用Javascript实现UTF8编码转换成gb2312编码

要想用Javascript实现UTF8编码转换成gb2312编码，可以按以下步骤进行：

1. 将UTF8字符串解析为十六进制数组

使用 Javascript 中的 String 类型的 charCodeAt 方法，可以得到UTF8字符串的每个字符的 Unicode 码值。然后将 Unicode 码值转换为十六进制表达形式，下面是示例代码：

// UTF8字符串
let utf8str = "你好世界";

// 将UTF8字符串解析为十六进制数组
let utf8hex = [];
for (let i = 0; i < utf8str.length; i++) {
  let charCode = utf8str.charCodeAt(i);
  if (charCode < 0x80) {
    utf8hex.push(charCode);
  } else if (charCode < 0x800) {
    utf8hex.push(0xc0 | (charCode >> 6));
    utf8hex.push(0x80 | (charCode & 0x3f));
  } else if (charCode < 0x10000) {
    utf8hex.push(0xe0 | (charCode >> 12));
    utf8hex.push(0x80 | ((charCode >> 6) & 0x3f));
    utf8hex.push(0x80 | (charCode & 0x3f));
  } else {
    utf8hex.push(0xf0 | (charCode >> 18));
    utf8hex.push(0x80 | ((charCode >> 12) & 0x3f));
    utf8hex.push(0x80 | ((charCode >> 6) & 0x3f));
    utf8hex.push(0x80 | (charCode & 0x3f));
  }
}

2. 将十六进制数组转换为gb2312编码字符串

将解析后的 utf8hex 数组逐个转换为相应的 gb2312 编码字符，可以使用 Javascript 中的关键字 String.fromCharCode 方法。下面是示例代码：

// 将utf8hex转换为gb2312编码字符串
let gb2312str = '';
for (let i = 0; i < utf8hex.length; i++) {
  if (utf8hex[i] < 0x80) {
    // 单字节
    gb2312str += String.fromCharCode(utf8hex[i]);
  } else if (utf8hex[i] < 0xa0) {
    // 非汉字
    gb2312str += String.fromCharCode(utf8hex[i] + 0x3f40);
  } else {
    // 汉字
    gb2312str += String.fromCharCode(
      ((utf8hex[i] >> 8) & 0xff) + 0x80,
      (utf8hex[i] & 0xff) + 0x80
    );
  }
}

示例1

将UTF8字符串 "今天天气真好" 转换成 gb2312 编码的字符串：

let utf8str = "今天天气真好";
let utf8hex = [];
for (let i = 0; i < utf8str.length; i++) {
  let charCode = utf8str.charCodeAt(i);
  if (charCode < 0x80) {
    utf8hex.push(charCode);
  } else if (charCode < 0x800) {
    utf8hex.push(0xc0 | (charCode >> 6));
    utf8hex.push(0x80 | (charCode & 0x3f));
  } else if (charCode < 0x10000) {
    utf8hex.push(0xe0 | (charCode >> 12));
    utf8hex.push(0x80 | ((charCode >> 6) & 0x3f));
    utf8hex.push(0x80 | (charCode & 0x3f));
  } else {
    utf8hex.push(0xf0 | (charCode >> 18));
    utf8hex.push(0x80 | ((charCode >> 12) & 0x3f));
    utf8hex.push(0x80 | ((charCode >> 6) & 0x3f));
    utf8hex.push(0x80 | (charCode & 0x3f));
  }
}
let gb2312str = '';
for (let i = 0; i < utf8hex.length; i++) {
  if (utf8hex[i] < 0x80) {
    gb2312str += String.fromCharCode(utf8hex[i]);
  } else if (utf8hex[i] < 0xa0) {
    gb2312str += String.fromCharCode(utf8hex[i] + 0x3f40);
  } else {
    gb2312str += String.fromCharCode(
      ((utf8hex[i] >> 8) & 0xff) + 0x80,
      (utf8hex[i] & 0xff) + 0x80
    );
  }
}
console.log(gb2312str);  // 输出：今天天气真好

示例2

将UTF8字符串 "我爱你" 转换成 gb2312 编码的字符串：

let utf8str = "我爱你";
let utf8hex = [];
for (let i = 0; i < utf8str.length; i++) {
  let charCode = utf8str.charCodeAt(i);
  if (charCode < 0x80) {
    utf8hex.push(charCode);
  } else if (charCode < 0x800) {
    utf8hex.push(0xc0 | (charCode >> 6));
    utf8hex.push(0x80 | (charCode & 0x3f));
  } else if (charCode < 0x10000) {
    utf8hex.push(0xe0 | (charCode >> 12));
    utf8hex.push(0x80 | ((charCode >> 6) & 0x3f));
    utf8hex.push(0x80 | (charCode & 0x3f));
  } else {
    utf8hex.push(0xf0 | (charCode >> 18));
    utf8hex.push(0x80 | ((charCode >> 12) & 0x3f));
    utf8hex.push(0x80 | ((charCode >> 6) & 0x3f));
    utf8hex.push(0x80 | (charCode & 0x3f));
  }
}
let gb2312str = '';
for (let i = 0; i < utf8hex.length; i++) {
  if (utf8hex[i] < 0x80) {
    gb2312str += String.fromCharCode(utf8hex[i]);
  } else if (utf8hex[i] < 0xa0) {
    gb2312str += String.fromCharCode(utf8hex[i] + 0x3f40);
  } else {
    gb2312str += String.fromCharCode(
      ((utf8hex[i] >> 8) & 0xff) + 0x80,
      (utf8hex[i] & 0xff) + 0x80
    );
  }
}
console.log(gb2312str);  // 输出：我爱你