JS: Determine Type of Object

How to determine the “type of an object”?

That is, how to find out if a object is function, array, date, regex, etc?

Check If Object is Array

Array.isArray(obj) → returns true for true array.

[see JS: Array-Like Object]

Check If Object is Function

typeof obj === "function"

Other

The most reliable and generic way to determine the type of object is this:

Object.prototype.toString.call ( obj )

The obj can be any value, such as 3.

The result is one of the following:

• "[object Undefined]"
• "[object Null]"
• "[object Array]"
• "[object String]"
• "[object Arguments]"
• "[object Function]"
• "[object Error]"
• "[object Boolean]"
• "[object Number]"
• "[object Date]"
• "[object RegExp]"
• "[object Object]"
• "[object JSON]"
• "[object Set]"
• "[object Map]"
• "[object GeneratorFunction]"
• "[object Generator]"

(there are more, such as WeakMap, WeakSet, Proxy, Promise, etc)

[see JS: Object.prototype.toString]

WARNING: If a obj is a object type, and has a symbol key property Symbol.toStringTag, either own property or inherited, and if its value is a string, let's say tag, then the result of Object.prototype.toString.call ( obj ) is the string:

"[object tag]"

[see JS: Symbol Tutorial]

For detail, see JS: Object.prototype.toString

How it Works

Object.prototype.toString.call(obj) is using the method Object.prototype.toString on the object obj.

[see JS: Function Call, Apply, Bind]

We don't want to do obj.toString() because object may have its own toString property therefore overrides the Object.prototype.toString.

[see JS: Object.prototype.toString]

Warning: Do Not Use 「typeof」, 「instanceof」, 「constructor」

Note: typeof does not work, because it doesn't distinguish different types of objects. See: JS: Data Types.

Note: do not use instanceof, because it doesn't work. See: JS: “instanceof” Operator.

Note: do not use property "constructor", because it doesn't work. See: JS: Property Key "constructor"