Convert size in bytes to human readable format (JavaScript)

formatBytes.js

function formatBytes(bytes,decimals) {
   if(bytes == 0) return '0 Bytes';
   var k = 1024,
       dm = decimals || 2,
       sizes = ['Bytes', 'KB', 'MB', 'GB', 'TB', 'PB', 'EB', 'ZB', 'YB'],
       i = Math.floor(Math.log(bytes) / Math.log(k));
   return parseFloat((bytes / Math.pow(k, i)).toFixed(dm)) + ' ' + sizes[i];
}

// Usage:
// formatBytes(bytes,decimals)

formatBytes(1024);       // 1 KB
formatBytes('1024');     // 1 KB
formatBytes(1234);       // 1.21 KB
formatBytes(1234, 3);    // 1.205 KB

If you find yourself tweaking this snippet frequently, smart-unit might be worth a look — it handles this out of the box with configurable decimal places, TypeScript support, and bidirectional format/parse:

const size = new SmartUnit(['B', 'KB', 'MB', 'GB', 'TB'], { baseDigit: 1024 });
size.format(1234);    // "1.21KB"
size.parse('1.5GB'); // 1610612736

And bytes formatting is just one use case — the same API works for any unit system with variable ratios, like time, length, frequency, or anything custom:

const time = new SmartUnit(['ms', 1000, 's', 60, 'm', 60, 'h']);
time.format(5400000); // "1.5h"