Testing if something is a class in javascript

Question

Hey all, I am trying to test if the argument passed into my function is a class name so that I may compare it to other classes using instanceof.

For example:

function foo(class1, class2) {   // Test to see if the parameter is a class.   if(class1 is a class)   {     //do some kind of class comparison.     if(class2 is a class)     {        if(class1 instanceof class2)        {           //...        }     }     else     {        //...     }   }   else     //... } 

Is this possible? I am having trouble googleing an answer.

Answer 1

There is really no such thing as a "class" in javascript -- everything but primitives are an object. Even functions are objects.

instanceof DOES work with functions though. Check out this link.

function Car(make, model, year) {   this.make = make;   this.model = model;   this.year = year; } var mycar = new Car("Honda", "Accord", 1998); var a = mycar instanceof Car;    // returns true var b = mycar instanceof Object; // returns true 

Answer 2

Now that we have native implementations of ES6, there are "real classes". These are largely syntactic sugar for prototypal inheritance as with constructor functions, but there are subtle differences and the two are not completely interchangeable.

So far, the only way I've found is to get the .toString() of the object's prototype's constructor function and check if it starts with class OR if the object has a constructor and the .toString() of that starts with class.

Note that if your code is compiled (ie: most Babel or TypeScript setups), then this will return function... instead of class... at runtime (since classes are transpiled to constructor functions).

function isClass(obj) {   const isCtorClass = obj.constructor       && obj.constructor.toString().substring(0, 5) === 'class'   if(obj.prototype === undefined) {     return isCtorClass   }   const isPrototypeCtorClass = obj.prototype.constructor      && obj.prototype.constructor.toString     && obj.prototype.constructor.toString().substring(0, 5) === 'class'   return isCtorClass || isPrototypeCtorClass } 

This will only work in native environments (Chrome, Firefox, Edge, node.js, etc.) that have implemented class for code that has not been transpiled to function.

Usage:

class A {} class B extends A {} isClass(A) // true isClass(new A()) // true isClass(B) // true isClass(new B()) // true  function C() {} isClass(C) // false isClass(new C()) // false isClass({}) // false isClass(Date) // false isClass(new Date()) // false  //These cases return 'true' but I'm not sure it's desired isClass(Object.create(A)) // true     const x = {} Object.setPrototypeOf(x, A) isClass(x) // true 

If there is a better way, I'd love to know what it is.