Base64编码是从二进制到字符的过程，可用于在HTTP环境下传递较长的标识信息。采用Base64编码具有不可读性，需要解码后才能阅读。
前端图片类的base64可以直接作为img的src属性源。如果是文字的编码与解码，就需要特定的方法了。下面是base64的编码与解码方法：

// base64.ts
/**
 * base64加密解密方法
 * @example
 * 使用静态方法
 * import Base64 from '/A/home/helper/base64';
 * Base64.encode('123');
 * Base64.decode('YWFh');
 */
class Base64 {
  static _keyStr = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/=';
  static encode(input) {
    let output = '';
    let chr1, chr2, chr3, enc1, enc2, enc3, enc4;
    let i = 0;
    const _keyStr = this._keyStr;
    input = this._utf8_encode(input);
    while (i < input.length) {
      chr1 = input.charCodeAt(i++);
      chr2 = input.charCodeAt(i++);
      chr3 = input.charCodeAt(i++);
      enc1 = chr1 >> 2;
      enc2 = ((chr1 & 3) << 4) | (chr2 >> 4);
      enc3 = ((chr2 & 15) << 2) | (chr3 >> 6);
      enc4 = chr3 & 63;
      if (isNaN(chr2)) {
        enc3 = enc4 = 64;
      } else if (isNaN(chr3)) {
        enc4 = 64;
      }
      output =
        output +
        _keyStr.charAt(enc1) +
        _keyStr.charAt(enc2) +
        _keyStr.charAt(enc3) +
        _keyStr.charAt(enc4);
    }
    return output;
  }

  static decode(input) {
    let output = '';
    let chr1, chr2, chr3;
    let enc1, enc2, enc3, enc4;
    let i = 0;
    const _keyStr = this._keyStr;
    input = input.replace(/[^A-Za-z0-9\+\/\=]/g, '');
    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;
  }

  static _utf8_encode(string) {
    string = string.replace(/\r\n/g, '\n');
    let utftext = '';
    for (let n = 0; n < string.length; n++) {
      const c = string.charCodeAt(n);
      if (c < 128) {
        utftext += String.fromCharCode(c);
      } else if (c > 127 && c < 2048) {
        utftext += String.fromCharCode((c >> 6) | 192);
        utftext += String.fromCharCode((c & 63) | 128);
      } else {
        utftext += String.fromCharCode((c >> 12) | 224);
        utftext += String.fromCharCode(((c >> 6) & 63) | 128);
        utftext += String.fromCharCode((c & 63) | 128);
      }
    }
    return utftext;
  }

  static _utf8_decode(utftext) {
    let string = '';
    let i = 0;
    let c = 0,
      c2 = 0,
      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;
  }
}

export default Base64;