Why does instanceof return false for some literals?

Question

"foo" instanceof String //=> false
"foo" instanceof Object //=> false
true instanceof Boolean //=> false
true instanceof Object //=> false
false instanceof Boolean //=> false
false instanceof Object //=> false

// the tests against Object really don't make sense

Array literals and Object literals match...

[0,1] instanceof Array //=> true
{0:1} instanceof Object //=> true

Why don't all of them? Or, why don't none of them?
And, what are they an instance of, then? Nothing()?

It's the same in FF3, IE7, Opera, and Chrome. So, at least it's consistent. ;)

---

Missed a few. ;)

12.21 instanceof Number //=> false
/foo/ instanceof RegExp //=> true

Answer 1

Literals are a different kind of object than objects created from within Javascript. From the Mozilla API docs:

var color1 = new String("green");
color1 instanceof String; // returns true
var color2 = "coral";
color2 instanceof String; // returns false (color2 is not a String object)

I can't find any way to construct literal types with code, perhaps it's not possible. This is probably why people use typeof "foo" === "string" instead of instanceof.

An easy way to remember things like this is asking yourself "I wonder what would be sane and easy to learn"? Whatever the answer is, Javascript does the other thing.

Answer 2

That's because those things are primitives, and unless they need to be used as objects (when you are calling methods on them, for example) they remain so. The only time they "become" objects is when they need to be wrapped. If you are familiar with the concept of "boxing" in .NET, then think of it in that way.

Here is an example - take a look at this code:

Number.prototype.times = function(func) {
   for(var index = 1; index <= this; index++) {
      func(index);
   }
};

So, the following code will fail:

3.times(print); // assume 'print' writes to standard out

3, by itself is a primitive. That said, the following will work:

(3).times(print); // assume 'print' writes to standard out

That would display the numbers 1, 2, and 3. Because of the parenthesis, the JavaScript interpreter will temporarily wrap the primitive 3 in a Number object, call the method, and then garbage collect the object since it isn't needed any longer.

Anyway, a full discussion of this can be found in "JavaScript: The Definitive Guide."

Answer 3

You can use constructor property:

'foo'.constructor == String // returns true
true.constructor == Boolean // returns true

Answer 4

Or you can just make your own function like so:

function isInstanceOf(obj, clazz){
  return (obj instanceof eval("("+clazz+")")) || (typeof obj == clazz.toLowerCase());
};

usage:

isInstanceOf('','String');
isInstanceOf(new String(), 'String');

These should both return true.