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' }
        ]
    }
]


const emails = nodes
.map(node => node.followers.map(user => user.email))
.reduce((emails, result) => [...emails, ...result], [])
.filter((email, i, self) => self.indexOf(email) === i)
.filter(mail => mail)

alert(emails)