How to fill in missing elements in an array describing a range

Presuming the starting array is already sorted, you might want to copy the first element in the source array into a new one and then iteratively push “default objects” into the destination array or copy from the source array when there is a matching object. Repeat until you reach the end of your range.

To check if there is a matching object, you might want to index first the source array by whatever parameter you want to iterate over (in your case: the “joined” field). This means that this parameter has to be unique.

function getIndex(srcArray, field) {
    var i, l, index;
    index = [];
    for(i = 0, l = srcArray.length; i < l; i++) {
        index[srcArray[i][field]] = srcArray[i];
    }
    return index;
}

This creates a hashmap of your array. Searching an element by it’s key (in your case the “joined” field) is very simple:

if(index['2013-06-05'] !== undefined) { 
    /* the element indexed by "2013-06-05" exists */ 
}

Now that you have an index, you can iterate over the range you expect to have. For this, get the first and the last dates in your source array:

//get boundaries
var first = new Date(src[0].joined);
var last = new Date(src[src.length-1].joined);

You also need to know how to construct your default objects:

function createDefault(datestr) {
    return {users: 0, joined: datestr};
}

Now declare a destination array and iterate over the range:

var dest = [];
for(var d = first; d.getTime() <= last.getTime(); d.setDate(d.getDate()+1)) {
    datestr = dateToYMD(d); //dateToYMD is a helper function I declared that formats a date
    if(index[datestr]) {
        //this date exists in your data, copy it
        dest.push(index[datestr]);
    } else {
        //this date does not exist, create a default
        dest.push(createDefault(datestr));
    }
}

I made a jsfiddle that logs the destination array to console.
http://jsfiddle.net/uNasR/