class Greeter {
  constructor(message) {
      this.greeting = message;
  }
  greet() {
      return "Hello, " + this.greeting;
  }
}

let greeter = new Greeter("world");

In google chrome console I run this:

greeter

Greeter {greeting: 'world'}

Object.keys(greeter)

I got this which is fine.

['greeting']

Object.getPrototypeOf(greeter)

{constructor: ƒ, greet: ƒ}

Object.keys(Object.getPrototypeOf(greeter))

I got an empty array!

[]

I expect it to return

['greet']

Why is this behavior?