JSFiddle - React, Tailwind, and code Playground

JavaScript

const employees = [
      {
        title: 'John Doe',
        departments: [
          { name: 'Marketing', slug: 'marketing'},
          { name: 'Sales', slug: 'sales'},
          { name: 'Administration', slug: 'administration'},
        ]
      },
      {
        title: 'John Doe Junior',
        departments: [
          { name: 'Operations', slug: 'operations'},
          { name: 'Sales', slug: 'sales'},
        ]
      },
      {
        title: 'Rick Stone',
        departments: [
          { name: 'Operations', slug: 'operations'},
          { name: 'Marketing', slug: 'marketing'},
        ]
      },
 ];
    
const isInDepartment = departmentToCheck => employer => employer.departments.find(department => department.slug == departmentToCheck);

var departments = [];
function check(departments, name) {
  return departments.some(object => name === object.department);
}

employees.forEach((employee) => {
    employee.departments.forEach((department) => {
        let found = check(departments, department.slug);
        if (!found) { 
        	departments.push({ department: department.slug }); 
        }
		});
});
console.log(departments);

departments.forEach((department) => {
//		console.log(department);
    //employees.filter(isInDepartment(department));
});