Interesting question, impressive code.

I only found one thing that could be considered a bug:
`nameNumber('15.000')` will return *"one hundred fiftyundefined thousand"*, this could be fixed by changing your regex to         

    if (!number.match(/^(-?)[\d,]+$/)) {
        throw new Error('invalid number: ' + number);
    }

Other than that I think you went a little overboard in some places to achieve the ultimate DRYness.

This:

    var digits = ['', 'one', 'two', 'three', 'four', 'five', 'six', 'seven', 'eight', 'nine'];
    var tens = ['', 'ten', 'twenty', 'thirty', 'forty', 'fifty', 'sixty', 'seventy', 'eighty', 'ninety'];
    var teens = { 0: 'ten', 1: 'eleven', 2: 'twelve', 3: 'thirteen', 5: 'fifteen', 8: 'eighteen' };

    function nameDigit(digit) {
        return digits[digit];
    }

    function nameTenToNineteen(number) {
        return teens[number] || digits[number] + 'teen';
    }

(ignore the removed newlines) puts the `'teen'` back in the `teens` array, you no longer need a temporary variable or encode that you do not have the add `'teen'` for 10,11 and 12. This has less characters, and less complexity.

The same ( though I am not sure this would reduce character count here ) can be said for `scalePrefix`, it took me a while to figure that this was supposed to return `m`illlion, `b`illion etc. If you were to add `''`, `'thousand'` and `'illion'` at the end of existing entries, then your `nameTripletScale` could be

    function nameTripletScale(digitCount) {
        return scalePrefix[digitCount / 3 ];
    }

Note how it sneakily removes an offensive ternary :P It would also no longer require the magical `+2` in `if (digitCount / 3 > scalePrefix.length + 2) {`

In `extractSymbols` you mix parsing, conversion, validation and error handling, I usually don't like that but the code is succinct enough that I can not offer a reasonable alternative.

The same goes for `function append(item) {` in `nameNumber`, it looks a bit busy but removing it looks worse.

One last item is `nameDigit(digit)`, it requires more characters than say digits[digit]` and about half of the time you actually use direct array access instead of calling `nameDigit`, I would just drop `nameDigit` entirely.