I must be missing something quite obvious here because something rather strange is happening
I have a bit of js code that goes pretty much like this
setTimeout(myFn(), 20000);
If I m correct when I hit that line, after 20 seconds myFn should run right?
in my case myFn is an ajax call and it happens quite fast ( not at 20seconds and I just dont understand why. Any ideas or pointers?
Try
setTimeout(myFn,20000);
When you say setTimeout(myFn(),20000) your telling it to evaluate myFn() and call the return value after 20 seconds.
The problem is that myFn() is a function call not function pointer.
You need to do:
setTimeout(myFn, 20000);
Otherwise the myFn will be run before the timer is set.
No, the correct line would be setTimeout(myFn, 20000);
In yours, you're actually calling the myFn without delay, on the same line, and its result is scheduled to run after 20 seconds.
Remove the (). If you put them, the function is called directly. Without them, it passed the function as argument.
Related
I am new to js.
I am trying to make a function that only calls input function f every 50 milliseconds.
wrote the code in the fiddle using setTimeout.
but isn't doing exactly what I wanted it to do
can you tell me how to fix it.
providing code below
https://jsfiddle.net/1pqznbgt/
function callSeconds(f){
setTimeout(function(){
alert("testing");
}, 500)
}
You are looking for setInterval instead of setTimeout (setTimeout will execute once, setInterval will execute at an interval)
The 500 in your sample means 500ms rather than 50ms
You will want to call f rather than your anonymous function, so:
What you are looking for is something like:
function callSeconds(f){
setInterval(f, 50);
}
The problem I see is that every time you call an alert() you are stopping execution of JavaScript and each browser can implement the alert differently . setTimeout() only executes once so if you call the function it will wait 500ms and issue an alert if you want a piece of code to operate continually you must use the setInterval() function but you still have the problem of how the browser chooses to handle the alert() function, normally an alert pauses execution of the current code, what effect that has on a timed execution function I do not know. I would try execution on something that does not pause or stop execution of the script. I also notice that in the code segment you have not invoked the parent function that would call the setTimeout function.
You can do one of 2 things here.
First, don't use setTimeout.
Instead use setInterval (f,50).
Or recursively call the function as so:
function foo (f){
f();
setTimeout (foo,50);
}
I was writing a long polling script and ran into a too much recursion error which hung the browser. My goal is to call the same function every 1000ms using setTimeout(). Yes, I could use setInterval() but it is going to be a long polling script and will be waiting for a server response.
I fixed this by removing the () from the function I was calling within the same function.
My script looks like:
function messagePolling(){
console.log("polled")
setTimeout(messagePolling(),1000) // <--- removing `()` from the function works as intended
}
messagePolling();
What is the logic behind this? messagePolling is a function after all isn't it.
You're absolutely right - messagePolling is a function. However, messagePolling() is not a function. You can see that right in your console:
// assume messagePolling is a function that doesn't return anything
messagePolling() // -> undefined
So, when you do this:
setTimeout(messagePolling(), 1000)
You're really doing this:
setTimeout(undefined, 1000)
But when you do this:
setTimeout(messagePolling, 1000)
You're actually passing the function to setTimeout. Then setTimeout will know to run the function you passed - messagePolling - later on. It won't work if it decides to call undefined (the result of messagePolling()) later, right?
Written as
setTimeout(messagePolling(),1000) the function is executed immediately and a setTimeout is set to call undefined (the value returned by your function) after one second. (this should actually throw an error if ran inside Node.js, as undefined is not a valid function)
Written as setTimeout(messagePolling,1000) the setTimeout is set to call your function after one second.
When you type messagePolling you are passing the function to setTimeout as a parameter. This is the standard way to use setTimeout.
When you type messagePolling() you are executing the function and passing the return value to setTimeout
That being said, this code looks odd to me. This function just runs itself. It's going to keep running itself indefinitely if you do this.
Anywhere a function name contains "()" it is executed immediately except when it is wrapped in quotes i.e is a string.
This doesn't seem right, but this is how Firefox appears to be acting.
setTimeout(print(),5000);
function print(){
console.log(1);
}
Before the 5 seconds are up, after a link is clicked and before the next page begins to render, 1 is printed to the console. Any ideas? Thanks!
Edit: retitled, to make it obvious that there is a bug in the example code, not in firefox behavior.
No, the problem is that you are executing the print function immediately. Remove the brackets so you pass the function object rather than execute it.
setTimeout(print,50000);
Firefox will call print when it gets an expression that calls it, such as print().
You are calling print and passing its return value (undefined) to setTimeout.
Drop the () to pass the function itself.
