What characters are valid for JavaScript variable names?

Question

Which characters can be used for naming a JavaScript variable?

I want to create a small "extension library" for my non-JavaScript users here at work (who all seem to be squeamish when it comes to the language). I love how jQuery and Prototype have both use the $ dollar sign, and since I use jQuery, I'm looking for another nice single-character symbol to use.

I realize that I could just test out a number of characters, but I'm hoping to narrow down my list of characters to start with (in consideration of future integration with another popular library, perhaps).

Answer 1

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
/^(?!(?:do|if|in|for|let|new|try|var|case|else|enum|eval|false|null|undefined|NaN|this|true|void|with|break|catch|class|const|super|throw|while|yield|delete|export|import|public|return|static|switch|typeof|default|extends|finally|package|private|continue|debugger|function|arguments|interface|protected|implements|instanceof)$)[$A-Z\_a-zxaaxb5xbaxc0-xd6xd8-xf6xf8-u02c1u02c6-u02d1u02e0-u02e4u02ecu02eeu0370-u0374u0376u0377u037a-u037du0386u0388-u038au038cu038e-u03a1u03a3-u03f5u03f7-u0481u048a-u0527u0531-u0556u0559u0561-u0587u05d0-u05eau05f0-u05f2u0620-u064au066eu066fu0671-u06d3u06d5u06e5u06e6u06eeu06efu06fa-u06fcu06ffu0710u0712-u072fu074d-u07a5u07b1u07ca-u07eau07f4u07f5u07fau0800-u0815u081au0824u0828u0840-u0858u08a0u08a2-u08acu0904-u0939u093du0950u0958-u0961u0971-u0977u0979-u097fu0985-u098cu098fu0990u0993-u09a8u09aa-u09b0u09b2u09b6-u09b9u09bdu09ceu09dcu09ddu09df-u09e1u09f0u09f1u0a05-u0a0au0a0fu0a10u0a13-u0a28u0a2a-u0a30u0a32u0a33u0a35u0a36u0a38u0a39u0a59-u0a5cu0a5eu0a72-u0a74u0a85-u0a8du0a8f-u0a91u0a93-u0aa8u0aaa-u0ab0u0ab2u0ab3u0ab5-u0ab9u0abdu0ad0u0ae0u0ae1u0b05-u0b0cu0b0fu0b10u0b13-u0b28u0b2a-u0b30u0b32u0b33u0b35-u0b39u0b3du0b5cu0b5du0b5f-u0b61u0b71u0b83u0b85-u0b8au0b8e-u0b90u0b92-u0b95u0b99u0b9au0b9cu0b9eu0b9fu0ba3u0ba4u0ba8-u0baau0bae-u0bb9u0bd0u0c05-u0c0cu0c0e-u0c10u0c12-u0c28u0c2a-u0c33u0c35-u0c39u0c3du0c58u0c59u0c60u0c61u0c85-u0c8cu0c8e-u0c90u0c92-u0ca8u0caa-u0cb3u0cb5-u0cb9u0cbdu0cdeu0ce0u0ce1u0cf1u0cf2u0d05-u0d0cu0d0e-u0d10u0d12-u0d3au0d3du0d4eu0d60u0d61u0d7a-u0d7fu0d85-u0d96u0d9a-u0db1u0db3-u0dbbu0dbdu0dc0-u0dc6u0e01-u0e30u0e32u0e33u0e40-u0e46u0e81u0e82u0e84u0e87u0e88u0e8au0e8du0e94-u0e97u0e99-u0e9fu0ea1-u0ea3u0ea5u0ea7u0eaau0eabu0ead-u0eb0u0eb2u0eb3u0ebdu0ec0-u0ec4u0ec6u0edc-u0edfu0f00u0f40-u0f47u0f49-u0f6cu0f88-u0f8cu1000-u102au103fu1050-u1055u105a-u105du1061u1065u1066u106e-u1070u1075-u1081u108eu10a0-u10c5u10c7u10cdu10d0-u10fau10fc-u1248u124a-u124du1250-u1256u1258u125a-u125du1260-u1288u128a-u128du1290-u12b0u12b2-u12b5u12b8-u12beu12c0u12c2-u12c5u12c8-u12d6u12d8-u1310u1312-u1315u1318-u135au1380-u138fu13a0-u13f4u1401-u166cu166f-u167fu1681-u169au16a0-u16eau16ee-u16f0u1700-u170cu170e-u1711u1720-u1731u1740-u1751u1760-u176cu176e-u1770u1