Javascript- Retrieve specific key values from JSON object

You are getting the only last key pair because after terminating the line scope against previous key is removed.

<script>
var final = {};
for (var key in user){
final[key] = {"displayName" : user[key]["displayName"],"fullName" : user[key]["fullName"],"profilepic" : user[key]["profilepic"]}

} 
</script>

You are overwriting final[key] in every line and get only the last assignment.

You could create a new object, if there is no one and assign first an object

final[key] = final[key] || {};

and later assign the properties to it.

final[key].displayName = user[key]["displayName"];
final[key].fullName = user[key]["fullName"];
final[key].profilepic = user[key]["profilepic"];

var user = { "987654321": { "displayName": "Wayne", "profilepic": "https://test1.com", "fullName": "Bruce Wayne", "Address": "1st Main 1st Cross", "dob": "Apr 1986", "likes": "Sports" }, "123456789": { "displayName": "wade", "profilepic": "https://test2.com", "fullName": "wade wilson", "Address": "31 Main 1st Cross", "dob": "Sep 1993", "likes": "Movies" } },
    final = {};

for (var key in user) {
    final[key] = final[key] || {};
    final[key].displayName = user[key]["displayName"];
    final[key].fullName = user[key]["fullName"];
    final[key].profilepic = user[key]["profilepic"];
}

console.log(final);

The reason for this is you're three times reassigning a new object (which is created whenever you use {...}.

What you probably want is something like this:

for (var key in user) {
    // Create a new object only once
    var newEntry = {};

    // Store a reference to the original entry
    var oldEntry = user[key]

    // Copy members
    newEntry.displayName = oldEntry.displayName;
    newEntry.fullName = oldEntry.fullName;
    newEntry.profilepic = oldEntry.profilepic;

    // Assign the object
    final[key] = newEntry;
}

Rather than using the . operator you could of course use brackets and string indices, but I think using . is easier to read.

Note that you could also store the fields to copy in an additional array, which could be useful to be more dynamic/selective:

// Members to copy
var copyStuff = ['displayName', 'fullName', 'profilePic'];

for (var key in user) {
    // Create a new object only once
    var newEntry = {};

    // Store a reference to the original entry
    var oldEntry = user[key]

    // copy members
    for (var subkey in copyStuff)
        newEntry[subkey]= oldEntry[subkey];

    // Assign the object
    final[key] = newEntry;
}