This question already has answers here:
What does $(function() {} ); do?
(6 answers)
Closed 9 years ago.
I find the following code in an html document:
<script type="text/javascript">
$(function () {
...
});
I cannot see any intrinsic events like onload = and would like to know how this code is called?
What is the real name and scope of this function and can I call any function defined inside? How?
Whenever you see $ before a function, or $(...).function(...) that usually denotes jQuery.
In the following fiddle I use this code, which is executed on load:
$(function () {
alert("hi!");
});
See here: http://jsfiddle.net/VMZkW/
It is just an anonymous function . In javascript you don't really need to give it a name and because of that after executing once you can never refer it again.
You can have anonymous functions that can be used several times, but not this one. To reuse an anonymous function you just return it to something.
And being an annonymous function it does not create any scope or naming issues and it can access everything according to where it is defined. So you can call outer function too from inside.
Related
This question already has answers here:
jQuery $(function() {}) vs (function () {})($) [duplicate]
(3 answers)
Closed 3 years ago.
What is the difference between (function(){})(); and $(document).ready(function ())
First: (function(){})();
Second: $(document).ready(function ())
I have a question about it.
if I use it in the first option, there is an error when I click it. The Error is that it does not work and there is no error.
but if I use the second one, there is no error. It is not working.
what is the difference between to two ?
This will execute immediatly:
(function(){
console.log("Called immediately invoked function expression");
})();
Where as, the function passed to jQueries $.ready() function will execute when the document can be safely manipulated:
$(document).ready(function () {
console.log("The document is safe to be interacted with");
});
The reason the first method causes errors is likely because the HTML document is not ready for interaction at the time that your function is called (ie immednaitly).
The second approach will however ensure that (in most cases), any scripts, HTML, or other resources (which the JavaScript defined in that function might depend on) are loaded and present before that function is invoked.
This question already has answers here:
Why can I use a function before it's defined in JavaScript?
(7 answers)
Closed 5 years ago.
I'm currently a teacher's assistant in a web development course. Today, a student asked for help with his homework, in which he'd used setInterval, passing as the first parameter a function which he didn't define until a few lines of code later. I told him that wouldn't work, as the function would be undefined by the time the interval setting code was reached.
To my surprise, it worked perfectly. I've been trying to research this and am coming up blank: does JavaScript actually wait until the first execution of the callback to even see if the function name passed to it exists? That seems so counter-intuitive, but I can't imagine any other reason it would have worked. Where can I find out more about this unexpected behavior?
It depends:
If its a function expression :
//callback not defined ( exists but undefined)
var callback=function(){};
//callback defined
If its a function declaration :
//callback is defined
function callback(){}
//callback is defined
This is called hoisting, so vars and functions are moved to the top.
It also depends on the passed function too:
setInterval(callback,0);//doesnt work, callback is *undefined* in this moment
setInterval(function(){ callback();},100);//does work as callback is just called before being referenced.
var callback=function(){};
This question already has answers here:
In JavaScript, does it make a difference if I call a function with parentheses?
(5 answers)
Closed 7 years ago.
I'm trying to understand why this code below executes my function as soon as the document is loaded:
var list = document.getElementsByTagName('li');
function yep() {
window.alert('yep');
}
list[0].onclick = yep();
But this does not:
list[0].onclick = yep;
Why does () make a difference when executing a function in this situation?
The parenthesis () execute function immediately. On your second line you are assigning the value of list[0].onclick to the function name but not executing it.
Putting () after a reference to a function means that you want to call the function. Leaving them off means you just want to work with the reference to the function as a value in and of itself.
yep is a reference to a function.
yep() is a directive telling the Javascript engine to execute the yep function.
That's why one executes immediately and the other does not.
This question already has answers here:
Immediate function invocation syntax
(3 answers)
Closed 9 years ago.
In order to use the $ symbol in jquery and not have to use jQuery.functionname, we use this
(function($) {
})(jQuery);
(In drupal, you actually have to specify this implicitly).
I don't understand this javascript syntax, why is there an initial parentheses? How is the (jQuery) at the end used?
It's just an anonymous function with an argument that's automatically invoked.
For example, if we were to expand it out a bit you'd end up with something like this:
var anon = function($) {
...
};
anon(jQuery);
The $ is a valid identifer in JavaScript and we pass in the existing jQuery object into the function for use through $, as it could be replaced later.
All that's doing is declaring an anonymous function and executing it immediately, passing in one argument (jQuery) into the function. That argument is given the name $ which can be used throughout the scope of the function.
The brackets around the function aren't strictly necessary in all contexts; see the comment under this answer for details. The gist is that they're needed here to make the function behave like an expression instead of a statement (function declaration).
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();