Iterate over a collection of objects in javascript and return true from that function

Your collection is an object and not an array, so this would be a way to do it:

var present_user = function(user){
    for (var k in collection) {
        if (collection[k]['screen_name'] == user) return true;
    }
    return false;
};

If your outer object keys are all numbers, you should be using an array instead:

var collection = [{screen_name:"justin"}, {screen_name:"barry"}];

Then iterate with:

function present_user(user) {
    for(var i=0; i < collection.length; i++) {
        if(collection[i].screen_name === user) return true;
    }
}

You could loop the object collection too (with for..in, see mVChr's answer), but in this case it looks like you really should be using an array.