Develop Reference

When, if ever, should you use `jQuery` over `$`?

I was looking through some code and noticed:
button: function(e) {
e.preventDefault();
var $target = jQuery(e.target);
var link = $target.attr('href');
I was just a little unclear about the line var $target = jQuery(e.target);.
Why use jQuery here?

Usually you would like to use jQuery instead of $, when the latter conflicts with a globar variable from another library.
For instance, see this list: What JavaScript libraries are known to use the global dollar sign: window.$?
In that case, jQuery provides the noConflict() method, which:
Relinquish jQuery's control of the $ variable.
The documentation also states:
In jQuery's case, $ is just an alias for jQuery, so all functionality is available without using $. If you need to use another JavaScript library alongside jQuery, return control of $ back to the other library with a call to $.noConflict(). Old references of $ are saved during jQuery initialization; noConflict() simply restores them.

Can we replace $ in prototype.js with some thing else like $p or $proto

I am using jquery and prototype.js. There is conflict in $.It could be resolved by using var $j = jQuery.noConflict(), but i have a plenty of file that are using $ for jquery so that would not be easy to remove. even i am using some plugings that also needs to be rename.(like jquery.dataTables.bootstrap.js in which i have to change from $ to $j)
So my question is can we rename $ in prototype.js to something else, so it will be easy.I am using prototype of version 1.5.1.
Thanks

Simple answer, no - the some of the PrototypeJS modules (Array, Enumerable, RegExp, String, etc) depend on the $() function being available, and expect it to return an extended element instead of a jQuery collection.
However you can wrap the jQuery code in a immediately executed function to work around your problem.
for example
(function($){
//jQuery code in here that uses $()
})(jQuery);
This is basically a Javascript closure that renames the jQuery() method to $() inside that context

changing $ to $p causing some problem(like calling jquery hide or show function, prototype.js function gets called). So i what did to solve the conflict is to open a prototype.js related things in new tab and using jquery.noConflict().

difference between $ and $$

I used $ instead of document.getElementById() in javascript and i don't know what is the use of $$. Can any one please tell me what is the use of $$?
Here is the code:
var link_object = $$('a[class="menu_item"]');
if (window.location.href.search('inident.do') >= 0) {
link_object.each(function (elem) {
if (elem.innerHTML == 'Create an Incident') {
elem.style.fontWeight = 'bold';
elem.style.color = 'black';
}
});
}

The single $ is usually a short name for jQuery object.
The double $$ could be anything.
In Angular, seems to designate a private identifier.
You can do something like this when you hate your colleagues :
$$$$($$$[$]);

$ is the selector in jquery. $$ doesn't have any specific meaning in jquery
so you can use it in your own way.
like this
function $$(){
alert('hello');
}
$$();
There may be other libraries like jquery which use $ or may be use $$.
since $ is valid symbol in javascript for variable and function names
it is the best way to simplify the dom selection instead of using long
document.getElementById() like functions.
contribution of Mr. Robg
In jQuery, $ is a function that takes different types arguments. It will accept functions, arrays, objects (native and host), strings or nothing at all. If it gets a string, it will work with a selector or HTML

$$ doesn't have any particular meaning for jQuery. But if you are using some library who can conflict with jQuery, you can change the name of $ function to anything you want by using jQuery noConflict function;
You should look for something like this early in your code
var $$ = jQuery.noConflict();

Are you sure this is jQuery ? It looks like mootools.
In mootools, $$() is the element selector function. It works similar to $() in jQuery.
$('id')
returns the element with id 'id'.
$$('selector')
returns a collection of the elements corresponding to the selector passed.

What is the meaning of "$" sign in JavaScript

In the following JavaScript code there is a dollar ($) sign. What does it mean?
$(window).bind('load', function() {
$('img.protect').protectImage();
});

Your snippet of code looks like it's referencing methods from one of the popular JavaScript libraries (jQuery, ProtoType, mooTools, and so on).
There's nothing mysterious about the use of $ in JavaScript. $ is simply a valid JavaScript identifier. JavaScript allows upper- and lower-case letters (in a wide variety of scripts, not just English), numbers (but not at the first character), $, _, and others.¹
Prototype, jQuery, and most javascript libraries use the $ as the primary base object (or function). Most of them also have a way to relinquish the $ so that it can be used with another library that uses it. In that case you use jQuery instead of $. In fact, $ is just a shortcut for jQuery.
¹ For the first character of an identifier, JavaScript allows "...any Unicode code point with the Unicode property “ID_Start”..." plus $ and _; details in the specification. For subsequent characters in an identifier, it allows anything with ID_Continue (which includes _) and $ (and a couple of control characters for historical compatibility).

