JavaScript datatype function validity

Question

Here is a simple example of a function and its datatype:

Code Snippet:

function hello(){
    console.log("hello");
}
console.log(typeof(hello)); //function

Query:

In JavaScript these are the datatypes:

1. Object
2. Primitives - String, number, boolean, null, undefined, symbol(ECMA6)

When there is no datatype named 'function' then how in the world am I getting the datatype as function (though logically its correct). Also, textually it is written that datatype of function is object. So, I must have got result as object.

Can someone clarify this confusion?

The typeof operator returns a string indicating the type of the unevaluated operand., Not really a data type — Rayon
– Rayon, Commented Sep 13, 2016 at 7:19
Because "function" is what the spec says to return? (Which makes sense, whereas typeof null returns "object".) — nnnnnn
– nnnnnn, Commented Sep 13, 2016 at 7:19
Yes both of these cases are confusing. Also, is there any other way to test the datatype of a variable other than typeof -- as it returns spec or indicates type of unevaluated operand? (As per the above 2 comments) — Deadpool
– Deadpool, Commented Sep 13, 2016 at 7:25

georg · Accepted Answer · 2016-09-13 07:26:27Z

4

Yes, you're right, there's no function type, it's just a hack in the typeof operator that returns "function" for objects that are callable.

Reference: http://www.ecma-international.org/ecma-262/6.0/#sec-typeof-operator

Generally speaking, typeof does return the actual type name, with two exceptions:

typeof null returns object, but the type of null is Null
typeof <func> returns function, but the type of <func> is Object

edited Sep 13, 2016 at 7:26

answered Sep 13, 2016 at 7:19

georg

216k57 gold badges324 silver badges401 bronze badges

Sign up to request clarification or add additional context in comments.

4 Comments

Deadpool Over a year ago

This means that the correct answer should have been object only, but in the current scenario due to some specifications -- "It is giving function", but in truth it should have been "Object"?

Deadpool Over a year ago

Also, add typeof(Nan) === "number" in the exception list!

Deadpool Over a year ago

But, I thought its real semantic meaning is "not a number"! Then how can its datatype be a "number"! Confused ...

georg Over a year ago

@Peterson: yep, NaN is a "Number" which is not a number. Confusing indeed, but that's how it works.

EternalLight · Accepted Answer · 2016-09-13 07:26:20Z

2

From the "You don't know JS book" by Kyle Simpson:

So what’s the seventh string value that typeof can return?

typeof function a(){ /* .. */ } === "function"; // true

It’s easy to think that function would be a top-level built-in type in JS, especially given this behavior of the typeof operator. However, if you read the spec, you’ll see it’s actually somewhat of a “subtype” of object . Specifically, a function is referred to as a “callable object”— an object that has an internal [[Call]] property that allows it to be invoked.

It was made to differentiate "callable" objects from "non-callable" ones.

answered Sep 13, 2016 at 7:26

EternalLight

1,3281 gold badge11 silver badges20 bronze badges

2 Comments

Deadpool Over a year ago

Seeing this, I believe function should have been a datatype itself or at-least instead of returning function --- typeof return should have been "callable object" -- That would have made better sense!

EternalLight Over a year ago

Functions have some object behavior - it has prototype, properties (like "length" - the number of arguments, "name", etc) and methods (including object-specific "toString", "valueOf"), and that's might be the reason why they are a subtype of object instead of a separate type. The "function" word is likely used to avoid confusion for people coming from another languages.

Collectives™ on Stack Overflow

JavaScript datatype function validity

2 Answers 2

4 Comments

2 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

4 Comments

2 Comments

Your Answer

Sign up or log in

Post as a guest

Related