How to check if a Javascript function is a constructor

Question

I noticed not all the Javascript functions are constructors.

var obj = Function.prototype;
console.log(typeof obj === \'function\'); //true
obj(); //OK
new          


        

Answer 1

For question 1, what about this helper?

Function.isConstructor = ({ prototype }) => Boolean(prototype) && Boolean(prototype.constructor)

Function.isConstructor(class {}); // true
Function.isConstructor(function() {}); // true
Function.isConstructor(() => {}); // false
Function.isConstructor("a string"); // false

For question 2, the arrow function is the solution. It cannot be used as a constructor since it does not rely on the same scope as a regular function and does not have a prototype (definition of instances, similar to class definition for real OOP)

const constructable = function() { console.log(this); };
const callable = () => { console.log(this); };

constructable(); // Window {}
callable(); // Window {}
new constructable(); // aConstructableFunction {}
new callable(); // Uncaught TypeError: callable is not a constructor