Filtering High-Earning Departments in TypeScript

When analyzing employee data, a common requirement is to extract insights based on departments. One such use case is identifying departments with above-average salaries.

Let's walk through a simple but powerful function written in Typescript that accomplishes just that.

The Problem

You're given a list of employees with their name, department, and salary. You want to return a list of department names where the average salary exceeds a specified threshold.

The Data Structure

Here's the Employee type we're working with:

type Employee = {
  name: string;
  department: string;
  salary: number;
};

The Function: getDepartmentsWithHighAverageSalary

export const getDepartmentsWithHighAverageSalary = (
  users: Employee[] | null | undefined,
  salaryThreshold: number
): string[] => {
  if (!Array.isArray(users)) return [];

  const deptStats = users.reduce<
    Record<string, { total: number; count: number }>
  >((acc, user) => {
    if (!acc[user.department]) {
      acc[user.department] = { total: 0, count: 0 };
    }

    acc[user.department].total += user.salary;
    acc[user.department].count += 1;

    return acc;
  }, {});

  return Object.entries(deptStats)
    .filter(([_, { total, count }]) => total / count > salaryThreshold)
    .map(([department]) => department);
};

How It Works

1. Input validation: First, we check if the users input is a valid array. If not, we return an empty array.

2. Aggregate data by department: using .reduce, we build a lookup object (deepStats) that accumulates:

   • total salary per department
   • number of employees per department

3. Calculate averages: We use Object.entries to iterate over each department's data and filter out those whose average salary is below the threshold.

4. Return result: Finally, we return just the department names that meet the criteria.

Example Usage

const employees: Employee[] = [
  { name: "Alice", department: "Engineering", salary: 120000 },
  { name: "Bob", department: "Engineering", salary: 110000 },
  { name: "Charlie", department: "HR", salary: 60000 },
  { name: "Diana", department: "HR", salary: 65000 },
];

const highPaying = getDepartmentsWithHighAverageSalary(employees, 100000);
console.log(highPaying); // ["Engineering"]

Type Safety + Edge Case Handling

This function gracefully handles null or undefined input, thanks to the Array.isArray check. It's also fully typed with TypeScript for better DX (developer experience) and fewer runtime surprises.

Final Thoughts

This pattern—grouping and aggregating data—is ver common in real-world applications like reporting dashboards, HR systems, or analytics engines.

You can easily adapt this logic for things like:

• Filtering top-performing sales regions
• Calculating average product ratings
• Analyzing time-tracking data