isString.js

String.isString = (function () {
    var strToString     = ('').toString,
        hasBind         = Function.prototype && Function.prototype.bind,
        strToStrCall    = hasBind && strToString.call.bind(strToString),
        isString        = function (str) {
        /*@cc_on
          @if (@_jscript_version >= 5) @*/
            try {
                hasBind ? strToStrCall(str) : strToString.call(str);
                return true;
            } catch (e) {
                return false;
            }
          /*@end
        @*/
        };

    return function (str) {
        return  typeof str === 'string' ||
                (str && typeof str === 'object' &&
                /*@cc_on
                    @if (@_jscript_version < 5.5)
                        /^\s*function\s*String\(\)\s*{\s*\[native code\]\s*}\s*$/.test(str.constructor)
                    @else @*/
                        isString(str)
                    /*@end
                @*/
                ) || false;
    };
})();