How is a Javascript string not an object?

user_12597 0
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It's not the setup to a joke, i'm really asking.

Douglas Crockford is fond of saying that in the javascript prototypal object-oriented language there is no need for new.

He explains that new was simply added to give people coming from class-based (i.e. "classical") object oriented programming languages some level of comfort:

JavaScript, We Hardly new Ya

JavaScript is a prototypal language, but it has a new operator that tries to make it look sort of like a classical language. That tends to confuse programmers, leading to some problematic programming patterns.

You never need to use new Object() in JavaScript. Use the object literal {} instead.

Okay, fine:

  • new bad
  • {} good

But then commenter Vítor De Araújo pointed out that the two are not the same. He gives an example showing that a string is not like an object:

A string object and a string value are not the same thing:

js> p = "Foo"
Foo
js> p.weight = 42
42
js> p.weight // Returns undefined

js> q = new String("Foo")
Foo
js> q.weight = 42
42
js> q.weight
42

The string value cannot have new properties. The same thing is valid for other types.

What is going on here that an string is not an object? Am i confusing javascript with some other languages, where everything is an object?

javascript
2 Answers

The most important difference between a string and an object is that objects must follow this rule for the == operator:

An expression comparing Objects is only true if the operands reference the same Object.

So, whereas strings have a convenient == that compares the value, you're out of luck when it comes to making any other immutable object type behave like a value type. (There may be other differences too, but this is the only one that causes JavaScript developers excitement on a daily basis). Examples:

"hello" == "hello"
-> true
new String("hello") == new String("hello") // beware!
-> false
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