Finding Variable Type in JavaScript

Question

In Java, you can use instanceOf or getClass() on a variable to find out its type.

How do I find out a variable's type in JavaScript which isn't strongly-typed?

For example, how do I know if the bar is a Boolean or a Number, or a String?

function foo(bar) {
    // what do I do here?
}

Answer 1

Use typeof:

> typeof "foo"
"string"
> typeof true
"boolean"
> typeof 42
"number"

So you can do:

if(typeof bar === 'number') {
   //whatever
}

Be careful though if you define these primitives with their object wrappers (which you should never do, use literals where ever possible):

> typeof new Boolean(false)
"object"
> typeof new String("foo")
"object"
> typeof new Number(42)
"object"

The type of an array is still object. Here you really need the instanceof operator.

Update:

Another interesting way is to examine the output of Object.prototype.toString:

> Object.prototype.toString.call([1,2,3])
"[object Array]"
> Object.prototype.toString.call("foo bar")
"[object String]"
> Object.prototype.toString.call(45)
"[object Number]"
> Object.prototype.toString.call(false)
"[object Boolean]"
> Object.prototype.toString.call(new String("foo bar"))
"[object String]"
> Object.prototype.toString.call(null)
"[object Null]"
> Object.prototype.toString.call(/123/)
"[object RegExp]"
> Object.prototype.toString.call(undefined)
"[object Undefined]"

With that you would not have to distinguish between primitive values and objects.