fill one javascript array with another js array

Question

I have an array listOfFriends of objects that contain [{"name" : "fName lName", "uid" : "0102030405"}], ...

I want to cut down the size of this array , since I only need the uids , so I would like to make another array with only the uids

var listOfFriends, listOfFriendsUIDs;
//listOfFriends gets filled
for(var i = 0; i < listOfFriends.length; i++){
 listOfFriendsUIDs[i].uid = listOfFriends[i].uid; // this line isn't working
 //listOfFriendsUIDs is still undefined untill the start of this loop
}

Answer 1

You can use Array.prototype.map to create new array out of IDs:

var listOfFriendsUIDs = listOfFriends.map(function(obj) { return obj.uid; });

Check the browser compatibility and use shim if needed.

DEMO: http://jsfiddle.net/x3UkJ/

• VisioN: @ScottSelby Yes, absolutely.
• Scott Selby: that's some pretty sexy code :) , still learning js , thanks
• Alnitak: @VisioN strictly speaking that's Array.prototype.map - Array.map would be a (non-existent) class method, not an instance method.
• VisioN: @Alnitak Yes, I have used Array.map just to be shorter.
• Alnitak: suggest you use myArray.map or write it in full to avoid ambiguity

Answer 2

Try to use projection method .map()

var listOfFriendsUIDs = listOfFriends.map(function(f){ return f.uid;});

Answer 3

listOfFriendsUIDs[i] is not defined, so you can't access its uid property. You either need:

// creates an array of UID strings: ["0102030405", ""0102030405", ...]
listOfFriendsUIDs[i] = listOfFriends[i].uid;

or

// creates an array of objects with a uid property: [{uid:"0102030405"}, ...]
listOfFriendsUIDs[i] = {}
listOfFriendsUIDs[i].uid = listOfFriends[i].uid;