Micro-feature from ES6, now in Firefox Aurora and Nightly: binary and octal numbers

A couple years ago when SpiderMonkey’s implementation of strict mode was completed, I observed that strict mode forbids octal number syntax. There was some evidence that novice programmers used leading zeroes as alignment devices, leading to unexpected results:

var sum = 015 + // === 13, not 15!
// sum === 210, not 212

But some users (Mozilla extensions and server-side node.js packages in particular) still want octal syntax, usually for file permissions. ES6 thus adds new octal syntax that won’t trip up novices. Hexadecimal numbers are formed with the prefix 0x or 0X followed by hexadecimal digits. Octal numbers are similarly formed using 0o or 0O followed by octal digits:

var DEFAULT_PERMS = 0o644; // kosher anywhere, including strict mode code

(Yes, it was intentional to allow the 0O prefix [zero followed by a capital O] despite its total unreadability. Consistency trumped readability in TC39, as I learned when questioning the wisdom of 0O as prefix. I think that decision is debatable, and the alternative is certainly not “nanny language design”. But I don’t much care as long as I never see it. 🙂 I recommend never using the capital version and applying a cluestick to anyone who does.)

Some developers also need binary syntax, which ECMAScript has never provided. ES6 thus adds analogous binary syntax using the letter b (lowercase or uppercase):

var FLT_SIGNBIT  = 0b10000000000000000000000000000000;
var FLT_EXPONENT = 0b01111111100000000000000000000000;
var FLT_MANTISSA = 0b00000000011111111111111111111111;

Try out both new syntaxes in Firefox Aurora or, if you’re feeling adventurous, in a Firefox nightly. Use the profile manager if you don’t want your regular Firefox browsing history touched.

If you’ve ever needed octal or binary numbers, hopefully these additions will brighten your day a little. 🙂