用js实现base64算法

下面是使用 JavaScript 实现 Base64 编码和解码的示例代码：

// Base64 编码
function base64Encode(str) {
  const base64EncodeChars = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";
  let out = "", i = 0, len = str.length, c1, c2, c3;
  while (i < len) {
    c1 = str.charCodeAt(i++) & 0xff;
    if (i == len) {
      out += base64EncodeChars.charAt(c1 >> 2);
      out += base64EncodeChars.charAt((c1 & 0x3) << 4);
      out += "==";
      break;
    }
    c2 = str.charCodeAt(i++);
    if (i == len) {
      out += base64EncodeChars.charAt(c1 >> 2);
      out += base64EncodeChars.charAt(((c1 & 0x3) << 4) | ((c2 & 0xF0) >> 4));
      out += base64EncodeChars.charAt((c2 & 0xF) << 2);
      out += "=";
      break;
    }
    c3 = str.charCodeAt(i++);
    out += base64EncodeChars.charAt(c1 >> 2);
    out += base64EncodeChars.charAt(((c1 & 0x3) << 4) | ((c2 & 0xF0) >> 4));
    out += base64EncodeChars.charAt(((c2 & 0xF) << 2) | ((c3 & 0xC0) >> 6));
    out += base64EncodeChars.charAt(c3 & 0x3F);
  }
  return out;
}

// Base64 解码
function base64Decode(str) {
  const base64DecodeChars = new Array(
    -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
    -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
    -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, 62, -1, -1, -1, 63,
    52, 53, 54, 55, 56, 57, 58, 59, 60, 61, -1, -1, -1, -1, -1, -1,
    -1, 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15,
    16, 17, 18, 19, 20, 21, 22, 23, 24, 25, -1, -1, -1, -1, 63, -1,
    26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41,
    42, 43, 44, 45, 46, 47, 48, 49, 50, 51, -1, -1, -1, -1, -1, -1
  );
  let c1, c2, c3, c4;
  let i = 0, len = str.length, out = "";
  while (i < len) {
    do {
      c1 = base64DecodeChars[str.charCodeAt(i++) & 0xff];
    } while (i < len && c1 == -1);
    if (c1 == -1) break;
    do {
      c2 = base64DecodeChars[str.charCodeAt(i++) & 0xff];
    } while (i < len && c2 == -1);
    if (c2 == -1) break;
    out += String.fromCharCode((c1 << 2) | ((c2 & 0x30) >> 4));
    do {
      c3 = str.charCodeAt(i++) & 0xff;
      if (c3 == 61) return out;
      c3 = base64DecodeChars[c3];
    } while (i < len && c3 == -1);
    if (c3 == -1) break;
    out += String.fromCharCode(((c2 & 0XF) << 4) | ((c3 & 0x3C) >> 2));
    do {
      c4 = str.charCodeAt(i++) & 0xff;
      if (c4 == 61) return out;
      c4 = base64DecodeChars[c4];
    } while (i < len && c4 == -1);
    if (c4 == -1) break;
    out += String.fromCharCode(((c3 & 0x03) << 6) | c4);
  }
  return out;
}

使用方法：

const str = "Hello, world!";
const encodedStr = base64Encode(str); // "SGVsbG8sIHdvcmxkIQ=="
const decodedStr = base64Decode(encodedStr); // "Hello, world!"