Format a number

Write formatNumber(value, options) without Intl or toLocaleString. It formats a number for display with a thousands separator and a fixed number of decimals.

options is optional, and so is each of its properties: decimals (default 0), thousands (the group separator, default ","), and decimal (the decimal point, default ".").

Round to exactly decimals digits after the point, adding zeros when needed (7 with 2 decimals is 7.00), and with no point at all when decimals is 0. Halves round away from zero: 2.5 becomes 3, and -2.5 becomes -3.

Split the whole-number part into groups of three digits, counting from the right, with thousands between groups: 1234567 is 1,234,567, and 999 has no separator. The digits after the point are never grouped.

Negative numbers start with -. A number that rounds to zero has no minus sign: -0.4 is 0, not -0. You can assume value is finite and smaller than 1e21 in absolute value.

formatNumber(1234567.891, { decimals: 2 }); // "1,234,567.89"
formatNumber(-1234.5);                      // "-1,235"
formatNumber(0.5, { decimals: 3 });         // "0.500"
formatNumber(1234567.891, { decimals: 2, thousands: ".", decimal: "," });
// "1.234.567,89"

What the tests check

1. 1groups the whole part in threes from the right

   expect(formatNumber(1234567)).toBe("1,234,567");
   expect(formatNumber(1000)).toBe("1,000");
   expect(formatNumber(999)).toBe("999");
   expect(formatNumber(0)).toBe("0");
   expect(formatNumber(12345678901)).toBe("12,345,678,901");
   

2. 2rounds to the given decimals, padding with zeros

   expect(formatNumber(3.14159, { decimals: 3 })).toBe("3.142");
   expect(formatNumber(1234.5, { decimals: 2 })).toBe("1,234.50");
   expect(formatNumber(7, { decimals: 1 })).toBe("7.0");
   expect(formatNumber(1234.56)).toBe("1,235");
   

3. 3rounds halves away from zero

   expect(formatNumber(2.5)).toBe("3");
   expect(formatNumber(-2.5)).toBe("-3");
   expect(formatNumber(0.125, { decimals: 2 })).toBe("0.13");
   expect(formatNumber(-0.125, { decimals: 2 })).toBe("-0.13");
   

4. 4never groups the digits after the point

   expect(formatNumber(1234.56789, { decimals: 5 })).toBe("1,234.56789");
   expect(formatNumber(0.123456, { decimals: 6 })).toBe("0.123456");
   

5. 5puts a minus sign in front of negative numbers

   expect(formatNumber(-1234567.891, { decimals: 2 })).toBe("-1,234,567.89");
   expect(formatNumber(-123)).toBe("-123");
   expect(formatNumber(-123456)).toBe("-123,456");
   

6. 6never shows -0

   expect(formatNumber(-0.4)).toBe("0");
   expect(formatNumber(-0.001, { decimals: 2 })).toBe("0.00");
   expect(formatNumber(-0)).toBe("0");
   

7. 7uses the given separators

   expect(formatNumber(1234567.891, { decimals: 2, thousands: ".", decimal: "," })).toBe("1.234.567,89");
   expect(formatNumber(1234567, { thousands: " " })).toBe("1 234 567");
   expect(formatNumber(1234.5, { decimals: 1, thousands: "" })).toBe("1234.5");
   expect(formatNumber(1234.5, { decimals: 1, decimal: "," })).toBe("1,234,5");
   

8. 8works without Intl or toLocaleString

   const toLocaleString = Number.prototype.toLocaleString;
   const NumberFormat = Intl.NumberFormat;
   Number.prototype.toLocaleString = () => {
     throw new Error("formatNumber must not use toLocaleString");
   };
   Intl.NumberFormat = function () {
     throw new Error("formatNumber must not use Intl.NumberFormat");
   };
   let results;
   try {
     results = [
       formatNumber(1234567.891, { decimals: 2 }),
       formatNumber(-9876543.21, { decimals: 1, thousands: " ", decimal: "," }),
     ];
   } finally {
     // Put the built-ins back even when formatNumber throws, before expect runs.
     Number.prototype.toLocaleString = toLocaleString;
     Intl.NumberFormat = NumberFormat;
   }
   expect(results).toEqual(["1,234,567.89", "-9 876 543,2"]);
   

Learn more

• Numbers & mathLesson
• Anchors, boundaries & lookaroundLesson