Edit in JSFiddle

var length = typeof '문자열' === 'string' && '문자열'.length;
var isString1 = typeof (new String('문자열')) === 'string' && (new String('문자열')).length > 0;
var isString2 = typeof (new String('문자열')) === 'object' && (new String('문자열')).length > 0;

var isString3 = typeof new String('문자열').valueOf() === 'string' && (new String('문자열')).length > 0;

alert(length);
alert(isString1);
alert(isString2);
alert(isString3);