Make all objects in an array have the same attributes

conform.js

/**
Conform. We have a collection of objects with the possibility of having unique attributes.
We would like to ensure that each object in the collection has the same attributes. Handy for passing objects
to a CSV function that expects all objects to conform:

Example: var coll = [{a : "a", b : ""}, {a : "a", c : "c"}, {a : "a", d : "d", f : "f"}];
conform(coll);

@param array array of objects. 
*/

function conform(collection) {

    "use strict";

    var keys = [], i = 0, j, cur;

    if (Array.isArray(collection)) {
        //loop through each object and get all of their keys, add them to keys if it isn't there already
        for (i = 0; i < collection.length; i = i + 1) {
            if (typeof collection[i] === "object") {
                keys = keys.concat(Object.keys(collection[i]));
            }
        }

        //now make sure that each object has all of the attributes in keys
        for (i = 0; i < collection.length; i = i + 1) {
            cur = collection[i];
            for (j = 0; j < keys.length; j = j + 1) {
                if (!cur.hasOwnProperty(keys[j])) {
                    cur[keys[j]] = null;
                }
            }
        }
    }
}