const users = [

        { name: 'John', age: 30, designation: 'Developer' },

        { name: 'Wilson', age: 26, designation: 'Designer' },

        { name: 'Riya', age: 45, designation: 'Business analyst' },

        { name: 'John', age: 30, designation: 'Developer' },

        { name: 'Sandeep', age: 22, designation: 'Tech Lead' },

    ];

​

    //duplicate_emp = { 30: 1, 26:1, 45:1, 22:1 }

​

    const output = users.reduce((acc, curr) => {

        if(acc[curr.age]){

            acc[curr.age] = ++acc[curr.age];

        }else{

            acc[curr.age] = 1; 

        }

        return acc;

    }, {});

    console.log(output);