Question

I have an array of Facebook users (userList) and I want to store the number of mutual friends for each user in the array as a property (mfCount). I have checked that I am getting the correct number of mutual friends if I put in an individual user, but I’m not sure why I can’t add this value to each user in the array?

function getfriends() {
FB.api('/me/friends', function(response) {
    userList = userList.concat(response.data);
    userCount = response.data.length;
    for( i=0; i<response.data.length; i++) {
      userId = response.data[i].id;
      FB.api('/me/mutualfriends/'+userId+'/', function(response) {
        userList[i].mfCount = response.data.length;
        userCount--;
        if(userCount === 0) { display_results();}
      });
    } 
  });

}

Answer 1

Have a look at the implementation below.
I’ve broken it out into multiple functions to separate each step.
When you’re dealing with loops and callbacks, it becomes very important to keep track of what scope your anonymous functions are being defined in.

You can theoretically do it all in a one-liner like you were writing…
…but it gets very, very confusing as you go further and further into nested-callbacks.

One solution would be to make every variable inside each function 100% global, so that only i needs to have an enclosed reference. That’s not really pretty, though.

Look through each function and take note of what parameters are going into the functions each step calls (or closures for callbacks). They’re all needed (whether you separate them this way, or through closures in a one-liner or whatever).

The following worked just fine for me, inside of the Facebook developer sandbox (first time using the API).
The logs were for my benefit to see how the data was coming out, and to keep a basic stack-trace.

var userList = [],
    userCount = 0;

function getfriends () {
    //console.log("getFriends");
    var url = "/me/friends";
    FB.api(url, function (response) {
        if (response.error && response.error.message) { return false; }
        userList = userList.concat(response.data);
        userCount = response.data.length;
        compareAllFriends();
    });
}

function compareAllFriends () {
    //console.log("compareAllFriends");
    var i = 0, l = userCount, userID;
    for (; i < l; i += 1) {
        userID = userList[i].id;
        compareFriendsWith (i, userID);
    }
}
function compareFriendsWith (i, id) {
    //console.log("compareFriendsWith", i, id);
    var path = "/me/mutualfriends/",
        url = path + id + "/";

    FB.api(url, (function (i) {
        return function (response) {
            //console.log(i, response);
            var numFriends = (response.data) ? response.data.length : 0;
            setMutualFriends(i, numFriends);
            userCount -= 1;
            //console.log(userCount);
            if (userCount === 0) {
                display_results();
                //console.log("DISPLAYING");
            }
        };
    }(i)));
}

function setMutualFriends (i, friendcount) { userList[i].mfCount = friendcount; }