How to check if a variable is an ES6 class declaration?

Question

I am exporting the following ES6 class from one module:

export class Thingy {
  hello() {
    console.log("A");
  }

  world() {
    console.log("B");
  }
}

And importing it from another module:

import {Thingy} from "thingy";

if (isClass(Thingy)) {
  // Do something...
}

How can I check whether a variable is a class? Not a class instance, but a class declaration?

In other words, how would I implement the isClass function in the example above?

Answer 1

If you want to ensure that the value is not only a function, but really a constructor function for a class, you can convert the function to a string and inspect its representation. The spec dictates the string representation of a class constructor.

function isClass(v) {
  return typeof v === 'function' && /^\s*class\s+/.test(v.toString());
}

---

Another solution would be to try to call the value as a normal function. Class constructors are not callable as normal functions, but error messages probably vary between browsers:

function isClass(v) {
  if (typeof v !== 'function') {
    return false;
  }
  try {
    v();
    return false;
  } catch(error) {
    if (/^Class constructor/.test(error.message)) {
      return true;
    }
    return false;
  }
}

The disadvantage is that invoking the function can have all kinds of unknown side effects...