Allow an object property to be accessed as a method or… non method?

overloaded-method.js

// Build the asserter
let assert = {
  get modal() {
    let modal = selector => ({
      get exists() {
        return assert.modalExists(selector);
      },
    });
    Object.assign(modal, {
      get exists() {
        return assert.modalExists();
      },
    });
    return modal;
  },
  
  modalExists(selector = '') {
    return !!document.querySelector(`${selector}.modal`);
  },
};

// Testing the asserter
console.log(assert.modal.exists); // => false

let modal = document.createElement('div');
modal.className = 'modal';
document.body.appendChild(modal);
console.log(assert.modal.exists); // => true
console.log(assert.modal('error').exists); // => false

modal.className = 'modal error';
console.log(assert.modal('.error').exists); // => true