Late addition: with Number.toLocaleString() now available on everything bar IE 10 & below, this works, albeit rather long-winded:
var n = 100.67287;
console.log(n.toLocaleString(undefined, {
minimumFractionDigits: 2,
maximumFractionDigits: 2
}));
Using undefined or ‘default’ for the language code will use the browser default language to format the number.
See developer.mozilla.org/en/docs/Web/JavaScript/Reference/Global_Objects/Number/toLocaleString for full details.
If you’re free to extend the Number prototype, you could defined Number.toLocaleFixed().