Why when calling fadeIn() onLoad does the browser run through the loop immediately. In other words there is an issue with either the setInterval or the Opacityto().
function Opacityto(elem,v){
elem.style.opacity = v/100;
elem.style.MozOpacity = v/100;
elem.style.KhtmlOpacity = v/100;
elem.style.filter=" alpha(opacity ="+v+")";}
function fadeIn(){
elem=document.getElementById('nav');
for (i=0;i==100;i++){
setInterval(Opacityto(elem,i),100);}
}
I think someone will tell me this can be done easiest with jQuery but I'm interested in doing it with javascript.
Thanks!HelP!
You've got several problems with your fadeIn() function:
A. Your for loop condition is i==100, which is not true on the first iteration and thus the body of the for loop will never be executed (the i++ won't ever happen). Perhaps you meant i<=100 or i<100 (depending on whether you want the loop to run 101 or 100 times)?
B. Your setInterval code has a syntax error EDIT: since you've updated your question to remove the quotes - setInterval expects either a string or a function reference/expression. So you need to either pass it the name of a function without parentheses and parameters, or a function expression like the anonymous function expression you can see in my code down below. in the way you try to build the string you are passing it. You've got this:
"Opacityto("+elem,i+")"
but you need this:
"Opacityto(elem," + i + ")"
The latter produces a string that, depending on i, looks like "Opacityto(elem,0)", i.e., it produces a valid piece of JavaScript that will call the Opacityto() function.
C. You probably want setTimeout() rather than setInterval(), because setInterval() will run your Opacityto() function every 100ms forever, while setTimeout() will run Opacityto() exactly once after the 100ms delay. Given that you are calling it in a loop I'm sure you don't really want to call setInterval() 100 times to cause your function Opacityto() to be run 100 times every 100ms forever.
D. Even fixing all of the above, your basic structure will not do what you want. When you call setInterval() or setTimeout() it does not pause execution of the current block of code. So the entire for loop will run and create all of your intervals/timeouts at once, and then when the 100ms is up they'll all be triggered more or less at once. If your intention is to gradually change the opacity with each change happening every 100ms then you need the following code (or some variation thereon):
function fadeIn(i){
// if called with no i parameter value initialise i
if (typeof i === "undefined") {
i = -1;
}
if (++i <= 100) {
Opacityto(document.getElementById('nav'), i);
setTimeout(function() { fadeIn(i); }, 100);
}
}
// kick it off:
fadeIn();
What the above does is defines fadeIn() and then calls it passing no parameter. The function checks if i is undefined and if so sets it to -1 (which is what happens if you call it without passing a parameter). Then it increments i and checks if the result is less than or equal to 100 and if so calls Opacityto() passing a reference to the element and i. Then it uses setTimeout() to call itself in 100ms time, passing the current i through. Because the setTimeout() is inside the if test, once i gets big enough the function stops setting timeouts and the whole process ends.
There are several other ways you could implement this, but that's just the first that happened as I started typing...
My guess is that there is a nasty comma inside the setInterval, messing the argument list:
"Opacityto("+elem,i+")"
^^^
here
You could try quoting the comma
+ "," +
but eval is evil so don't do that. The good way is to pass a real callback function:
function make_opacity_setter(elem, i){
return function(){
OpacityTo(elem, i);
};
}
...
setTimeout( make_opacity_setter(elem, i), 1000);
Do note that the intermediate function-making-function is needed to avoid the nasty interaction between closures and for-loops.
BTW, when you do
setInterval(func(), 1000)
you call func once yourself and then pass its return value to setInterval. since setInterval receives a junk value instead of a callback it won't work as you want to.
It's not the first time I've used setTimeout(), but I can't figure out what the problem is. The code part of the setTimeout() is executing correctly, but it is executing immediately without the delay. If anyone can see the problem, that would help. Here's the code:
if(token==1){
img1.src=ssImages[imgNum];
num1=0;
num2=10;
setTimeout('crossFade()',2500);
}
Are you sure this is the code? If it executes immediately there are usually two reasons:
The developer thought the time is specified in seconds - but 2500 is fine, that's 2.5 seconds.
He calls the function immediately (e.g. setTimeout(foo(), 1234));
But none of the reasons apply to your code so check the rest of the code if there are any other calls to that function.
Anyway, you should really pass a function instead of a string:
setTimeout(crossFade, 2500);
Or, if you need to specify any arguments:
setTimeout(function() {
crossFade(...);
}, 2500);
I agree with Theifmaster. The window. setTimeout method takes two arguments:
1) Function OR Expression
2) Time in ms
In your code you provide a string or an Expression :
setTimeout('crossFade()',....)
This is generally discouraged as with the use of eval. You should pass a function - either named:
setTimeout(crossFade,....)
OR as suggested anonymous:
setTimeout(function(){crossFade()},....
This is about all you can do to trouble shoot this code unless you provide an example ok jsfiddle for us to see the context this is called.