Develop Reference

(Why) does jQuery .click() require a callback function?

I have the following jQuery code:
function next() {
//some code here
}
function previous() {
//some code here
}
$("#next").click(function(){
next();
});
$("#previous").click(function(){
previous();
});
This works, but this doesn't:
$("#next").click(next());
$("#previous").click(previous());
Why is this happening? Is there a problem in my code, or is this just a thing with jQuery? Note: #next and #previous refer to two buttons in my html file.

The callback should be a reference to the function.
Why $("#next").click(next()); doesn't work?
func() is a function call and not a reference, which is why it is called immediately.
This,
$("#next").click(function(){
next();
});
is a preferable way in case you need to pass arguments.
Else,
$("#next").click(next) //notice just the signature without ()

This works (if the functions next and previous are defined):
$("#next").click(next);
$("#previous").click(previous);
In this case the next and previous are also callback functions, the difference between the two is,
when you call this line
$("#next").click(next()); the function is executed immediately, and you are passing the result of the next function to the eventHandler of jQuery.
and in this case
$("#next").click(next); you are passing the function next to the EventHandler of jQuery.
Btw.: in the jQuery API Documentation (https://api.jquery.com/click/) it shows all parameters for the click function and the required types it states: "...handler Type: Function( Event eventObject ) A function to execute each time the event is triggered. ..."

try like this you will get your answer,
function next() {
//some code here
}
function previous() {
//some code here
}
$("#next").click(next);
$("#previous").click(previous);
working demo jsfiddle Example

What is going on there is a little bit obscured by the syntax of anonymous functions function() { ... }. What you are doing by that is passing a function, without calling it. And I want to explain how this works:
If you have a simple function
function next() { return 5 };
It will simply return the value 5, if you call it from somewhere:
a = next(); // value of a will be 5
But what you can do too, is to pass the whole function to a. This is possible, because functions in JavaScript are actually objects:
a = next;
b = a(); // value of b will be 5
If you look at the syntax, it shows you, that putting parentheses () at the end of a function invokes it, and returns the return value. While the naked string, without parentheses hands you the function itself.
So what is a callback now, and what does click() like to get as a parameter? A callback function is a function, that gets called later; we actually hand it over, to get called later. click() would like to get such a function as parameter, and it should be clear now, that we have to pass the function without parentheses, to enable click() to call it later, instead of just passing a 5 to it.
$("#next").click(next);
So how does then the initial syntax with the anonymous function work?
function() { next(); }
actually wraps your next() into another function, which is anonymous – because it does not have a name – but is working in the same way as a named function. You can even set a variable by it:
a = function() { next(); } // a will be the anonymous function that calls next()
But calling that function a() will return nothing, because the anonymous function does not return a value (To be exactly: every function call in JavaScript is returning at least undefined, but that's a technical detail).
It can even be called immediately by putting parenthesis at the end of it:
a = function() { return next(); }() // value of a will be 5
Adding the return there will make sure, the return value of next() will be passed through the anonymous function.
This should make clear why
$("#next").click(function(){ next(); });
is working, and why
$("#next").click(next());
is not, but
$("#next").click(next);
will be a good solution.

$("#next").click(next); would work. Notice parenthesis are not required as the function/callback handler should be passed as a parameter.

Anonymous Callback Function Clarification

I'm brushing up on callback functions and came across the following passage from http://javascriptissexy.com/understand-javascript-callback-functions-and-use-them/#
"When we pass a callback function as an argument to another function, we are only passing the function definition. We are not executing the function in the parameter. In other words, we aren’t passing the function with the trailing pair of executing parenthesis () like we do when we are executing a function.
And since the containing function has the callback function in its parameter as a function definition, it can execute the callback anytime."
Can someone explain that? Here are two examples they provided.
​//The item is a callback function
$("#btn_1").click(function() {
alert("Btn 1 Clicked");
});
Here is another example:
var friends = ["Mike", "Stacy", "Andy", "Rick"];
​
friends.forEach(function (eachName, index){
console.log(index + 1 + ". " + eachName); // 1. Mike, 2. Stacy, 3. Andy, 4. Rick​
});
"Note that the callback function is not executed immediately. It is “called back” (hence the name) at some specified point inside the containing function’s body. So, even though the first jQuery example looked like this:
//The anonymous function is not being executed there in the parameter. ​
​//The item is a callback function
$("#btn_1").click(function() {
alert("Btn 1 Clicked");
});
the anonymous function will be called later inside the function body. Even without a name, it can still be accessed later via the arguments object by the containing function."
For the first example with jquery, what are they saying exactly. If the #btn_1 element is clicked, will the anonymous function be executed? I am assuming it will be executed if the button is clicked, but the wording from the passage was confusing?
Similarly, for the second example, do they not need to call the function that they passed as an argument bc its anonymous?

