A vanilla JS extend() function - http://gomakethings.com/vanilla-javascript-version-of-jquery-extend/

example.js

var defaults = {
    number: 1,
    bool: true,
    magic: 'real',
    animal: 'whale',
    croutons: 'delicious'
};

var options = {
    number: 2,
    magic: 'real',
    animal: 'porpoise',
    bool: false,
    random: 42
};

var settings = extend(defaults, options);

console.log(settings);
// Returns: Object{animal: "porpoise", bool: false, croutons: "delicious", magic: "real", number: 2, random: 42}

extend.js

/**
 * Merge defaults with user options
 * @private
 * @param {Object} defaults Default settings
 * @param {Object} options User options
 * @returns {Object} Merged values of defaults and options
 */
var extend = function ( defaults, options ) {
    var extended = {};
    var prop;
    for (prop in defaults) {
        if (Object.prototype.hasOwnProperty.call(defaults, prop)) {
            extended[prop] = defaults[prop];
        }
    }
    for (prop in options) {
        if (Object.prototype.hasOwnProperty.call(options, prop)) {
            extended[prop] = options[prop];
        }
    }
    return extended;
};

How about this little code down here:

  function extendDefaults(source, properties) {
         var property;
         for(property in properties) {
             if(properties.hasOwnProperty(property)) {
                 source[property] = properties[property];
             }
         }
         return source;
     }