Base64 encode in javascript

base64encode.js

function base64encode (s) {
  // from https://gist.github.com/stubbetje/229984
  const base64 = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/='.split('')
  const l = s.length
  let o = ''
  for (let i = 0; i < l; i++) {
    const byte0 = s.charCodeAt(i++) & 0xff
    const byte1 = s.charCodeAt(i++) & 0xff
    const byte2 = s.charCodeAt(i) & 0xff
    o += base64[byte0 >> 2]
    o += base64[((byte0 & 0x3) << 4) | (byte1 >> 4)]
    const t = i - l
    if (t >= 0) {
      if (t === 0) {
        o += base64[((byte1 & 0x0f) << 2) | (byte2 >> 6)]
        o += base64[64]
      } else {
        o += base64[64]
        o += base64[64]
      }
    } else {
      o += base64[((byte1 & 0x0f) << 2) | (byte2 >> 6)]
      o += base64[byte2 & 0x3f]
    }
  }
  return o
}