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

javascript naming-conventions invalid-characters

I noticed that in Internet Explorer (but, unfortunately, not in the other browsers I tested), you can use some Unicode variable names. This made my day, and I was absolutely delighted that I could write fun Unicode-laden code like this:

var ктоείναι草泥马 = "You dirty horse.",
    happy☺n☺mat☺p☺eia = ":)Yay!",
    ಠ_ಠ = "emoticon";

alert(ктоείναι草泥马 + happy☺n☺mat☺p☺eia + ಠ_ಠ);

For some reason, though, ◎ܫ◎, ♨_♨ and ☺ are not valid variable names.

Why do ಠ_ಠ and 草泥马 work, but ◎ܫ◎, ♨_♨ and ☺ don't?

EDIT: Test it out in your browser on JSFiddle. I've tested it in Internet Explorer 9, Chrome, Firefox, and Opera. So far, it seems to only work in Internet Explorer 9. (I don't know about Internet Explorer 8 and below.) Let me know if it works in another browser.

@JamWaffles You need to have a look at aaencode

Does _☺ work? Also, perhaps you should try and work ;_; in there somewhere :)

@JamWaffles I guess you could exploit weird characters to use as many 1-character identifiers as possible in a minifier. Although for 2-byte unicode characters you loose that advantage anyway.

@Cyrille You can, because ∆ is a unicode letter. Same with θ, π, λ, Σ and so on.

I'd like to see Javascript and Java 9 use Math.π as an alias for Math.PI. I am not sure I'd go as far as to use the symbols used in the Scalaz library for Scala, but I think we should all be using Unicode web pages and source code files by now, and I would be happy with using mathematical symbols for mathematical concepts.

cHao

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.

Wow, JS variables are allowed to include zero-width characters? This provides some scary obfuscation abilities... Thank goodness there's no Underhanded Javascript Contest.

@David: The zero-width joiner and non-joiner characters are special in that they affect the rendering of adjacent characters in e.g. Arabic. For such scripts, they need to be present in order to ensure proper display of the intended text.

Oh, I know what they're for. I guess the real question is whether ab and a<ZWNJ>b are different variable names or the same one. If they're different, that's very strange, and exploitable; if they're the same, then that makes sense, and the ZWNJ is indeed only a rendering hack rather than a meaningful part of the variable name.

@Daniel Just tried this: var ab = 'ab'; a‌b = 'a<ZWNJ>b'; alert(ab); and it alerted ab with this warning: "Warning: assignment to undeclared variable ab". Since many pages are filled with warnings like that, it could likely go unnoticed, even by someone who was actually looking at the error console. I'm using Notepad++ with "Show all characters" mode on, and it's not showing the ZWNJ at all. I had to use a hex editor to put it in.

Looks like Stackoverflow edited out the actual ZWNJ characters from my comment, but I think you can see what I mean. I should have mentioned that I'm on Aurora, which (for those who aren't familiar with Mozilla's "channels") is a preview of what will be Firefox 8. Also, why did I not think to put this snippet on jsfiddle?

Peter Mortensen

EcmaScript 262, section 7.6 says names must start with $, _, or a Unicode letter, and after that may contain either those characters, or Unicode combining marks, Unicode digits, or Unicode connector punctuation (and a couple of format-control characters that are language specific.)

So, the difference between allowed and not allowed identifiers in your cases is probably whether the leading character is considered a "letter".

So ಠ is a unicode letter but ◎ isn't? And even so, it doesn't seem to work in browsers other than IE, so I would imagine the ECMAScript standard wouldn't mean much in this case.

I expect what we're seeing here is EcmaScript attempting to make the language more accessible to non-english speakers. I would expect to see support in other browsers emerge in due course.

The offending character and it is a U+25CE BULLSEYE which is in Category So = Symbol, other, and hence indeed not a Unicode letter. What do you mean "it doesn't work in browsers other than IE?"

@Ray see the jsFiddle link in my question and the comment that goes along with it.

I tried the fiddle, and thanks for posting it. The first variable is fine in Chrome, but the second is not, because it contains the happy face character U+263A. This is a symbol from category So, and Chrome is obeying the ECMAScript standard which prohibits these characters in identifiers. So your first comment to this answer is, I think, right on; IE is not obeying the standard if it lets that variable be declared.

Mathias Bynens

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 6.3:

https://i.imgur.com/YEE9a.png

P.S. If you were to summarize all these rules in a single ASCII-only regular expression for JavaScript, it would be over 9,000 characters long. Here it is:

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I just discovered that var await = 123 is perfectly legal!

Ben Aston

For object properties, you can use square-bracket syntax:

window["◎ܫ◎"] = true;
alert(window["◎ܫ◎"]); // alerts "true"

Not quite the same thing. You can't declare a function-scoped variable this way; it only works for globals, as they're properties of the global object. I suppose if you really really wanted to, you could say var local = { '◎ܫ◎' => true, (other vars...) };...but it's probably not worth the trouble.

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

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