2^3 = 8
log₂8 = 3

-- javascript
---------------------------------------------------

Math.log2(8) = Math.log2(2 * 2 * 2) = 3
Math.log1024(1024) = 1 <- KB
Math.log1024(1024 * 1024) = 2 <- MB
Math.log1024(1024 * 1024 * 1024) = 3 <- GB

but doesn't have base 1024 log function in built-in javascript.

The numerical value for logarithm to the base 10 can be calculated with the following identity.
log10 (x) = log₂(x) / log₂(10)

Can be derived. Like below
log1024 (x) = log₂(x) / log₂(1024)

so we can override the Math.log method to accept an optional base argument

Math.log = (function()
{
    var log = Math.log;
    return function(base, n)
    {
        return log(n) / (base ? log(base) : 1);
    };
})();

Math.log(10, 100)
2
Math.log(1024, 1024)
1
Math.log(1024, 1024 * 1024)
2
Math.log(1024, 1024 * 1024 * 1024)
3
Math.log(1024, 1024 * 1024 * 24.7)
2.4626439136697313
Math.log(1024, 1024 * 1024 * 58.2)
2.586294724802092


var _unit = ["Bytes", "KB", "MB", "GB", "TB", "PB", "EB", "ZB", "YB"];
var _fileSize = 1024 * 1024 * 58.7264;

_fileSize / Math.pow(1024, (_i = Math.floor(Math.log(1024, _fileSize))));
58.7264

var _n = _fileSize / Math.pow(1024, (_i = Math.floor(Math.log(1024, _fileSize))));
_n.toFixed(2) + " " + _unit[_i];
"58.73 MB"