const isUnique = (value, index, self) => self.indexOf(value) === index;
const unique = items => items.filter(isUnique);
const mapBy = (items, key) => items.map(item => item[key]);
const uniqueKeys = (items, key) => unique(mapBy(items, key));
const someKeys = (item, keys) => keys.reduce((result, key) => {
	result[key] = item[key];
	return result;
}, {});