Why are there two kinds of JavaScript strings?

Question

This one just stabbed me hard. I don\'t know if it\'s the case with all browsers (I don\'t have any other competent browser to test with), but at least Firefox has two kind

Answer 1

Another important thing to remember is the following:

typeof "my-string" // "string"
typeof String('my-string') // 'string'
typeof new String("my-string") // "object". 

Therefore, when testing whether an argument or variable is a string you need to do at least the following.

function isString(arg) {
  if (!arg) {
    return false;
  }
  return typeof arg == "string" || arg.constructor == String;
}

The above function will still fail if you pass in a String object from another frame/iframe. That is:

frames[0].myStringVar.constructor != frames[1].myStringVar.constructor

That is because the constructor String is different for each window context. Therefore, a foolproof isString method would be

function isString(obj) {
   return Object.prototype.toString.call(obj) == "[object String]";
}