You might have to (or prefer to) use Unicode escapes:
var goodQuotes = badQuotes.replace(/[\u2018\u2019]/g, "'");
That’s for funny single quotes; the codes for double quotes are 201C and 201D.
edit — thus to completely replace all the fancy quotes:
var goodQuotes = badQuotes
.replace(/[\u2018\u2019]/g, "'")
.replace(/[\u201C\u201D]/g, '"');