I'm passing the click event into my function but doesn't seem to be working?
$('body').on("click", youclick(event));
function youclick(e){
console.log("testing");
e.stopPropagation();
}
Just pass the reference of that function itself. And you don't need to pass the event, you can receive that through global scope,
Try,
$('body').on("click", youclick);
function youclick(e){
console.log("testing");
e.stopPropagation();
}
As #Rajaprabhu suggested. When you are using a callback function(youclick(e)), the callback argument(youclick) is passed as the function name itself without arguments or even the opening or closing braces.
It means that a reference of the function is passed in the function call.
How CallBack Function works?
Because functions are first-class objects in JavaScript, we can treat
functions like objects, so we can pass functions around like variables
and return them in functions and use them in other functions. 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. 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. This allows us to execute the callback functions at any point
in the containing function.
It is important to 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.
$('body').on("click", youclick);
function youclick(e){
console.log("testing");
e.stopPropagation();
}
Related
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.
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.
This is a simple question. Here is my code:
$(document).ready( function () {
func1( "foo", callback);
function callback(param){
alert(param+" is my name");
}
function func1(name, cb) {
cb(name); // alerts "foo is my name"
callback("bar"); // alerts "bar is my name"
}
});
I want to know:
Which one of the function calls inside func1 is the correct callback and why?
Or are they both correct?
Isn't callback("bar"); a normal function call?
Callbacks are meant to let a caller specify what a function should do at some defined point in that function's execution. The function being called shouldn't know the name of that callback function ahead of time. So they'll often be passed to a function as an argument and the function that's supposed to call the callback should just invoke that argument.
When you call callback("bar") in func1, you're totally missing the point of callbacks. You may be invoking the function that you happen to use as a callback, but the point of callbacks is that func1 isn't supposed to know about that. It's just supposed to call the function that's been passed in as an argument (cb). When I'm calling func1 I should be able to pass a completely different callback function and func1 should just call that function without knowing what its name is (it may not even have one!).
The "correct" way is cb(name).
callback("bar"); is directly invoking the callback function where as cb(name); calls the reference passed to the func1,
cb(name); seems to be the correct way here.
First one. Function calls another one which has been pased as a parameter.
It seems like most jquery methods follow this this form for callbacks:
$(SUBJECT).method(function() {
//do stuff
}, /*callback here*/ function(){
//do stuff
});
like for instance
$(foo).click(function() {
$(bar).fadeIn(300, function(){
//call back here
});
});
fiddle
This is best explained by a code example, so can anyone explain why (in technical terms) the anonymous function passed to test doesn't get called after the jQuery hide event?
UPDATE: Not that is really matters for this example what this is referring to, but for clarity lets say test function is in the global scope and this is an anchor element.
test(this, function() {
alert('Called by anonymous function!');
});
function test(object, callback) {
$(object).hide('slow', callback);
}
Changing:
$(object).hide('slow', callback);
To:
$(object).hide('slow', callback());
works. Is this because callback isn't a named function in the current context or global window object?
Your code should look more like this
test(this, function() {...});
You're not calling an anonymous function in your code, but I'm not sure what that syntax actually does
Check out this fiddle of a working example http://jsfiddle.net/L4NxD/2
Edit, made more edits to the fiddle to better duplicate original code. Use http://jsfiddle.net/L4NxD/1 and just http://jsfiddle.net/L4NxD/ to get earlier versions.
This is the correct way to do it.
http://jsfiddle.net/6dFm6/6/
Calling remove after the hide animation will remove the object and result in the animation callback not being called.
The reason it works in jsfiddle.net/6dFm6/1 is because the callback is executed at runtime and the value is passed and called during the callback event.
See these articles for varying level of clarity on what is returned by 'functionName()' as the callback. It appears 'undefined' is returned by the call, however the execution still happens at the correct time.
When to use () after a callback function name?
In JavaScript, does it make a difference if I call a function with parentheses?
Callback function - use of parentheses
do I need parenthesis when using a named function as an jquery ajax success callback
javascript syntax: function calls and using parenthesis
I would like to write a function that accepts an anonymous function as a parameter. For example:
run('param1', function(){
alert('execute this');
});
function run(param1, callback) {
//now execute the callback parameter as a function
}
How can I achieve something like this?
callback() would invoke it.
If you need to supply a context, do callback.apply(this, arguments). When you use .apply be aware of the current execution context, basically know what this will refer to, or your code will not work as expected if you are feeding a literal that references this inside it's function body.