780-u17b3u17d7u17dcu1820-u1877u1880-u18a8u18aau18b0-u18f5u1900-u191cu1950-u196du1970-u1974u1980-u19abu19c1-u19c7u1a00-u1a16u1a20-u1a54u1aa7u1b05-u1b33u1b45-u1b4bu1b83-u1ba0u1baeu1bafu1bba-u1be5u1c00-u1c23u1c4d-u1c4fu1c5a-u1c7du1ce9-u1cecu1cee-u1cf1u1cf5u1cf6u1d00-u1dbfu1e00-u1f15u1f18-u1f1du1f20-u1f45u1f48-u1f4du1f50-u1f57u1f59u1f5bu1f5du1f5f-u1f7du1f80-u1fb4u1fb6-u1fbcu1fbeu1fc2-u1fc4u1fc6-u1fccu1fd0-u1fd3u1fd6-u1fdbu1fe0-u1fecu1ff2-u1ff4u1ff6-u1ffcu2071u207fu2090-u209cu2102u2107u210a-u2113u2115u2119-u211du2124u2126u2128u212a-u212du212f-u2139u213c-u213fu2145-u2149u214eu2160-u2188u2c00-u2c2eu2c30-u2c5eu2c60-u2ce4u2ceb-u2ceeu2cf2u2cf3u2d00-u2d25u2d27u2d2du2d30-u2d67u2d6fu2d80-u2d96u2da0-u2da6u2da8-u2daeu2db0-u2db6u2db8-u2dbeu2dc0-u2dc6u2dc8-u2dceu2dd0-u2dd6u2dd8-u2ddeu2e2fu3005-u3007u3021-u3029u3031-u3035u3038-u303cu3041-u3096u309d-u309fu30a1-u30fau30fc-u30ffu3105-u312du3131-u318eu31a0-u31bau31f0-u31ffu3400-u4db5u4e00-u9fccua000-ua48cua4d0-ua4fdua500-ua60cua610-ua61fua62aua62bua640-ua66eua67f-ua697ua6a0-ua6efua717-ua71fua722-ua788ua78b-ua78eua790-ua793ua7a0-ua7aaua7f8-ua801ua803-ua805ua807-ua80aua80c-ua822ua840-ua873ua882-ua8b3ua8f2-ua8f7ua8fbua90a-ua925ua930-ua946ua960-ua97cua984-ua9b2ua9cfuaa00-uaa28uaa40-uaa42uaa44-uaa4buaa60-uaa76uaa7auaa80-uaaafuaab1uaab5uaab6uaab9-uaabduaac0uaac2uaadb-uaadduaae0-uaaeauaaf2-uaaf4uab01-uab06uab09-uab0euab11-uab16uab20-uab26uab28-uab2euabc0-uabe2uac00-ud7a3ud7b0-ud7c6ud7cb-ud7fbuf900-ufa6dufa70-ufad9ufb00-ufb06ufb13-ufb17ufb1dufb1f-ufb28ufb2a-ufb36ufb38-ufb3cufb3eufb40ufb41ufb43ufb44ufb46-ufbb1ufbd3-ufd3dufd50-ufd8fufd92-ufdc7ufdf0-ufdfbufe70-ufe74ufe76-ufefcuff21-uff3auff41-uff5auff66-uffbeuffc2-uffc7uffca-uffcfuffd2-uffd7uffda-uffdc][$A-Z\_a-zxaaxb5xbaxc0-xd6xd8-xf6xf8-u02c1u02c6-u02d1u02e0-u02e4u02ecu02eeu0370-u0374u0376u0377u037a-u037du0386u0388-u038au038cu038e-u03a1u03a3-u03f5u03f7-u0481u048a-u0527u0531-u0556u0559u0561-u0587u05d0-u05eau05f0-u05f2u0620-u064au066eu066fu0671-u06d3u06d5u06e5u06e6u06eeu06efu06fa-u06fcu06ffu0710u0712-u072fu074d-u07a5u07b1u07ca-u07eau07f4u07f5u07fau0800-u0815u081au0824u0828u0840-u0858u08a0u08a2-u08acu0904-u0939u093du0950u0958-u0961u0971-u0977u0979-u097fu0985-u098cu098fu0990u0993-u09a8u09aa-u09b0u09b2u09b6-u09b9u09bdu09ceu09dcu09ddu09df-u09e1u09f0u09f1u0a05-u0a0au0a0fu0a10u0a13-u0a28u0a2a-u0a30u0a32u0a33u0a35u0a36u0a38u0a39u0a59-u0a5cu0a5eu0a72-u0a74u0a85-u0a8du0a8f-u0a91u0a93-u0aa8u0aaa-u0ab0u0ab2u0ab3u0ab5-u0ab9u0abdu0ad0u0ae0u0ae1u0b05-u0b0cu0b0fu0b10u0b13-u0b28u0b2a-u0b30u0b32u0b33u0b35-u0b39u0b3du0b5cu0b5du0b5f-u0b61u0b71u0b83u0b85-u0b8au0b8e-u0b90u0b92-u0b95u0b99u0b9au0b9cu0b9eu0b9fu0ba3u0ba4u0ba8-u0baau0bae-u0bb9u0bd0u0c05-u0c0cu0c0e-u0c10u0c12-u0c28u0c2a-u0c33u0c35-u0c39u0c3du0c58u0c59u0c60u0c61u0c85-u0c8cu0c8e-u0c90u0c92-u0ca8u0caa-u0cb3u0cb5-u0cb9u0cbdu0cdeu0ce0u0ce1u0cf1u0cf2u0d05-u0d0cu0d0e-u0d10u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