averageSalary.ts

function averageSalary(employees: Employee[], conditions: Predicate[]): number {
  let total = 0;
  let count = 0;

  employees.forEach((e) => {
    if(conditions.every(c => c(e))){
      total += e.salary;
      count += 1;
    }
  });
  return (count === 0) ? 0 : total / count;
}

//...

expect(averageSalary(empls, [(e) => e.salary > 50, (e) => sales.works(e)])).toEqual(150);