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what does the function name starts with '$' mean? [duplicate]

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What is the purpose of the dollar sign in JavaScript?
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Closed 2 years ago.
I quite often see JavaScript with variables that start with a dollar sign. When/why would you choose to prefix a variable in this way?
(I'm not asking about $('p.foo') syntax that you see in jQuery and others, but normal variables like $name and $order)

Very common use in jQuery is to distinguish jQuery objects stored in variables from other variables.
For example, I would define:
var $email = $("#email"); // refers to the jQuery object representation of the dom object
var email_field = $("#email").get(0); // refers to the dom object itself
I find this to be very helpful in writing jQuery code and makes it easy to see jQuery objects which have a different set of properties.

In the 1st, 2nd, and 3rd Edition of ECMAScript, using $-prefixed variable names was explicitly discouraged by the spec except in the context of autogenerated code:
The dollar sign ($) and the underscore (_) are permitted anywhere in an identifier. The dollar sign is intended for use only in mechanically generated code.
However, in the next version (the 5th Edition, which is current), this restriction was dropped, and the above passage replaced with
The dollar sign ($) and the underscore (_) are permitted anywhere in an IdentifierName.
As such, the $ sign may now be used freely in variable names. Certain frameworks and libraries have their own conventions on the meaning of the symbol, noted in other answers here.

As others have mentioned the dollar sign is intended to be used by mechanically generated code. However, that convention has been broken by some wildly popular JavaScript libraries. JQuery, Prototype and MS AJAX (AKA Atlas) all use this character in their identifiers (or as an entire identifier).
In short you can use the $ whenever you want. (The interpreter won't complain.) The question is when do you want to use it?
I personally do not use it, but I think its use is valid. I think MS AJAX uses it to signify that a function is an alias for some more verbose call.
For example:
var $get = function(id) { return document.getElementById(id); }
That seems like a reasonable convention.

I was the person who originated this convention back in 2006 and promoted it on the early jQuery mailing list, so let me share some of the history and motivation around it.
The accepted answer gives this example:
var $email = $("#email"); // refers to the jQuery object representation of the dom object
var email_field = $("#email").get(0); // refers to the dom object itself
But that doesn't really illustrate it well. Even without the $, we would still have two different variable names here, email and email_field. That's plenty good right there. Why would we need to throw a $ into one of the names when we already have two different names?
Actually, I wouldn't have used email_field here for two reasons: names_with_underscores are not idiomatic JavaScript, and field doesn't really make sense for a DOM element. But I did follow the same idea.
I tried a few different things, among them something very similar to the example:
var email = $("#email"), emailElement = $("#email")[0];
// Now email is a jQuery object and emailElement is the first/only DOM element in it
(Of course a jQuery object can have more than one DOM element, but the code I was working on had a lot of id selectors, so in those cases there was a 1:1 correspondence.)
I had another case where a function received a DOM element as a parameter and also needed a jQuery object for it:
// email is a DOM element passed into this function
function doSomethingWithEmail( email ) {
var emailJQ = $(email);
// Now email is the DOM element and emailJQ is a jQuery object for it
}
Well that's a little confusing! In one of my bits of code, email is the jQuery object and emailElement is the DOM element, but in the other, email is the DOM element and emailJQ is the jQuery object.
There was no consistency and I kept mixing them up. Plus it was a bit of a nuisance to keep having to make up two different names for the same thing: one for the jQuery object and another for the matching DOM element. Besides email, emailElement, and emailJQ, I kept trying other variations too.
Then I noticed a common pattern:
var email = $("#email");
var emailJQ = $(email);
Since JavaScript treats $ as simply another letter for names, and since I always got a jQuery object back from a $(whatever) call, the pattern finally dawned on me. I could take a $(...) call and just remove some characters, and it would come up with a pretty nice name:
$("#email")
$(email)
Strikeout isn't perfect, but you may get the idea: with some characters deleted, both of those lines end up looking like:
$email
That's when I realized I didn't need to make up a convention like emailElement or emailJQ. There was already a nice convention staring at me: take some characters out of a $(whatever) call and it turns into $whatever.
var $email = $("#email"), email = $email[0];
// $email is the jQuery object and email is the DOM object
and:
// email is a DOM element passed into this function
function doSomethingWithEmail( email ) {
var $email = $(email);
// $email is the jQuery object and email is the DOM object
// Same names as in the code above. Yay!
}
So I didn't have to make up two different names all the time but could just use the same name with or without a $ prefix. And the $ prefix was a nice reminder that I was dealing with a jQuery object:
$('#email').click( ... );
or:
var $email = $('#email');
// Maybe do some other stuff with $email here
$email.click( ... );

