fastCurrencyFormatter.js

// Motivation:
var formatted = new Intl.NumberFormat('de-DE', {
  style: 'currency',
  currency: 'EUR',
}).ormat(1332123.55);
// This will format currency however 1 million iterations take 27.000 ms
// Caching the formatter is faster at 440 ms
var formatter = new Intl.NumberFormat('de-DE', {
  style: 'currency',
  currency: 'EUR',
});
var formatted = formatter.format(1332123.55);
// This improvement inspired a fast formatter version which is ~6x faster ~70ms:
ultraFastFormat(1332123.55);
// This is a proof of concept and can be used as a basis for a fully fleged formatting library
function ultraFastFormat(num) {
  // Handle negative numbers by working with the absolute value.
  const isNegative = num < 0;
  const absNum = Math.abs(num);

  // Multiply by 100 and round to get a whole number (in cents)
  const totalCents = Math.round(absNum * 100);
  const fraction = totalCents % 100;
  let integerPart = Math.floor(totalCents / 100);
  
  let intStr = "";
  // Unroll common cases based on the magnitude of the integer part:
  if (integerPart < 1000) {
    // 0 to 999: single group, no separator.
    intStr = integerPart.toString();
  } else if (integerPart < 1000000) {
    // 1,000 to 999,999: exactly two groups.
    const a = Math.floor(integerPart / 1000);
    const b = integerPart % 1000;
    intStr = a.toString() + "." + (b < 10 ? "00" + b : b < 100 ? "0" + b : b.toString());
  } else if (integerPart < 1000000000) {
    // 1,000,000 to 999,999,999: exactly three groups.
    const a = Math.floor(integerPart / 1000000);
    const rem = integerPart % 1000000;
    const b = Math.floor(rem / 1000);
    const c = rem % 1000;
    intStr = a.toString() + "." +
             (b < 10 ? "00" + b : b < 100 ? "0" + b : b.toString()) + "." +
             (c < 10 ? "00" + c : c < 100 ? "0" + c : c.toString());
  } else {
    // For numbers with 4 or more groups, fall back to the generic loop.
    const groups = [];
    do {
      groups.unshift(integerPart % 1000);
      integerPart = Math.floor(integerPart / 1000);
    } while (integerPart > 0);
    intStr = groups[0].toString();
    for (let i = 1; i < groups.length; i++) {
      const g = groups[i];
      intStr += "." + (g < 10 ? "00" + g : g < 100 ? "0" + g : g.toString());
    }
  }
  
  // Format the fractional part using a simple ternary operator.
  const fracStr = fraction < 10 ? "0" + fraction : fraction.toString();
  
  // Prepend a minus sign if the original number was negative.
  return (isNegative ? '-' : '') + intStr + ',' + fracStr + ' €';
}

// Example usage:
console.log(ultraFastFormat(1332123.55));  // "1.332.123,55 €"
console.log(ultraFastFormat(-98765.4));     // "-98.765,40 €"