JS: Determine Type of Object

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:

Reflect.apply ( Object.prototype.toString , obj, [] )

or

Object.prototype.toString.call ( obj )

The obj can be any value, object type or not, such as 3.

const x = /123/;

console.log (
Object.prototype.toString.call ( x ) === "[object RegExp]"
);

console.log (
Reflect.apply ( Object.prototype.toString , x, [] ) === "[object RegExp]"
);

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]"

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 Does it Work

Reflect.apply ( Object.prototype.toString , obj, [] )

Applies the function Object.prototype.toString and passing obj as thisBinding, with no argument.

Object.prototype.toString.call(obj) does the same.

[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]