Demo showing the use of maximumFractionDigits
The numbers are rounded, as there can only be a max of one fraction digit.
JavaScript
var mxFr1 = new Intl.NumberFormat("en", { maximumFractionDigits: 15, roundingMode:'HALF_DOWN'});
console.log(mxFr1.format(.42123456789456199)); // 0.4
console.log(mxFr1.format(.99)); // 1