context-problem.ts

class Human {
  private age: number = 42;

  getAge() {
    return this.age;
  }
}

let f = new Human().getAge
console.log(f());

x-answer1.ts

class Human {
  private age: number = 42;

  constructor() {
    this.getAge = this.getAge.bind(this);
  }

  getAge() {
    return this.age;
  }
}

let f = new Human().getAge
console.log(f());

x-answer2.ts

class Human {
  private age: number = 42;

  getAge() {
    return this.age;
  }
}

Human.prototype = bind(Human.prototype);

let f = new Human().getAge
console.log(f());

function bind(obj) {
  return new Proxy(obj, {
    get(target, name, receiver) {
       const x = target[name];
       return typeof x === 'function' ? x.bind(receiver) : x;
    }
  });
}

x-answer3.ts

class Human {
  private age: number = 42;

  @autobind getAge() {
    return this.age;
  }
}

let f = new Human().getAge
console.log(f());

function autobind(target, key, { value: method, configurable, enumerable }: PropertyDescriptor) {
  if (typeof method !== 'function') {
    throw new SyntaxError('@autobind can only be used on methods');
  }

  return {
    configurable,
    enumerable,

    get() {
      const boundMethod = method.bind(this);

      Object.defineProperty(this, key, {
        configurable: true,
        writable: true,
        enumerable: false,
        value: boundMethod
      });

      return boundMethod;
    },
  };
}