Use filter and map, with a Set to remove the repeated values:
var employee=[{"firstName":"Zahir","lastName":"Alam","Age":25,"Company":"Switchme","Role":"Developer","Department":"Tech","Head":{"Id":3,"Name":"Sourasis Roy"}},{"firstName":"Amith","lastName":"Manniken","Age":25,"Company":"Switchme","Role":"Developer","Department":"Tech","Head":{"Id":3,"Name":"Sourasis Roy"}},{"firstName":"Sourasis","lastName":"Roy","Age":28,"Company":"Switchme","Role":"CTO"},{"firstName":"Aditya","lastName":"Mishra","Age":29,"Company":"Switchme","Department":"Tech","Role":"CEO"},{"firstName":"Priti","lastName":"Lata","Age":24,"Company":"Switchme","Role":"HR"},{"firstName":"Sumita","lastName":"Nath","Age":24,"Company":"Switchme","Role":"HLA Head","Department":"Crm"},{"firstName":"Tarini","lastName":"Khanna","Age":22,"Company":"Switchme","Role":"Content Writer"},{"firstName":"Abhisek","lastName":"Soni","Age":23,"Company":"Switchme","Role":"HLA","Department":"Crm","Head":{"Id":5,"Name":"Sumita Nath"}},{"firstName":"Ankit","lastName":"Pump","Age":23,"Company":"Switchme","Role":"HLA","Department":"Crm","Head":{"Id":5,"Name":"Sumita Nath"}},{"firstName":"Pogo","lastName":"Laal","Age":23,"Company":"Switchme","Role":"Designer"},{"firstName":"Sabina","lastName":"Sekh","Age":28,"Company":"Switchme","Role":"HLA Head","Department":"Crm"},{"firstName":"Sanjay","lastName":"Poudal","Age":24,"Company":"Switchme","Role":"HLA Head","Department":"Crm","Head":{"Id":10,"Name":"Sabina Sekh"}}];
var repeated = [...new Set(employee.map(({ Department }) => Department).filter(Boolean))];
$("div.all").text(repeated.join(", "));<script src="https://code.jquery.com/jquery-3.3.1.js"></script>
<h3>2. List out all department name </h3>
<div class="all"></div>7
solved I want to print only the repeated values once from an associative array