Most JS senior candidates didn't know why this won't work

foo.js

function foo() {
  setTimeout(function(){
    this.magic();
  }, 1);
}
foo.prototype.magic = () => console.log('magic');
new foo();

// it won't work. if you know why and look for VanillaJS/Angular position in London (Wimbledon) then send me your CV on ... (I don't work there anymore)

this haven't got a magic method inside the setTimeout function since it's an other context.
This should work though:

function foo() {
let that = this;
  setTimeout(function(){
    that.magic();
  }, 1);
}

foo.prototype.magic = () => console.log('magic');
new foo();

Yes, you've spotted it. That was a honeypot question for candidates.