The source code

    /**
     * @class Ext.util.JSON
     * Modified version of Douglas Crockford"s json.js that doesn"t
     * mess with the Object prototype
     * http://www.json.org/js.html
     * @singleton
     */
    Ext.util.JSON = new (function(){
        var useHasOwn = !!{}.hasOwnProperty,
            isNative = function() {
                var useNative = null;

                return function() {
                    if (useNative === null) {
                        useNative = Ext.USE_NATIVE_JSON && window.JSON && JSON.toString() == '[object JSON]';
                    }

                    return useNative;
                };
            }(),
            pad = function(n) {
                return n < 10 ? "0" + n : n;
            },
            doDecode = function(json){
                return eval("(" + json + ')');
            },
            doEncode = function(o){
                if(typeof o == "undefined" || o === null){
                    return "null";
                }else if(Ext.isArray(o)){
                    return encodeArray(o);
                }else if(Object.prototype.toString.apply(o) === '[object Date]'){
                    return Ext.util.JSON.encodeDate(o);
                }else if(typeof o == "string"){
                    return encodeString(o);
                }else if(typeof o == "number"){
                    return isFinite(o) ? String(o) : "null";
                }else if(typeof o == "boolean"){
                    return String(o);
                }else {
                    var a = ["{"], b, i, v;
                    for (i in o) {
                        if(!useHasOwn || o.hasOwnProperty(i)) {
                            v = o[i];
                            switch (typeof v) {
                            case "undefined":
                            case "function":
                            case "unknown":
                                break;
                            default:
                                if(b){
                                    a.push(',');
                                }
                                a.push(doEncode(i), ":",
                                        v === null ? "null" : doEncode(v));
                                b = true;
                            }
                        }
                    }
                    a.push("}");
                    return a.join("");
                }
            },
            m = {
                "\b": '\\b',
                "\t": '\\t',
                "\n": '\\n',
                "\f": '\\f',
                "\r": '\\r',
                '"' : '\\"',
                "\\": '\\\\'
            },
            encodeString = function(s){
                if (/["\\\x00-\x1f]/.test(s)) {
                    return '"' + s.replace(/([\x00-\x1f\\"])/g, function(a, b) {
                        var c = m[b];
                        if(c){
                            return c;
                        }
                        c = b.charCodeAt();
                        return "\\u00" +
                            Math.floor(c / 16).toString(16) +
                            (c % 16).toString(16);
                    }) + '"';
                }
                return '"' + s + '"';
            },
            encodeArray = function(o){
                var a = ["["], b, i, l = o.length, v;
                    for (i = 0; i < l; i += 1) {
                        v = o[i];
                        switch (typeof v) {
                            case "undefined":
                            case "function":
                            case "unknown":
                                break;
                            default:
                                if (b) {
                                    a.push(',');
                                }
                                a.push(v === null ? "null" : Ext.util.JSON.encode(v));
                                b = true;
                        }
                    }
                    a.push("]");
                    return a.join("");
            };

        this.encodeDate = function(o){
            return '"' + o.getFullYear() + "-" +
                    pad(o.getMonth() + 1) + "-" +
                    pad(o.getDate()) + "T" +
                    pad(o.getHours()) + ":" +
                    pad(o.getMinutes()) + ":" +
                    pad(o.getSeconds()) + '"';
        };

        /**
         * Encodes an Object, Array or other value
         * @param {Mixed} o The variable to encode
         * @return {String} The JSON string
         */
        this.encode = function() {
            var ec;
            return function(o) {
                if (!ec) {
                    // setup encoding function on first access
                    ec = isNative() ? JSON.stringify : doEncode;
                }
                return ec(o);
            };
        }();

        /**
         * Decodes (parses) a JSON string to an object. If the JSON is invalid, this function throws a SyntaxError unless the safe option is set.
         * @param {String} json The JSON string
         * @return {Object} The resulting object
         */
        this.decode = function() {
            var dc;
            return function(json) {
                if (!dc) {
                    // setup decoding function on first access
                    dc = isNative() ? JSON.parse : doDecode;
                }
                return dc(json);
            };
        }();

    })();
    /**
     * Shorthand for {@link Ext.util.JSON#encode}
     * @param {Mixed} o The variable to encode
     * @return {String} The JSON string
     * @member Ext
     * @method encode
     */
    Ext.encode = Ext.util.JSON.encode;
    /**
     * Shorthand for {@link Ext.util.JSON#decode}
     * @param {String} json The JSON string
     * @param {Boolean} safe (optional) Whether to return null or throw an exception if the JSON is invalid.
     * @return {Object} The resulting object
     * @member Ext
     * @method decode
     */
    Ext.decode = Ext.util.JSON.decode;