JS - Humanize and dehumanize byte-unit inputs (with support for both SI and binary byte prefixes)

index.mjs

// Note:  I'm ignoring fractional bytes (e.g., 4.2 B is invalid input; 4.2 KiB or 4.2 KB are valid, tho ).
export const humanizeBytes = function(bytes, si = false, digits = 1){
    let basis = si ? 1000 : 1024

    let scale = (Math.log(Math.max(1,bytes)) / Math.log(basis))|0
    let coeff = bytes / (basis ** scale)
    let unit = scale == 0 ? "B" : ("KMGTP"[scale-1] + (si ? "" : "i") + "B")

    return `${coeff.toFixed(scale == 0 ? 0 : digits)} ${unit}`
}
export const dehumanizeBytes = function(humanText){

    let matched = humanText.match(/^([0-9]+) ?(B|K|KB|KiB|M|MB|MiB|G|GB|GiB|T|TB|TiB|P|PB|PiB)?$/) 
      || humanText.match(/^([0-9]+\.[0-9]+) ?(K|KB|KiB|M|MB|MiB|G|GB|GiB|T|TB|TiB|P|PB|PiB)$/)

    if(!matched)
      return null
    
    let [coeff, scale] = matched.slice(1, 3)
    coeff = parseFloat(coeff)||0
    scale = typeof scale === "undefined" ? 1 : (scale.length>2 ? 1024 : 1000) ** ("KMGTP".indexOf(scale[0])+1)
    
    return Math.trunc(coeff * scale)
}

In order to better support Petabytes/Pebibytes, you should adapt it to "BigNumber"/"BigInt" (be it the JS native or third-party libraries), because such scale isn't well-represented by IEEE 754.

Also, in order to support higher scales (such as Yottabytes/Yobibytes... but I wonder what exactly are you planning to do with such large scales), add them to the RegExps, as well as to the hardcoded string (but I guess you already know it, given you're dealing with daunting Yottabytes of data... who are you working for, a multinational ISP provider? Perhaps a Big Tech employee? Help hiring me?).