let AnimalMixin = {
    eats() {
        return `${this.name} eats.`;
    }
};

class Employee {
    constructor(name, age, salary, jobTitle) {
        this.name = name;
        this.age = age;
        this.salary = salary;
        this.jobTitle = jobTitle;
    }

    description() {
        return `A person named ${this.name} who is ${this.age} years old`;
    }
}

Object.assign(Employee.prototype, AnimalMixin);

let employee = new Employee("Aurel", 30, 15000, "CEO boss");
console.log(employee.eats()); // Aurel eats.