JSFiddle - React, Tailwind, and code Playground

JavaScript

var nodes = [
    { 
        name: 'java',
        followers: [
            { name: 'ABC', email: 'abc@abc.com' },
            { name: 'IJK', email: 'ijk@ijk.com' },
            { name: 'LMN', email: 'lmn@lmn.com' }
        ]
    },
    { 
        name: 'javascript',
        followers: [
            { name: 'ABC', email: 'abc@abc.com' },
            { name: 'IJK', email: 'ijk@ijk.com' },
            { name: 'XYZ', email: 'xyz@xyz.com' }
        ]
    },
    { 
        name: 'programming',
        followers: [
            { name: 'XYZ', email: 'abc@abc.com' },
            { name: 'IJK', email: 'ijk@ijk.com' },
            { name: 'PQR' }
        ]
    }
];

var emails = nodes.reduce(function(store, node) {
		return node.followers.reduce(function(store, follower) {
				follower.email && (store[follower.email] = true);
				return store;
    }, store);
}, {});

console.log(Object.keys(emails));