Format bytes

Write formatBytes(bytes, decimals = 1). It turns a size in bytes (a whole number, 0 or more) into a short label like 1.5 KB.

The units are B, KB, MB, GB, TB, and PB, and each is 1024 times the one before: 1 KB is 1024 bytes, and 1 MB is 1024 KB. Use the largest unit in which the size is at least 1, but never go past PB.

Round to decimals decimal places, with halves rounding up, then drop trailing zeros and a trailing point: 1.50 shows as 1.5, and 2.0 as 2. If rounding reaches 1024 and there's a larger unit, use the larger unit instead: 1048575 bytes is 1023.999 KB, which rounds to 1024.0 KB at one decimal, so it shows as 1 MB.

Put one space between the number and the unit.

formatBytes(512);        // "512 B"
formatBytes(1536);       // "1.5 KB"
formatBytes(2048);       // "2 KB"
formatBytes(1234567, 2); // "1.18 MB"
formatBytes(1048575);    // "1 MB"

What the tests check

1. 1shows sizes under 1024 in bytes

   expect(formatBytes(0)).toBe("0 B");
   expect(formatBytes(1)).toBe("1 B");
   expect(formatBytes(1000)).toBe("1000 B");
   expect(formatBytes(1023)).toBe("1023 B");
   

2. 2uses 1024 per unit

   expect(formatBytes(1024)).toBe("1 KB");
   expect(formatBytes(1024 ** 2)).toBe("1 MB");
   expect(formatBytes(5 * 1024 ** 3)).toBe("5 GB");
   expect(formatBytes(1024 ** 4)).toBe("1 TB");
   

3. 3rounds to one decimal by default

   expect(formatBytes(1536)).toBe("1.5 KB");
   expect(formatBytes(1100)).toBe("1.1 KB");
   expect(formatBytes(123456789)).toBe("117.7 MB");
   

4. 4drops trailing zeros

   expect(formatBytes(2048)).toBe("2 KB");
   expect(formatBytes(2048, 2)).toBe("2 KB");
   expect(formatBytes(1536, 3)).toBe("1.5 KB");
   expect(formatBytes(1280, 2)).toBe("1.25 KB");
   

5. 5rounds to the given number of decimals

   expect(formatBytes(1234567, 2)).toBe("1.18 MB");
   expect(formatBytes(1234567, 0)).toBe("1 MB");
   expect(formatBytes(1536, 0)).toBe("2 KB");
   

6. 6moves up a unit when rounding reaches 1024

   expect(formatBytes(1048575)).toBe("1 MB");
   expect(formatBytes(1024 ** 3 - 1)).toBe("1 GB");
   expect(formatBytes(1048575, 3)).toBe("1023.999 KB");
   

7. 7stops at PB

   expect(formatBytes(1024 ** 5)).toBe("1 PB");
   expect(formatBytes(1024 ** 6)).toBe("1024 PB");
   expect(formatBytes(2 * 1024 ** 6)).toBe("2048 PB");
   

Learn more

• Numbers & mathLesson