isString.max.js

// The closure permits to capture references of the intrinsic objects
// before other scripts get the chance to tamper with them. Therefore
// it needs to be loaded before any potentially hazardous arbitrary code.
String.isString = (function() {
    var strToString     = ('').toString,
// mitigate Function.prototype.call mangling
        hasBind         = Function.prototype && Function.prototype.bind,
        strToStrCall    = hasBind && strToString.call.bind(strToString),
// The try-catch-finally construct may render the function holding it
// unoptimizable. It's being extracted to avoid the inherent performance hit.
        isString        = function(str) {
        /*@cc_on
// Furthermore IE didn’t support it until version 5.
          @if (@_jscript_version >= 5) @*/
          try {
// String.prototype.toString throws if str's [[StringData]] internal slot is
// not a String value.
            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
// IE didn't support Function.prototype.call until version 5.5.
                  @if (@_jscript_version < 5.5)
                    /^\s*function\s*String\(\)\s*{\s*\[native code\]\s*}\s*$/.test(str.constructor)
                  @else @*/
                    isString(str)
                  /*@end
                @*/
// fallback for falsy values
                ) || false;
    };
})();