//get email id
let users = [{
        userID: "1",
        userEmail: "user1@email.com"
    },
    {
        userID: "2",
        userEmail: "user2@email.com"
    },
    {
        userID: "3",
        userEmail: "user3@email.com"
    }
]
let getEmailID = users.reduce(function (previous, current) {
    previous.push(current.userEmail);
    return previous;
}, []);

console.log(getEmailID)