In both examples, you are passing an anonymous function as a parameter.
$("#btn_1").click(function() {
alert("Btn 1 Clicked");
});
jQuery's click method takes a function as its first parameter. So imagine that click's function definition is this:
function click(fn) {
// fn will contain a reference to any
// function passed as the first parameter to click
// merely calling fn does nothing, because you are just 'calling'
// the reference.
fn;
// Since what is inside of fn is a function, you can execute it
// with the () syntax
fn();
}
// Now, you have many ways to pass a function as the first parameter to the function
// 1. As an anonymous function:
click(function() {
console.log("Hi");
});
// 2. As a named function:
click(function hello() {
console.log("Hi");
});
// 3. As a reference to a function declaration
function hiThere() {
console.log("Hi");
}
click(hiThere);
// 4. As a variable that holds an anonymous function inside
var howdy = function () {
console.log("howdy");
};
click(howdy);
Just imagine that functions are like variables, but they have content inside that can be executed with () at the end.
function hi() {
console.log('bye');
}
hi; // Calls the reference, but does not execute it. This does nothing.
hi.toString(); // Returns the function as a string
hi(); // Executes the code within the function
Whenever you declare a named function, you can do stuff with it according to its name, like you would do with variables. Of course, unlike variables, they hold executable code inside, and not values.
You can't reference an anonymous function, because it's well... anonymous. UNLESS, you hold it inside of something that has a name, like a var.
var iHoldAFunctionInside = function () {
console.log('Im not so anonymous now');
};
iHoldAFunctionInside(); // Logs "Im not so anonymous now"
And that is why you can pass an anonymous function as a parameter to a function, and it can execute it as a callback. Because the parameter now 'holds' the anonymous function inside of it:
function iExecuteYourCallback(callback) {
// callback contains the anonymous function passed to it
// Similar to doing:
// var callback = function () { };
callback();
}
iExecuteYourCallback(function() {
console.log('Im a callback function!');
});
Hope this helps clear things a bit.

In javascript functions are first class members to you can pass a function as an parameter and the called function can accept it as a named argument.
A simple example can be as below
function testme(callback) {
//here the argument callback refers to the passed function
//the timer is used just to delay the execution of the callback
setTimeout(function () {
//the passed function is called here
callback();
}, 1000)
}
testme(function () {
alert('x')
})
Demo: Fiddle
In your examples, yes the first callback will be executed once the element with id btn_1 is clicked.

Passing parameters to callbacks JavaScript

I would like for a callback function to fire after a click event. Currently I have the JavaScript
$('#btnSubmit').click(function ()
{
$('#testDiv').hide('slow', onComplete('test'));
});
var onComplete = function (t)
{
$('#hiddenDiv').hide();
alert(t);
}
The callback function is supposed to be fired after the hiding of #testDiv. However, the onComplete function fires first. If I remove the parameters on onComplete and just give it a reference and not invoke it, then the function fires at the right time, but I can't pass parameters to it. How can I pass parameters to onComplete and not have it fire before the div is finished hiding?
fiddle here

You have to actually have a anonymous function there wrapping your onComplete():
$('#btnSubmit').click(function () {
$('#testDiv').hide('slow', function () {
onComplete('test')
});
});
Demo here
In the jQUery docs:
complete
Type: Function()
A function to call once the animation is complete.
When adding onComplete() without a wrapping function the function will be called immediately, otherwise you need to reference it just with onComplete, but then you cannot pass your value unless you use .bind() to pass your parameter.

Use anonymous function:
$('#btnSubmit').click(function () {
$('#testDiv').hide('slow', function () {
onComplete('test')
});
});

You can use .bind() :
$('#testDiv').hide('slow', onComplete.bind(null, 'test'));
The first parameter is the value this will have in the onComplete function. All other parameters are the argument list.

Calling a function inside itself using setTimeout

I want to call a function within itself like this:
$(document).ready (
function ready() {
var tester = $.ajax({
async: false,
url: "test_parse.php"
}).responseText;
document.getElementById('test').innerHTML = tester;
setTimeout(ready(), 3000);
}
);
But every time I do this my browser just keeps loading and eventually Apache shuts down (obviously not my expected result). Could you help me to figure out a solution?

setTimeout takes a function reference:
setTimeout(ready, 3000);
not
setTimeout(ready(), 3000);
And that being said, I would also do this:
$(document).ready (
function ready() {
var tester = $.ajax({
url: "test_parse.php",
success: function (data) {
document.getElementById('test').innerHTML = data;
setTimeout(ready, 3000);
}
})
}
);
Because async: false will lock up the browser until that data returns from the server

This is wrong:
setTimeout(ready(), 3000);
This is right:
setTimeout(ready, 3000);
ready() is actually invoking the function. ready is simply a reference to the function, which is what you want.

setTimeout expects a function reference as the first parameter, you have a function invocation which is passing the result of calling ready().
This is causing an infinite loop.
You need to pass in "ready", not "ready()"
setTimeout(ready, 3000);
And if you're trying to queue ajax requests that happen in a structured order, you'll want to fire the setTimeout on success after the previous ajax call, not immediately, otherwise you'll have ajax results returning and updating at arbitrary intervals depending on how the server responds to each request.
$.ajax({
// your ajax settings
}).success(function () {
// fire off the next one 3 secs after the last one completes
setTimeout(ready, 3000);
});
This is better than using the async: false setting, which blocks the browser.

Why are you trying to call the function within itself? Surely all you need to do is move setTimeout outside of the function, then call ready() every 3000ms? What'l do you want your output to be?

how to call a function with passing parameter?

i want to pass the event to function in javascript suppose. i am fire function from two input. then how i can call the event with passing parameter.
can i do this using calling and without binding event in jQuery.

I'm not exactly sure what you're asking, but here's an example of passing a function with parameters as a callback:
setTimeout(function() {
return myFunction(false, 3);
}, 1000);
You wrap your function call in an anonymous function wrapper.

Example:
function DoCSSStuff(param1,param2)
{
$(param1).css('height', param2);
};
That way you could call the css function to manipulate height with passed parameters.