You can loop over properties of an Object using for..in
var obj = this.skins.hologramDark, key; // remember to var
for (key in obj) { // loop over each key
if (obj.hasOwnProperty(key)) { // if the key is on the obj (not inherited)
if (typeof obj[key] === 'object') { // and the property is another object
obj[key].default = 'foo'; // set the property default
}
}
}