In the context of AngularJS, the $ prefix is used only for identifiers in the framework's code. Users of the framework are instructed not to use it in their own identifiers:
Angular Namespaces $ and $$
To prevent accidental name collisions with your code, Angular prefixes names of public objects with $ and names of private objects with $$. Please do not use the $ or $$ prefix in your code.
Source: https://docs.angularjs.org/api

Stevo is right, the meaning and usage of the dollar script sign (in Javascript and the jQuery platform, but not in PHP) is completely semantic. $ is a character that can be used as part of an identifier name. In addition, the dollar sign is perhaps not the most "weird" thing you can encounter in Javascript. Here are some examples of valid identifier names:
var _ = function() { alert("hello from _"); }
var \u0024 = function() { alert("hello from $ defined as u0024"); }
var Ø = function() { alert("hello from Ø"); }
var $$$$$ = function() { alert("hello from $$$$$"); }
All of the examples above will work.
Try them.

The $ character has no special meaning to the JavaScript engine. It's just another valid character in a variable name like a-z, A-Z, _, 0-9, etc...

Since _ at the beginning of a variable name is often used to indicate a private variable (or at least one intended to remain private), I find $ convenient for adding in front of my own brief aliases to generic code libraries.
For example, when using jQuery, I prefer to use the variable $J (instead of just $) and use $P when using php.js, etc.
The prefix makes it visually distinct from other variables such as my own static variables, cluing me into the fact that the code is part of some library or other, and is less likely to conflict or confuse others once they know the convention.
It also doesn't clutter the code (or require extra typing) as does a fully specified name repeated for each library call.
I like to think of it as being similar to what modifier keys do for expanding the possibilities of single keys.
But this is just my own convention.

${varname} is just a naming convention jQuery developers use to distinguish variables that are holding jQuery elements.
Plain {varname} is used to store general stuffs like texts and strings.
${varname} holds elements returned from jQuery.
You can use plain {varname} to store jQuery elements as well, but as I said in the beginning this distinguishes it from the plain variables and makes it much easier to understand (imagine confusing it for a plain variable and searching all over to understand what it holds).
For example :
var $blah = $(this).parents('.blahblah');
Here, blah is storing a returned jQuery element.
So, when someone else see the $blah in the code, they'll understand it's not just a string or a number, it's a jQuery element.

As I have experienced for the last 4 years, it will allow some one to easily identify whether the variable pointing a value/object or a jQuery wrapped DOM element
Ex:
var name = 'jQuery';
var lib = {name:'jQuery',version:1.6};
var $dataDiv = $('#myDataDiv');
in the above example when I see the variable "$dataDiv" i can easily say that this variable pointing to a jQuery wrapped DOM element (in this case it is div). and also I can call all the jQuery methods with out wrapping the object again like $dataDiv.append(), $dataDiv.html(), $dataDiv.find() instead of $($dataDiv).append().
Hope it may helped.
so finally want to say that it will be a good practice to follow this but not mandatory.

While you can simply use it to prefix your identifiers, it's supposed to be used for generated code, such as replacement tokens in a template, for example.

Angular uses is for properties generated by the framework. Guess, they are going by the (now defunct) hint provided by the ECMA-262 3.0.

$ is used to DISTINGUISH between common variables and jquery variables in case of normal variables.
let you place a order in FLIPKART then if the order is a variable showing you the string output then it is named simple as "order" but if we click on place order then an object is returned that object will be denoted by $ as "$order" so that the programmer may able to snip out the javascript variables and jquery variables in the entire code.

If you see the dollar sign ($) or double dollar sign ($$), and are curious as to what this means in the Prototype framework, here is your answer:
$$('div');
// -> all DIVs in the document. Same as document.getElementsByTagName('div')!
$$('#contents');
// -> same as $('contents'), only it returns an array anyway (even though IDs must be unique within a document).
$$('li.faux');
// -> all LI elements with class 'faux'
Source:
http://www.prototypejs.org/api/utility/dollar-dollar

The reason I sometimes use php name-conventions with javascript variables:
When doing input validation, I want to run the exact same algorithms both client-side,
and server-side. I really want the two side of code to look as similar as possible, to simplify maintenance. Using dollar signs in variable names makes this easier.
(Also, some judicious helper functions help make the code look similar, e.g. wrapping input-value-lookups, non-OO versions of strlen,substr, etc. It still requires some manual tweaking though.)

