Can someone explain the dollar sign in Javascript?

Question

The code in question is here:

var $item = $(this).parent().parent().find('input');

What is the purpose of the dollar sign in the variable name, why not just exclude it?

Solution 1

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.

Solution 2

In your example the $ has no special significance other than being a character of the name.

However, in ECMAScript 6 (ES6) the $ may represent a Template Literal

var user = 'Bob'
console.log(`We love ${user}.`); //Note backticks
// We love Bob.

Solution 3

The $ sign is an identifier for variables and functions.

https://web.archive.org/web/20160529121559/http://www.authenticsociety.com/blog/javascript_dollarsign

That has a clear explanation of what the dollar sign is for.

Here's an alternative explanation: http://www.vcarrer.com/2010/10/about-dollar-sign-in-javascript.html

Solution 4

The dollar sign is treated just like a normal letter or underscore (_). It has no special significance to the interpreter.

Unlike many similar languages, identifiers (such as functional and variable names) in Javascript can contain not only letters, numbers and underscores, but can also contain dollar signs. They are even allowed to start with a dollar sign, or consist only of a dollar sign and nothing else.

Thus, $ is a valid function or variable name in Javascript.

Why would you want a dollar sign in an identifier?

The syntax doesn't really enforce any particular usage of the dollar sign in an identifier, so it's up to you how you wish to use it. In the past, it has often been recommended to start an identifier with a dollar sign only in generated code - that is, code created not by hand but by a code generator.

In your example, however, this doesn't appear to be the case. It looks like someone just put a dollar sign at the start for fun - perhaps they were a PHP programmer who did it out of habit, or something. In PHP, all variable names must have a dollar sign in front of them.

There is another common meaning for a dollar sign in an interpreter nowadays: the jQuery object, whose name only consists of a single dollar sign ($). This is a convention borrowed from earlier Javascript frameworks like Prototype, and if jQuery is used with other such frameworks, there will be a name clash because they will both use the name $ (jQuery can be configured to use a different name for its global object). There is nothing special in Javascript that allows jQuery to use the single dollar sign as its object name; as mentioned above, it's simply just another valid identifier name.

Solution 5

Dollar sign is used in ecmascript 2015-2016 as 'template literals'. Example:

var a = 5;
var b = 10;
console.log(`Sum is equal: ${a + b}`); // 'Sum is equlat: 15'

Here working example: https://es6console.com/j3lg8xeo/ Notice this sign " ` ",its not normal quotes.

U can also meet $ while working with library jQuery.

$ sign in Regular Expressions means end of line.

View more solutions

Comments

[nitesh]

Note: By default, jQuery uses "$" as a shortcut for "jQuery". This has some effects on the use of other Javascript libraries. See docs.jquery.com/Using_jQuery_with_Other_Libraries

[Erik Kaplun]

Javascript does have types; and in any case, how is the dollar sign even related to that? It's just a character that happens to be a legal identifier in Javascript.

[F-3000]

In other words, $ is comparable to any acceptable symbol in variable/function names. Doing var $=function(){} is very same as doing var a=function(){}.

[RustyH]

Thimmayya your comment "By default, jQuery uses "$" as a shortcut for "jQuery"" should be in bold letters at the top of every page on Jquery's website, Their examples are horribly documented.

[TartanLlama]

While this link may answer the question, it is better to include the essential parts of the answer here and provide the link for reference. Link-only answers can become invalid if the linked page changes.

[Nomis]

... @TartanLlama If you downgraded my answer please read my updated answer. Thank you.

[TartanLlama]

Wasn't me! I just flagged it.

[Nomis]

... ok to the person who down-voted this, please explain why and read my edited answer. Thank you.

[TartanLlama]

It is very unlikely that the person who downvoted you will come back, especially since you are answering a 5-year-old question which already has an accepted answer.

[Nomis]

... @TartanLlama this just proves (again) that Stackoverflows anonymous downgrade feature is broken. Answering old questions with accepted answers is ok for me since I myself find it useful to see other answers when I browse around for answers.

[Slight]

I'm not sure I would call that link a "clear" explanation. Does it really take 6+ paragraphs to explain that $ is simply a valid character when defining function and variable names?

[Sander Garretsen]

Looks like the first link is working again. I have to agree with @Slight that alternative explanation is terrible.

[still_dreaming_1]

What about when $ is specified as a function parameter? I am looking at some code like jQuery(document).ready(function($){... Then the $ is used inside the function implementation.

[still_dreaming_1]

Oh, I think I get it. It is just the name of the parameter / variable, which has no special meaning outside of maybe a convention. Then the variable is referenced / used in the implementation.

[Throw Away Account]

@still_dreaming_1, you're wrong. I just started using Angular.js, and I decided to call the parameter to my function scope instead of $scope. It failed to work until I tacked the dollar sign back on. I have no idea what's going on here, but that dollar sign has to mean something.

[still_dreaming_1]

@ThrowawayAccount3Million It's just part of the name. In the case of $scope the $ is one of the characters in that name. Just as if you take off the e at the end of $scope, it won't work, so taking of the $ will also not work. In the case that you see something like $( "a" ).addClass( "test" ), the dollar sign is the entire name of whatever that is.

[Throw Away Account]

If JavaScript functions are lexically scoped, then how does it matter what you name the argument to your function? Since $scope is the parameter to a function you create, it shouldn't matter what you name it.

[蔡宗容]

The sample code in the link is quite easy to understand. Awesome!

[NicDD4711]

I think the question is pretty old, at that time didn't exist ECMAScript6. The meaning of $ as in the explanation / answere of @UpTheCreek is also very important. stackoverflow.com/a/41167804/3408530

[Justin Liu]

You can noconflict the jQuery object with this var $j = jQuery.noConflict(); now $j is the jQuery object.

[Orco]

Hi @cobbal; maybe this could be a separate question, but does the $ symbol have the same semantics in TypeScript?

[VLAZ]

"the $ has no special significance other than being a character of the name." it does have special significance. It indicates that the value for that variable is a jQuery wrapped object. It's a type of Hungarian notation. Thus you know how to operate with the value purely by the naming convention.

[thomasrutter]

This use of the dollar sign did not appear in the question. Is this the result of this answer being moved here from elsewhere?

[Andreas Fester]

@VLAZ Don't confuse language syntax with library conventions. Neither the question nor this answer mention jQuery, but only JavaScript - and in JavaScript, the $ sign does not have any special meaning, besides the template literal in ES6.

[VLAZ]

@AndreasFester "Neither the question nor this answer mention jQuery" the question most certainly shows using jQuery code. It may not mention it but the usage is consistent with how jQuery is used. The question doesn't need to mention jQuery if that's what it actually shows. OP is asking about it because they don't know it.