假设已有一个包含所有员工信息的数组employeeList，每个员工都有一个department属性，表示所在的部门。我们可以按部门分组，并计算每个部门的员工数量，然后找出拥有最多员工的部门，最后返回该部门的所有员工信息。

代码示例如下：

function getEmployeesWithHighestCount() {
  // 按部门分组并计算员工数量
  const departmentCounts = {};
  employeeList.forEach(employee => {
    const department = employee.department;
    if (!departmentCounts[department]) {
      departmentCounts[department] = 0;
    }
    departmentCounts[department] += 1;
  });

  // 找出员工最多的部门
  let maxCount = 0;
  let maxDepartment = '';
  for (const department in departmentCounts) {
    if (departmentCounts[department] > maxCount) {
      maxCount = departmentCounts[department];
      maxDepartment = department;
    }
  }

  // 返回员工最多的部门的所有员工信息
  const employeesWithHighestCount = employeeList.filter(employee => {
    return employee.department === maxDepartment;
  });
  return employeesWithHighestCount;
}

调用该函数将返回员工最多的部门的所有员工信息。