Why would a JavaScript variable start with a dollar sign? [duplicate]

Answer 1

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...

Answer 2

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

Answer 3

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.

Answer 4

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.

Answer 5

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

Answer 6

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.