A valid JavaScript identifier shuold must start with a letter,
underscore (_), or dollar sign ($);
subsequent characters can also
be digits (0-9). Because JavaScript is case sensitive,
letters
include the characters "A" through "Z" (uppercase) and the
characters "a" through "z" (lowercase).
Details:
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Grammar_and_types#Variables

What exactly does !function ($){...}(window.jQuery) do?

I'd like to know exactly what's going on here. I know what $(document).ready(function() {...}); does and when it comes into effect. Same goes for jQuery(function($) {...}.
But what does this do?
!function ($) {
$(function(){
var $window = $(window)
//normal jquery stuff
})
}(window.jQuery)
Is it loaded when jQuery is loaded instead of when the document is 'ready'?

It creates a closure in which the variable $ is assigned the value of window.jQuery.
The intention is to allow the uninformatively named variable $ to be used as a shortcut for jQuery without conflicting with the large number of other libraries and custom functions that also use $ as a variable name.

Using the ! operator before the function causes it to be treated as an expression
!function () {}()

The syntax you're looking at is used for setting up a jQuery closure. This is used to ensure that the jQuery $ variable is garuanteed to be available and correct within the code; ie it can't be overwritten in the global scope by anything else (which is possible if you're using multiple libraries, etc).
This technique is often used by jQuery plugin authors -- if you're interested in finding out more, the docs are here, and explain in more detail why you'd want to wrap your jQuery code in a function like this.
The only point of interest that's different in your example is that in the docs, the function is wrapped in brackets, whereas in the example you've given it's preceded by a !
The ! is a not operator, but doesn't actually get used for anything; I think it's just there instead of the brackets to save a single character of code. Probably helpful if you're into minifying javascript.

Not quite sure but I guess this is somewhat equivalent to (function(){})() approach and it's about js closures. And it ensures $ and jQuery are the same thing

The '!' is a 'not' operator. It doesn't do anything in the code. The only reason it is there is to signify that the function will execute immediately.
You may also see functions wrapped in parenthesis instead.
(function() {}());
Whatever is used is personal preference.

jQuery like custom functions

I have been wondering how I can create functions like jQuery. For example: $(ID).function()
Where ID is the id of an HTML element, $ is a function that return the document.getElementById reference of the element "ID" and function is a custom javascript function.
I'm creating a little library which implements some functions. And I want to use that sintax without using jQuery.
Now, my questions are: how I can implement that? What is the name of the tecnique that allow that?
Edit:
What I want to do is this:
HTMLElement.prototype.alertMe = function() {alert(this.value);}
Then, when I call document.getElementById('html_input_id').alertMe(), it must show an alertbox with the input value. But HTMLElement.prototype doesn't work in IE.

$ = function(id) {
return document.getElementById(id);
}

Okay, look, what you're asking has a lot of details and implications. The code for jQuery is open source, you can read it for the details; you'd do well to find a good Javascript book as well, the the O'Reilly Definitive Guide.
$ is just a character for names in JS, so as some of the other answers have shown, there's no reason you can't just write a function with that name:
var $ = function(args){...}
Since everyone and his brother uses that trick, you want to have a longer name as well, so you can mix things.
var EstebansLibrary = function(args){...}
var $ = EstebansLibrary; // make an alias
Since you end up doing different things with the entry point function, you need to know how JS uses arguments -- look up the arguments object.
You'll want to package this so that your internals don't pollute the namespace; you'll want some variant of the module pattern, which will make it something like
var EstebansLibrary = (function(){
// process the arguments object
// do stuff
return {
opname : implementation,...
}
})();
And you'll eventually want to be prepared for inheritance and that means putting those functions into the prototype object.

You can use prototype to assign a new function to the Element prototype.
Element.prototype.testFunction=function(str){alert(str)};
This would provide the function 'testFunction' to all HTML elements.
You can extend any base Object this way, i.e. Array, String etc.
This will work without any plugin at all - although that said I don't think it will work in IE. I believe libraries such as MooTools and jQquery create their own inheritance with DOM elements to ensure cross-browser compatibility, although don't quote me on that.

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.