Use toFixed to convert it to a string with some decimal places shaved off, and then convert it back to a number.
+(0.1 + 0.2).toFixed(12) // 0.3
It looks like IE's toFixed has some weird behavior, so if you need to support IE something like this might be better:
Math.round((0.1 + 0.2) * 1e12) / 1e12