js base64解密

decode (input) {
  var _keyStr = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/='
  var output = ''
  var chr1, chr2, chr3
  var enc1, enc2, enc3, enc4
  var i = 0
  var re = /[^A-Za-z0-9\+\/\=]/g;
  input = input.replace(re, '')
  while (i < input.length) {
    enc1 = _keyStr.indexOf(input.charAt(i++))
    enc2 = _keyStr.indexOf(input.charAt(i++))
    enc3 = _keyStr.indexOf(input.charAt(i++))
    enc4 = _keyStr.indexOf(input.charAt(i++))
    chr1 = (enc1 << 2) | (enc2 >> 4)
    chr2 = ((enc2 & 15) << 4) | (enc3 >> 2)
    chr3 = ((enc3 & 3) << 6) | enc4
    output = output + String.fromCharCode(chr1)
    if (enc3 !== 64) {
      output = output + String.fromCharCode(chr2)
    }
    if (enc4 !== 64) {
      output = output + String.fromCharCode(chr3)
    }
  }
  output = this._utf8_decode(output)
  return output
},
_utf8_decode(utftext) {
  let string = ''
  let i = 0
  let c = 0
  let c2 = 0
  let c3 = 0
  while (i < utftext.length) {
    c = utftext.charCodeAt(i)
    if (c < 128) {
      string += String.fromCharCode(c)
      i++
    } else if ((c > 191) && (c < 224)) {
      c2 = utftext.charCodeAt(i + 1)
      string += String.fromCharCode(((c & 31) << 6) | (c2 & 63))
      i += 2
    } else {
      c2 = utftext.charCodeAt(i + 1)
      c3 = utftext.charCodeAt(i + 2)
      string += String.fromCharCode(((c & 15) << 12) | ((c2 & 63) << 6) | (c3 & 63))
      i += 3
    }
  }
  return string
},