From another answer:
A little history
Remember, there is nothing inherently special about $. It is a variable name just like any other. In earlier days, people used to write code using document.getElementById. Because JavaScript is case-sensitive, it was normal to make a mistake while writing document.getElementById. Should I capital 'b' of 'by'? Should I capital 'i' of Id? You get the drift. Because functions are first-class citizens in JavaScript, you can always do this:
var $ = document.getElementById; //freedom from document.getElementById!
When Prototype library arrived, they named their function, which gets the DOM elements, as '$'. Almost all the JavaScript libraries copied this idea. Prototype also introduced a $$ function to select elements using CSS selector.
jQuery also adapted $ function but expanded to make it accept all kinds of 'selectors' to get the elements you want. Now, if you are already using Prototype in your project and wanted to include jQuery, you will be in problem as '$' could either refer to Prototype's implementation OR jQuery's implementation. That's why jQuery has the option of noConflict so that you can include jQuery in your project which uses Prototype and slowly migrate your code. I think this was a brilliant move on John's part! :)

That is most likely jQuery code (more precisely, JavaScript using the jQuery library).
The $ represents the jQuery Function, and is actually a shorthand alias for jQuery. (Unlike in most languages, the $ symbol is not reserved, and may be used as a variable name.) It is typically used as a selector (i.e. a function that returns a set of elements found in the DOM).

As all the other answers say; it can be almost anything but is usually "JQuery".
However, in ES6 it is a string interpolation operator in a template "literal" eg.
var s = "new" ; // you can put whatever you think appropriate here.
var s2 = `There are so many ${s} ideas these days !!` ; //back-ticks not quotes
console.log(s2) ;
result:
There are so many new ideas these days !!

The $() is the shorthand version of jQuery() used in the jQuery Library.

In addition to the above answers, $ has no special meaning in javascript,it is free to be used in object naming. In jQuery, it is simply used as an alias for the jQuery object and jQuery() function.
However, you may encounter situations where you want to use it in conjunction with another JS library that also uses $, which would result a naming conflict. There is a method in JQuery just for this reason, jQuery.noConflict().
Here is a sample from jQuery doc's:
<script src="other_lib.js"></script>
<script src="jquery.js"></script>
<script>
$.noConflict();
// Code that uses other library's $ can follow here.
</script>
Alternatively, you can also use a like this
(function ($) {
// Code in which we know exactly what the meaning of $ is
} (jQuery));
Ref:https://api.jquery.com/jquery.noconflict/

From the jQuery documentation describing the jQuery Core Object:
Many developers prefix a $ to the name of variables that contain jQuery
objects in order to help differentiate. There is nothing magic about
this practice – it just helps some people keep track of what different
variables contain.

Basic syntax is: $(selector).action()
A dollar sign to define jQuery A (selector) to "query (or find)" HTML elements A jQuery action() to be performed on the element(s)

Why/how is everything $() based in jQuery?

I know a bit of JavaScript, and can work fine with jQuery. I just don't get why everything is referenced from $(). My understanding is that $ is never needed in JavaScript (unlike for example PHP, where every variable is prefixed with $).
I've looked through the source code, and it doesn't really make sense. Is it just that $ is the function name (for example, it could have easily have been jQuery(), but they selected $?) I assume not, though, as I don't think $ is valid in function names in JavaScript?

$ is just a global variable that's also a reference to the jQuery function, it's $ on purpose so it's less to type. $ is perfectly valid for a function name in ECMAScript:
function $(){}; alert(typeof $);
Note that if you're using multiple libraries you can use function scope to avoid clashing dollar sign variables, eg:
jQuery.noConflict();
(function($){
$('body').hide();
})(jQuery);

$() is the same as jQuery(). Also, $ is a valid function name.