Question: According to [this](http://mathiasbynens.be/notes/javascript-identifiers), JavaScript allows Unicode characters as identifiers. However, this is how Node handles it.

```
  > var ∆ = 6;
  > ...

```

I've also put it in the [identifier validator](http://mothereff.in/js-variables), and it agrees that "∆" shouldn't be allowed.

I suppose my question is "What's so special about ∆?"

Answer: 
That isn't [U+0394 GREEK CAPITAL LETTER DELTA](http://codepoints.net/U+0394); it's [U+2206 INCREMENT](http://codepoints.net/U+2206), which is a Math Symbol, not a Letter.
JavaScript identifiers can contain any "Unicode Letter", which means

> 
> 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)”.
> 
> 
> 

Now, what you *can* do is this:

```
var \u0394 = 0;

```

and `0394` is the Unicode value for ∆. Clearly, that's not quite as satisfying, but it is syntactically OK.

*edit* — as usual SLaks is correct; you can in fact do this:

```
var Δ = 0;

```

when you've got the right version of Δ. (In my current font the math version is prettier.)