Can someone explain the dollar sign in Javascript?

问题:

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?

回答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.

回答2:

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

回答3:

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 start with a dollar sign.

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.

回答4:

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.

回答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.

回答6:

No reason. Maybe the person who coded it came from PHP. It has the same effect as if you had named it \"_item\" or \"item\" or \"item$$\".

Maybe it\'s some kind of Hungarian notation for the coder to note that the variable is a DOM element or something.

回答7:

Here is a good short video explanation: https://www.youtube.com/watch?v=Acm-MD_6934

According to Ecma International Identifier Names are tokens that are interpreted according to the grammar given in the “Identifiers” section of chapter 5 of the Unicode standard, with some small modifications. An Identifier is an IdentifierName that is not a ReservedWord (see 7.6.1). The Unicode identifier grammar is based on both normative and informative character categories specified by the Unicode Standard. The characters in the specified categories in version 3.0 of the Unicode standard must be treated as in those categories by all conforming ECMAScript implementations.this standard specifies specific character additions:

The dollar sign ($) and the underscore (_) are permitted anywhere in an IdentifierName.

Further reading can be found on: http://www.ecma-international.org/ecma-262/5.1/#sec-7.6

Ecma International is an industry association founded in 1961 and dedicated to the standardization of Information and Communication Technology (ICT) and Consumer Electronics (CE).