I've experimented with JavaScript and noticed this strange thing:

var s = "hello world!";
s.x = 5;
console.log(s.x); //undefined

Every type of variable in JavaScript is inherited from object. So it should be possible to add new attributes to every object.

Did I misunderstand something wrong?

I've experimented with JavaScript and noticed this strange thing:

var s = "hello world!";
s.x = 5;
console.log(s.x); //undefined

Every type of variable in JavaScript is inherited from object. So it should be possible to add new attributes to every object.

Did I misunderstand something wrong?

• javascript

• There are primitive types which don't inherit from Object. This is the case for string, number and boolean literals. – Felix Kling Commented Apr 4, 2011 at 13:22
• JS worst language type system ever – R.Moeller Commented May 16, 2014 at 13:10

6 Answers 6

A string in JavaScript isn't an instance of String. If you do new String('my string') then it will be. Otherwise it's a primitive, which is converted to a String object on the fly when you call methods on it. If you want to get the value of the string, you need to call toString(), as shown below:

var s = new String("hello world!");
s.x = 5;
console.log(s.x); //5
console.log(s); //[object Object]
console.log(s.toString()); //hello world!

String objects are objects and can be expanded, but string literals are not string objects and can not be expanded.

Example:

var s = 'asdf';
s.x = 42;
alert(s.x); // shows "undefined"

s = new String('asdf');
s.x = 1337;
alert(s.x); // shows "1337"

Skilldrick’s answer explains why it doesn’t work and therefore answers your question.

As a side note, it is possible to do this:

var s = {
  toString: function() { return "hello world!"; }
};
s.x = 5;
console.log(s.x); // 5
console.log('result: ' + s); // "result: hello world!";
console.log(String(s)); // "hello world!";

Your s is a string literal, not a string object. String literals are handled differently:

The reason you can't add properties or methods to a string literal is that when you try to access a literal's property or method, the Javascript interpreter temporarily copies the value of the string into a new object and then use that object's properties or methods. This means a String literal can only access a string's default properties or methods and those that have been added as prototypes.

Primitives ^{MDC docs} are immutable.

primitive, primitive value
A data that is not an object and does not have any methods.
JavaScript has 5 primitive datatypes: string, number, boolean, null, undefined.
With the exception of null and undefined, all primitives values have object equivalents which wrap around the primitive values, e.g. a String object wraps around a string primitive.
All primitives are immutable.

Try to do this:

var s = "hello world!";
s.prototype.x = 5;
console.log(s.x);