`this` is weird

this-is-weird.js

let obj = {
  fooFn: function() {
    console.log('foofn: hi!');
  },
  barFn: function() {
    console.log('barFn: calling fooFn');
    this.fooFn();
    console.log('barFn: called fooFn');
  }
}

console.log('global: this will work fine:');
obj.barFn();
console.log('global: but this will break');
f = obj.barFn;
f();
console.log('wat');