What Is the Dollar Sign in JavaScript, If Not Jquery

What is the dollar sign in Javascript, if not jQuery

1) Is it correct that dollar sign was not assigned, a few years back, or do I remember wrong?

That's correct and still true.

2) What is the dollar sign, if not jQuery?

Firefox and Chrome implement $, $$ and several others as helper commands. Both will set $$ to document.querySelectorAll(), and set $ to document.querySelector if window.$ hasn't been defined.

So what you're seeing isn't actually standard JavaScript, but a helper in the developer console of your browser. It's also not jQuery (as long as you're not on a page using jQuery). However, it's behaviour is close to the one of jQuery, concerning that querySelector (for single matches) and querySelectorAll (for multiple matches) give you almost the same strength as the jQuery selector.

Is dollar sign defined in Javascript even without jQuery?

No you can't. It's only available in developer console in some browsers without importing jQuery library.

See https://developer.chrome.com/docs/devtools/console/utilities/#querySelector-function:

$(selector) returns the reference to the first DOM element with the specified CSS selector. When called with one argument, this function is an alias for the document.querySelector() function.

Here is a simple example:

console.log($('#div'));

<div id="div"></div>

What is the purpose of the dollar sign in JavaScript?

A '$' in a variable means nothing special to the interpreter, much like an underscore.

From what I've seen, many people using jQuery (which is what your example code looks like to me) tend to prefix variables that contain a jQuery object with a $ so that they are easily identified and not mixed up with, say, integers.

The dollar sign function $() in jQuery is a library function that is frequently used, so a short name is desirable.

Reading a javascript script, why so many $ (dollar signs)?

I think you are reading a JavaScript library famously known as jQuery (or possibly another library). The $ is just a short form for a namespace or use as an identifier.

You can think of it like this jQuery('p') to select all the paragraphs on a page,
or for short form you can just write $('p').

Here is a link for jQuery tutorials/docs jQuery

Here is a list of standards section 7.6 describes it in detail ECMA Standard

What is the meaning of $ sign in JavaScript

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).

dollar sign is not defined

This is because the dollar sign ($) is for jQuery library usage.

To include it in your project, simply import it from their CDN:

<script
    src="https://code.jquery.com/jquery-3.2.1.min.js"
    integrity="sha256-hwg4gsxgFZhOsEEamdOYGBf13FyQuiTwlAQgxVSNgt4="
    crossorigin="anonymous"></script>

So all in all your <head> tag should look like this:

<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>My Site</title>
<meta name="keywords" content="" />
<meta name="description" content="" />
<link href="default.css" rel="stylesheet" type="text/css" media="all" />
<script
  src="https://code.jquery.com/jquery-3.2.1.min.js"
  integrity="sha256-hwg4gsxgFZhOsEEamdOYGBf13FyQuiTwlAQgxVSNgt4="
  crossorigin="anonymous"></script>
<script language="JavaScript" src="functions.js"></script>
</head>

Why does JQuery have dollar signs everywhere?

$ is just a shortcut for jQuery. The idea is that everything is done with the one global symbol (since the global namespaces is ridiculously crowded), jQuery, but you can use $ (because it's shorter) if you like:

// These are the same barring your using noConflict (more below)
var divs = $("div");       // Find all divs
var divs = jQuery("div");  // Also find all divs, because
console.log($ === jQuery); // "true"

If you don't want to use the alias, you don't have to. And if you want $ to not be an alias for jQuery, you can use noConflict and the library will restore $ to whatever it was before jQuery took it over. (Useful if you also use Prototype or MooTools.)

Why use $ (dollar sign) in the name of javascript variables?

The $ in the variable name is only part of the name, but the convention is to use it to start variable names when the variable represents a jQuery object.

var $myHeaderDiv = $('#header');
var myHeaderDiv = document.getElementById('header');

Now later in your code, you know the $myHeaderDiv is already a jQuery object, so you can call jQuery functions:

$myHeaderDiv.fade();

To get from the DOM-variable to the jQuery variable:

var $myHeaderDiv = jQuery(myHeaderDiv); //assign to another variable
jQuery(myHeaderDiv).fade(); //use directly

//or, as the $ is aliased to the jQuery object if you don't specify otherwise:
var $myHeaderDiv = jQuery(myHeaderDiv); //assign
$(myHeaderDiv).fade(); //use

To get from the jQuery variable to the DOM-variable.

var myHeaderDiv = $myHeaderDiv.get(0);

jQuery Dollar Sign Confusion

A dollar sign ($) is actually an alias for jQuery function. And according to the documentation, if you pass a callback as an argument to this function, it will be executed when the DOM is ready.

When it comes to the second part of your question (about why the second part of the code is not working): just check the selectors. For me it is working perfectly (see jsfiddle - it is without .button() method, because I am not loading jQuery UI), so this may be caused by incorrect selectors.

Why would a JavaScript variable start with a dollar sign?

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.

What Is the Dollar Sign in JavaScript, If Not Jquery