I asked John, his answer is: 
>"Since ECMA-262 v5 uses "fractionDigits" in  Number.prototype.toFixed (fractionDigits) and ECMA-402 uses "minimumFractionDigits" and "maximumFractionDigits" in Intl.NumberFormat.prototype.resolvedOptions () 
>I would think that you would want to go with "fractionDigits" to convey what you are doing."

So 'fractionDigits' seems the right naming for this.