constructor vs typeof to detect type in JavaScript - Stack Overflow

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In this question I did not see suggestions to use constructor.

So instead of typeof callback == "function"

I would use callback && (callback.constructor==Function).

To me it seems obvious that parison to memory pointers is always better than parison to strings in terms of both runtime performance and coding safety.

Why not use constructor to detect all types and forget about ugly typeof?

It works for all primitive types, functions and arrays:

undefined === undefined
null === null
[1,2,3].constructor == Array 
(1).constructor == Number
(true).constructor == Boolean
(()=>null).constructor == Function
'abc'.constructor == String
(new Date()).constructor == Date
else it's an object, where instanceof helps to detect it's parents if needed.

If string interning can be relied upon then runtime performance advantage goes away. But safe coding advantage still stays.

In this question I did not see suggestions to use constructor.

So instead of typeof callback == "function"

I would use callback && (callback.constructor==Function).

To me it seems obvious that parison to memory pointers is always better than parison to strings in terms of both runtime performance and coding safety.

Why not use constructor to detect all types and forget about ugly typeof?

It works for all primitive types, functions and arrays:

undefined === undefined
null === null
[1,2,3].constructor == Array 
(1).constructor == Number
(true).constructor == Boolean
(()=>null).constructor == Function
'abc'.constructor == String
(new Date()).constructor == Date
else it's an object, where instanceof helps to detect it's parents if needed.

If string interning can be relied upon then runtime performance advantage goes away. But safe coding advantage still stays.

• javascript
• types
• constructor
• detection
• typeof

Share Improve this question Follow edited May 23, 2017 at 12:26 CommunityBot 111 silver badge asked Oct 1, 2016 at 2:43 alpavalpav 3,06244 gold badges4040 silver badges4949 bronze badges 1

• Comparing function with function ? – Rayon Commented Oct 1, 2016 at 2:45

Add a ment  | 

2 Answers 2

Sorted by: Reset to default 4

instanceof is better because it works with inherited constructors. .constructor is a mutable property on an object, so it's not a good thing to check because one can simply change it. You can't change the instanceof something.

const x = new Date();
console.log("Date Constructor", x.constructor);
x.constructor = "herpderpderp";
console.log("Date Constructor", x.constructor);

You can also define your own functions for both tests that also work on primitives by using getPrototypeOf and isPrototypeOf. E.G.:

function typeOf(obj) {
    return Object.getPrototypeOf(obj).constructor;
}

typeOf(2) === Number // true
typeOf("cats") === String // true
class Foo {}
typeOf(new Foo()) === Foo // true

class Bar extends Foo {}
typeOf(new Bar()) === Bar // true
typeOf(new Bar()) === Foo // false    

var b = new Number(3)
if (typeOf(b) === Number) {
    console.log(b.valueOf() + 5)
}

and

function instanceOf(obj, type) {
    var objType = typeOf(obj)
    return (
        // Allow native instanceof in case of Symbol.hasInstance
        obj instanceof type ||
        // Handle case where is of type type
        typeOf(obj) === type ||
        // Handle general case
        type.isPrototypeOf(objType) || 
        // Handle special case where type.prototype acts as a
        // prototype of the object but its type isn't in the
        // prototype chain of the obj's type
        // OPTIONALLY remove this case if you don't want
        // primitives to be considered instances of Object
        type.prototype.isPrototypeOf(objType.prototype)

    );
}

instanceOf(3, Number) // true
instanceOf(new Number("2"), Number) // true
instanceOf(2, Number) // true, OPTIONAL with the last condition
                      // but is probably preferable as 2 does
                      // indeed get all methods of Objects
class Hat {}
instanceOf(new Hat(), Hat) // true
class Fedora extends Hat {}
instanceOf(new Fedora(), Fedora) // true
instanceOf(new Fedora(), Hat) // true
instanceOf(new Fedora(), Object) // true

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