Javascript 中的美元符号是什么，如果不是 jQuery

Question

I did some javascript / jQuery programming a few years ago, and I just started up again. Back then the dollar sign was used for all the jQuery functionality, and if no jQuery library was imported, the dollar sign was not defined.

Today, I started up Firefox, in a completely empty html file with no javascript libraries, and yet the dollar sign points to something. If I open the Firefox console and type '$' I get "function()" .

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

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

Answer 1

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.

Answer 2

It means nothing to the interpreter just like the underscore

From the ECMAScript specification:

The dollar sign ($) and the underscore (_) are permitted anywhere in an identifier. The dollar sign is intended for use only in mechanically generated code.

You may also check JavaScript Dollar Sign ($) - What is it for?

By convention, the dollar sign ($), the underscore (_) and even some ASCII character are permitted to be used anywhere in a JavaScript identifier (Source: Ecma Script documentation (7.6 Identifiers, ECMA-262, 3rd Ed.) the dollar sign is intended for use only in mechanically generated code. This means that we do not want to use the dollar sign ($) in our indentifier names, unless we are writing a framework. The following is a list of permitted characters that can be used anywhere in an identifier name:

IdentifierName ::
IdentifierStart
IdentifierName IdentifierPart
IdentifierStart ::
UnicodeLetter
$
_
UnicodeEscapeSequence
IdentifierPart ::
IdentifierStart
UnicodeCombiningMark
UnicodeDigit
UnicodeConnectorPunctuation
UnicodeEscapeSequence

EDIT:-

Actually dollar sign function has become the more-or-less de facto shortcut to document.getElementById() .

To confirm my point check this :

$(selector)

Returns a single element matching the given CSS selector. In old Firebug versions , this used to be equivalent to document.getElementById .

Answer 3

Dollar sign($) was not assigned, but some browser add function for special usage.

Like Google Chrome, if you type $ on the console, it will return :

function $(selector, [startNode]) { [Command Line API] }

This function assigned for Google Chrome Developer Tool, and let debug more easier.

if you type $('div') , it will return something like this:

e.fn.e.init[178]

and include every div DOM object in it.

BTW, after you click the right button on the mouse to select element, you can acceess angular.js scope by type $scope on the console

Answer 4

Note that $$ isn't quite document.querySelectorAll , because unlike that function it doesn't return a NodeList :

document.querySelectorAll("p") instanceof NodeList // true

$$("p") instanceof NodeList // false

Array.isArray($$("p")) // true

So $$(selector) is really more like Array.from(document.querySelectorAll(selector)) . This means that array methods like map and friends, not just forEach , are available when using $$ which is actually quite useful.

Answer 5

It may be anything, as $ is a valid variable name, just like dollar .

From ECMAScript :

Identifier ::
IdentifierName but not ReservedWord

IdentifierName ::
IdentifierStart
IdentifierName IdentifierPart

IdentifierStart ::
UnicodeLetter
$
_ 
\ UnicodeEscapeSequence

The simplest solution to see what it is and where it is defined would probably to type $() and to put a breakpoint on this line.

Answer 6

Just to complete the other answers here, MooTools also uses $ as a alias to document.getElementById .

It does check if the $ is taken and will then default to document.id .