JavaScript String :: Primitive or Object?

A colleague and I were chatting about JavaScript strings and he noted that they're not objects. He's half right.

JavaScript strings are primitives much like numbers and booleans, but they can also be objects ( like Numbers and Booleans ).

You can create a String object ( I've capitalized String ) by calling it's constructor --

var aStr = new String("I'm a String object");

You can also create a string like this without calling the constructor ( that is without the "new" operator ) --

var aStr = String("I'm a string primitive");

This is the same as calling --

var aStr = "I'm a string primitive";

which results in a string primitive or as I typically call it, a string constant or a string literal.

Typically, you don't have to worry about whether you're dealing with a string primitive or a String object because JavaScript automatically converts the primitive to a String object.

So, you can do things like getting the length of a string primitive --

var myStr = "I'm a string primitive";
alert(myStr.length);

or calling methods --

alert("string primitive".substring(0,7));

All this means is that you can access any property found in the String object from a primitive.

The difference between a primitive and a String object is when you're adding functionality to a String. You can't do it to primitives.

So, if you wanted to add a function to reverse the characters in a string, you'd do something like this by adding it to the String object's prototype property --

String.prototype.reverseString = function() {
var len = this.length;
var result = "";
var strArray = [];
var ct = 0;
for (var i=len-1;i>=0;i--) {
strArray[ct++] = this.charAt(i);
}
return strArray.join("");
}

If you called it,

"cola".reverseString();

it's the same as

var cola = new String("cola");
cola.reverseString();

Though you've added the method to the String object, you can use it with a primitive!

Note that JavaScript strings are immutable ( you can't change them ). This is why the method reverseString returns a new string from the array.

All of this is pretty obvious, but I thought I'd share my thoughts.
Enjoy!