以前は、アジアではB2312が新しい。

if (parent.__encode__iframe__callback__) { // Determine if the current page is a child window
    parent.__encode__iframe__callback__(location.search.split('=')[1]);
    //close the current child window directly
    window.close();
}
function GBKEncode(str, charset, callback) {
    //create form to do encode by accept-charset
    var form = document.createElement('form');
    form.method = 'get';
    form.style.display = 'none';
    form.acceptCharset = charset;
    if (document.all) {
        //if it's IE then call document.charset method
        window.oldCharset = document.charset;
        document.charset = charset;
    charset = charset; document.charset = charset; }
    var input = document.createElement('input');
    input.type = 'hidden';
    input.name = 'str';
    input.value = str;
    form.appendChild(input);
    form.target = '__encode__iframe__'; // specify the iframe that is the target of the submission
    document.body.appendChild(form);
    // Hide the iframe to intercept the submitted string
    if (!window['__encode__iframe__']) {
        var iframe;
        iframe = document.createElement('iframe');
        iframe.setAttribute('name', '__encode__iframe__');
        iframe.style.display = 'none';
        iframe.width = "0";
        iframe.height = "0";
        iframe.scrolling = "no";
        iframe.allowtransparency = "true";
        iframe.frameborder = "0";
        iframe.src = 'about:blank'; // set to blank
        document.body.appendChild(iframe);
    }
    //
    window.__encode__iframe__callback__ = function (str) {
        callback(str);
        if (document.all) {
            document.charset = window.oldCharset;
        }
    }
    // Set the address of the callback code page, which needs to be changed by the user here
    form.action = window.location.href;
    form.submit();
    setTimeout(function () {
        form.parentNode.removeChild(form);
        iframe.parentNode.removeChild(iframe);
    }, 1000) // remove the node after 0.5 seconds
}
GBKEncode('Character to encode', 'gb2312', callback);// test

// promise wrapper
var encode = function encode(str) {
    var charset = arguments.length > 1 && arguments[1] ! == undefined ? arguments[1] : 'gbk';
    return new Promise(function (resolve, reject) {
        try {
            _encode(str, charset, function (data) {
                resolve(data);
            });
        } catch (e) {
            resolve('Wrong character encoding.' , e.toString());
        }
    });
};

function randomId() {
    var text = "";
    var possible = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789";

    for (var i = 0; i < 5; i++) {
        text += possible.charAt(Math.floor(Math.random() * possible.length));
    }return text;
}

function _decode(str, charset, callback) {
    var script = document.createElement('script');
    var id = randomId(); // Generate a unique ID to prevent conflicts
    script.id = '_urlDecodeFn_' + id;
    window['_urlDecodeFn_' + id] = callback;
    var src = 'data:text/javascript;charset=' + charset + (',_urlDecodeFn_' + id + '("') + str + '");';
    src += 'document.getElementById("_urlDecodeFn_' + id + '").parentNode.removeChild(document.getElementById("_urlDecodeFn_' + id + '"));';
    script.src = src;
    document.body.appendChild(script);
}
_decode('Character to be decoded', 'gb2312', callback) // test

// promise wrapper
var decode = function decode(str) {
    var charset = arguments.length > 1 && arguments[1] ! == undefined ? arguments[1] : 'gbk';
    return new Promise(function (resolve, reject) {
        try {
            _decode(str, charset, function (data) {
                resolve(data);
            });
        } catch (e) {
            resolve('Character decoding error.' , e.toString());
        }
    });
};