What Characters Are Valid For JavaScript Variable Names

What characters are valid for JavaScript variable names?

To quote Valid JavaScript variable names, my write-up summarizing the relevant spec sections:

An identifier must start with $, _, or any character in the Unicode categories “Uppercase letter (Lu)”, “Lowercase letter (Ll)”, “Titlecase letter (Lt)”, “Modifier letter (Lm)”, “Other letter (Lo)”, or “Letter number (Nl)”.

The rest of the string can contain the same characters, plus any U+200C zero width non-joiner characters, U+200D zero width joiner characters, and characters in the Unicode categories “Non-spacing mark (Mn)”, “Spacing combining mark (Mc)”, “Decimal digit number (Nd)”, or “Connector punctuation (Pc)”.

I’ve also created a tool that will tell you if any string that you enter is a valid JavaScript variable name according to ECMAScript 5.1 and Unicode 6.1:

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P.S. To give you an idea of how wrong Anthony Mills' answer is: if you were to summarize all these rules in a single ASCII-only regular expression for JavaScript, it would be 11,236 characters long. Here it is:

// ES5.1 / Unicode 6.1
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Is it valid to use $ for variable name in JavaScript?

Yes, it has always been valid since the first edition of the ECMAScript language specification.

According to the 10th edition of the ECMAScript language specification published in 2019 (see: ECMAScript Language: Lexical Grammar - Names and Keywords), the grammar for variable names is:

IdentifierName::
  IdentifierStart
  IdentifierName IdentifierPart

IdentifierStart::
  UnicodeIDStart
  $
  _
  \ UnicodeEscapeSequence

IdentifierPart::
  UnicodeIDContinue
  $
  \ UnicodeEscapeSequence
  <ZWNJ>
  <ZWJ>

...

As you can see, it is valid to have $ anywhere in the variable name.

This has always been the case since the first edition of ECMAScript standard published in 1997. Section 4.5 Identifiers from the first edition says:

An identifier is a character sequence of unlimited length, where each character in the sequence must be a letter, a decimal digit, an underscore (_) character, or a dollar sign ($) character, and the first character may not be a decimal digit.

Why aren't ◎ܫ◎ and ☺ valid JavaScript variable names?

ಠ_ಠ
and 草泥马 only contain "letters" used in actual alphabets; that is, ಠ
is a symbol from the Kannada alphabet, and 草泥马 consists of Chinese characters.

◎ and ☺, however, are purely symbols; they are not associated with any alphabet.

The ECMAScript standard, chapter 7.6 (which all the browsers except Internet Explorer are following), states that an identifier must start with one of the following.

• a Unicode letter
• $ or _
• \ followed by a unicode escape sequence.

The following characters of an identifier must be one of the following.

• any of the characters permitted at the start
• a Unicode combining mark
• a Unicode digit
• a Unicode connector punctuation
• a zero-width-non-joiner
• a zero-width joiner

IE goes beyond the standard and is permissive enough to allow some symbols, such as ☺.

There’s a tool that will tell you if any string that you enter is a valid JavaScript variable name according to ECMAScript 5.1 and Unicode 6.1.

What ASCII characters are valid for Javascript variable names?

According to this article, this is all about javascript variables :

The general rules for constructing names for variables (unique identifiers) are:

Names can contain letters, digits, underscores, and dollar signs.
Names must begin with a letter
Names can also begin with $ and _
Names are case sensitive (y and Y are different variables)
Reserved words (like JavaScript keywords) cannot be used as names

You can use all ASCII codes. but you have to comply with above rules.

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.

JavaScript Variable Name Regex

String jsVarName = "1inva>idName".replaceAll("^[^a-zA-Z_$]|[^0-9a-zA-Z_$]", "_");

Note that since \w is [a-zA-Z_0-9], it can be simplified:

String jsVarName = "1inva>idName".replaceAll("^[^a-zA-Z_$]|[^\\w$]", "_")

^[^a-zA-Z_$] matches anything that is not [a-zA-Z_$] and appears at the beginning of the line. | is OR. [^0-9a-zA-Z_$] matches anything that is not [0-9a-zA-Z_$].

See regex tutorial for more info.

Viable JavaScript Variable Names

You can find lists of reserved words in JavaScript.

href is certainly fine as a variable name because href is an attribute of an a tag and in no way conflicts with JavaScript naming.

If you are ever in doubt as to whether or not a variable is already in use you can always open the developer tools (F12 in most browsers), go to the console, and type in the name. In this case you'll get:

> href
x ReferenceError: href is not defined

Nothing is using it, so it is yours to use without problem.

Just for kicks if you did enter a reserved word it would look like:

> finally
x SyntaxError: Unexpected token finally

Or if it was a native but already taken word it might look like:

> Node
function Node() { [native code] }

(Node is already defined, and its a native function)

Can I use `µ` as a javascript variable name

Yes, you can use it.
In example:

var π = Math.PI;

as well as Unicode escape sequences.

Basically the variable in Javascript can be anything that's valid according to ECMAScript 5.1 / Unicode 6.1.

Here is some online JavaScript variable name validator.

For the references, this page is for documenting the differences between ES5 specification and the requirements for ECMAScript implementations in web browsers.

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