say setTimeout(myFunction(),1000);, it will use the return value of the function as the callback for the timeout. If the return value for the function is not itself a function, you'll get an error because it will try to execute something that isn't executable after the timeout (a string, a number, undefined, etc).
In line 2, the function myFunction is not called, but passed as an argument to the setTimeout function, whereas in line 4 myFunction is called; to call a function, you always have to use parentheses, even if there are no arguments.
I think this example would make it clearer if i may,
function callback() {
console.log('this function runs on page loads.');
}
setTimeout(callback(), 2000);
Here callback() function will run immediately after page loads and won't wait 2 seconds.
function callback() {
console.log('this function runs after page loads.');
}
setTimeout(callback, 2000);
Here callback() function will run after 2 seconds.
I dont know why my code dont work. Please help!
$('nav').mouseout(setTimeout(function() {
$(this).removeClass('subm')
}, 1000));
Without setTimeout is work normaly.
setTimeout(...) is being called immediately. It returns the id number of the newly pending timeout. The timeout is only registered and called once here. The execution of your code is happening like this:
setTimeout(function() {
$(this).removeClass('subm')
}, 1000);
// = 2
$('nav').mouseout(2);
You need to pass .mouseout() a function that calls setTimeout each time. You also need to fix the this reference, which is different inside the setTimeout callback. This should fix both issues:
$('nav').mouseout(function() {
var self = this;
setTimeout(function() {
$(self).removeClass('subm')
}, 1000);
});
In javascript, like in most other languages, when you do this:
variable = some_function();
you're passing the return value of a function to a variable. Similarly when you do this:
a_function(another_function());
you're passing the return value of another function as an argument to a function.
This works the same in javascript, C, PHP, Ruby and even Fortran.
So, when you do this:
$('nav').mouseout(setTimeout(..));
You're passing the return value of setTimeout as an argument to mouseout. And setTimeout returns a number which can be used in clearTimeout. So you're basically doing this:
$('nav').mouseout(a_number);
What you want instead is to pass a function:
$('nav').mouseout(function(){setTimeout(..)});
Or if you find that hard to read then do this:
function handleMouseOut () {
setTimeout(...);
}
$('nav').mouseout(handleMouseOut); // note we're passing a function here
// not calling it
I have a function in javascript:
function test(){
...
if (){
setTimeout(test(), 1000);
}
}
according to the manual, i can call this with:
setTimeout(test(), 1000);
in fact, it calls, but do not wait the 1s. so I try to use it like the following and it works.
setTimeout(function(){test();}, 1000);
Anyone can explain this to me?
Which manual? Calling test() would call things returned by test() and pass it to setTimeout, so unless your test() is returning a function, this won't work.
You could use the anon function alternative, or you can pass it like setTimeout(test, 1000) without the ().
Another bad usage you might find along the way is to pass it as string like:
setTimeout("test()", 1000)
avoid this at all cost, as this is equivalent to calling eval and you'll run into scoping issue sooner or later.
You should be calling with
setTimeout(test, 1000);
and NOT with
setTimeout(test(), 1000);
In other words, the function you want to call after 1000 ms is test, not the result of calling test!
The reason why
setTimeout(function(){test();}, 1000);
works is that the function you call after 1000ms is a function that calls test, which is basically test itself. For lambda calculus geeks, that's called eta-reduction.
setTimeout expects a function reference. When you pass it this:
setTimeout(test(), 1000);
That is passing the result of calling the test function to setTimeout. Instead, pass it the reference to test itself:
setTimeout(test, 1000);
Want to see something fancy?
function test () {
var what_are_you = 'a closure';
return function () {
alert('I am '+what_are_you+'!')
}
}
setTimeout(test(), 1000);
Here, I am returning a function reference from a function call. See the article below for more info on that!
Documentation
setTimeout on MDN - https://developer.mozilla.org/en/DOM/window.setTimeout
Article about functions, function references - http://jszen.blogspot.com/2008/07/function-reference-vs-function-call.html