This question already has answers here:
When do I use parentheses and when do I not?
(5 answers)
Closed 6 years ago.
I have written a function and intend to call it in several circumstances. Here is the function:
var responsive = function(){
$('#menuicon').toggleClass('menuicon-res');
$('.fixed-menu-item').toggleClass('res');
}
In a certain case, calling the function works only when written WITHOUT parenthesis after it:
$('#menuicon').click(responsive);
In another, similar case, calling the function works only when written WITH parenthesis after it:
$('#top').click(function(){
$('html, body').animate({scrollTop: 0},500);
responsive();
});
I think I understand that the first case (w/o parenthesis) returns the function for callback, and the second case (w/ parenthesis) returns the output of the function. Can anyone help explain why the way to call my function changes in these two scenarios?
Thanks;
CPR
In the first scenario you are assigning the function to be run on your click event, and the click event will do the calling. In the second scenario you are calling the function yourself.
The first case you are talking about is hooking up your function as an event listener, which will call the passed function when an event is triggered
//this will execute when "#menuitem" is clicked
$('#menuicon').click(responsive);
Your second case you are calling it yourself, here just putting responsive without actually invoking it will do nothing.
$('#top').click(function(){
$('html, body').animate({scrollTop: 0},500);
responsive();
});
Related
This question already has answers here:
React Function Calling Methods
(3 answers)
Closed 8 months ago.
I have noticed that (at least in React) there are different ways to call a function. I'd say:
onClick={myFunction}
onClick={myFunction()}
onClick={()=>myFunction}
onClick={()=>myFunction()} /*Not sure if I've seen this type*/
Being the case all are correct, what are the difference between them?
onClick={myFunction}
Assigns myFunction to onClick.
onClick={myFunction()}
Calls myFunction immediately and assigns the return value (which needs to be another function) to onClick.
This is often an error caused by people not understanding how () works and wanting the previous syntax.
onClick={()=>myFunction}
Creates a new function and assigns it to onClick.
The new function mentions myFunction but doesn't do anything with it. It is a noop.
It is always a mistake.
onClick={()=>myFunction()}
Creates a new function, which calls myFunction, and assigns it to onClick.
This is usually a waste of resources. It is useful only if myFunction would do something with the event object that onClick passes to the event handler when called and you want to prevent that.
If you were to pass arguments to myFunction (which this example does not) then it would be more generally useful.
This question already has answers here:
When to use () after a callback function name? [duplicate]
(5 answers)
Closed 6 years ago.
Well I'm starting with jQuery and I'm wondering what is the difference between writing a function with or without parentheses inside a function.
For example if I have the following function:
function test(){
alert("pen pineapple apple pen");
}
This:
$(document).ready(test);
and this:
$(document).ready(test());
both show the same result: "pen pineapple apple pen"
Putting parentheses () at the end of a function causes that function to be invoked immediately, and use its return value in the expression.
This code $(document).ready(test); is using test as a callback function. It essentially says: when the document becomes ready, call the function that I'm providing you with (test).
This code $(document).ready(test()); is immediately invoking the function test, having it return a value, and then passing that value to the ready method. It's possible that test is returning a different function here, which in turn will act as the required callback function. It could also just be an error though, with someone inadvertently including the parentheses when they shouldn't have.
This question already has answers here:
What is a callback function?
(22 answers)
Closed 7 years ago.
$("p").click(function(){
// action goes here!!
});
In the above jquery code why we pass a function() to the event?
It's called a callback or an anonymous function that you want executed when the event happens. You'll notice it's not just jQuery, but almost all Javascript frameworks will expect one. It's usually whenever you are binding to events, executing functions that might take a while to return, or functions that execute other functions.
But you can also provide it a function name if you want.
function clickedMe(){
alert("Something clicked me");
}
$("p").click(clickedMe)
Because the click method is defined this way (the method parameter ).
See:
click
You have to understand its not a default JavaScript method, this method is defined in the JQuery so you have to call it as it is defined in the method.
Behind the scene actually when JQuery registering any event with DOM its only says to do one thing which is calling your provide method.
The simplest answer: according to documentation.
https://api.jquery.com/click/
When a user clicks a p element, jQuery is supposed to fire an event and execute something. How is it supposed to pass actions, if not using functions?
This question already has answers here:
setTimeout ignores timeout? (Fires immediately) [duplicate]
(3 answers)
Closed 8 years ago.
I'm working on a Chrome extension to fetch tweets and I figured that I could use the setInterval() function to make the script run every minute. First I tried giving it the function like this:
setInterval(myFunction(), interval);
But it would only execute my script once.
Then out of curiosity I tried declaring the function in the setInterval() function like so:
setInterval(function() {body of my function}, interval);
And that works, but is not a very pretty solution, does anybody any other way of doing this or am I just going to have to deal with it?
Just remove the brackets from the first call. The reason for this is that you need to pass in the function, not the result of the function (what it returns).
When you write the function's name with brackets, it calls the function. When you exclude the brackets, it simply refers to the function like a variable, and so you can pass in your function to the setInterval() function.
setInterval(myFunction, interval);
This question already has answers here:
Calling a function in JavaScript without parentheses
(7 answers)
Closed 9 years ago.
I have javascript script code here. When I execute my code as it is then it does not trigger func_two function. But when I change following code in func_one
if (this.remove) {
this.func_two;
}
to this
if (this.remove) {
this.func_two();
}
Then it does trigger second function. But I want to trigger it this way this.func_one. IS it possible to do it this way? How?
You have to put () when you call a function, you can't just, out of nowhere, decide that you want it to work another way.
Take a look at this answer, it may help you.
this.func_two;
This statement return the function. It does not call the function. To call the function you have to add () at the end. or you have to do it like:
f2=this.func_two;
f2();