fake-click.js

button.click(); // nothing

event = document.createEvent ('MouseEvents');
event.initEvent('click', true, true);
button.dispatchEvent(click) // nothing

// here I discover `event.isTrusted`

event = document.createEvent ('MouseEvents');
event.initEvent('click', true, true);
event.isTrusted = true;
console.log(event.isTrusted);
// false

event = document.createEvent ('MouseEvents');
event.initEvent('click', true, true);
Object.defineProperty(event, 'isTrusted', { value: true });
// Cannot redefine property: isTrusted

event = document.createEvent ('MouseEvents');
event.initEvent('click', true, true);
fake = Object.create(event, { isTrusted: { value: true }});
button.dispatchEvent(fake);
// Failed to execute 'dispatchEvent' on 'EventTarget': parameter 1 is not of type 'Event'.

event = document.createEvent ('MouseEvents');
event.initEvent('click', true, true);
fake = Object.create(
  Object.getPrototypeOf(event),
  { isTrusted: { value: true }}
);
button.dispatchEvent(fake);
// Failed to execute 'dispatchEvent' on 'EventTarget': parameter 1 is not of type 'Event'.

event = document.createEvent ('MouseEvents');
event.initEvent('click', true, true);
fake = new Proxy(event, {
  get(target, prop, receiver) {
    return prop === 'isTrusted' ? true : Reflect.get(...arguments);
  }
});
button.dispatchEvent(fake);
// Failed to execute 'dispatchEvent' on 'EventTarget': parameter 1 is not of type 'Event'.

afaik, you can't fake isTrusted on real Event objects. If you're trying to trick 3rd-party event listeners checking for this, you may try to wrap the listener functions themselves.