JavaScript To Convert Bytes To MB, KB, Etc

bytesToSize.js

// from http://scratch99.com/web-development/javascript/convert-bytes-to-mb-kb/
function bytesToSize(bytes) {
    var sizes = ['Bytes', 'KB', 'MB', 'GB', 'TB'];
    if (bytes == 0) return 'n/a';
    var i = parseInt(Math.floor(Math.log(bytes) / Math.log(1024)));
    if (i == 0) return bytes + ' ' + sizes[i];
    return (bytes / Math.pow(1024, i)).toFixed(1) + ' ' + sizes[i];
};

Using the Intl.NumberFormat for local friendly conversions

export function bytesToSize(bytes: number): string {
  const units = ['byte', 'kilobyte', 'megabyte', 'gigabyte', 'terabyte'];

  const navigatorLocal = navigator.languages && navigator.languages.length >= 0 ? navigator.languages[0] : 'en-US'
  const unitIndex      = Math.max(0, Math.min(Math.floor(Math.log(bytes) / Math.log(1024)), units.length - 1));

  return Intl.NumberFormat(navigatorLocal, {
    style: 'unit',
    unit : units[unitIndex]
  }).format(bytes / (1024 ** unitIndex))
}

The sizes should be named Mebibyte (MiB), Gibibyte (GiB) etc if you use 1024 for the calculation.

Or if you want to calculate Megabyte, Gigabyte etc. replace the '1024' in the code by '1000'.

@HaukeHa Thank you for noticing 👍 

I use the following code:

export function convertBytes(bytes: number, options: { useBinaryUnits?: boolean; decimals?: number } = {}): string {
  const { useBinaryUnits = false, decimals = 2 } = options;

  if (decimals < 0) {
    throw new Error(`Invalid decimals ${decimals}`);
  }

  const base = useBinaryUnits ? 1024 : 1000;
  const units = useBinaryUnits
    ? ["Bytes", "KiB", "MiB", "GiB", "TiB", "PiB", "EiB", "ZiB", "YiB"]
    : ["Bytes", "KB", "MB", "GB", "TB", "PB", "EB", "ZB", "YB"];

  const i = Math.floor(Math.log(bytes) / Math.log(base));

  return `${(bytes / Math.pow(base, i)).toFixed(decimals)} ${units[i]}`;
}

and basic tests:

import { describe, expect } from "vitest";
import { convertBytes } from "./bytes";

describe("convertBytes", () => {
  const fileSizeInBytes = 2048;

  describe("default conversion", () => {
    it("should convert to KB", () => {
      const resultKB = convertBytes(fileSizeInBytes);
      expect(resultKB).toBe("2.05 KB");
    });

    it("should convert to MB", () => {
      const resultMB = convertBytes(fileSizeInBytes * 1000);
      expect(resultMB).toBe("2.05 MB");
    });

    it("should throw an error for invalid decimals", () => {
      expect(() => convertBytes(fileSizeInBytes, { decimals: -1 })).toThrowError("Invalid decimals -1");
    });
  });

  describe("conversion to binary units", () => {
    it("should convert to KiB", () => {
      const resultKiB = convertBytes(fileSizeInBytes, { useBinaryUnits: true });
      expect(resultKiB).toBe("2.00 KiB");
    });

    it("should convert to MiB", () => {
      const resultMiB = convertBytes(fileSizeInBytes * 1024, { useBinaryUnits: true });
      expect(resultMiB).toBe("2.00 MiB");
    });
  });

  it("should handle very large bytes (Number.MAX_SAFE_INTEGER)", () => {
    const result = convertBytes(Number.MAX_SAFE_INTEGER);
    expect(result).toBe("9.01 PB");
  });
});

  const units = ["byte", "kilobyte", "megabyte", "terabyte", "petabyte"];

you lost "gigabyte"