Calculate the difference between two lists of posts.

app.js

var _ = require('lodash')

var posts_one = [
	  { id: 1, title: 'foo'}
	, { id: 2, title: 'bar'}
	, { id: 2, title: 'bar'}
	, { id: 3, title: 'baz'}
	, { id: 4, title: 'merp'}
	, { id: 4, title: 'merp'}
	, { id: 5, title: 'flakes'}
]

var posts_two = [
	  { id: 1, title: 'foo'}
	, { id: 2, title: 'bar'}
	, { id: 3, title: 'baz'}
	, { id: 3, title: 'baz'}
	, { id: 4, title: 'merp'}
	, { id: 5, title: 'flakes'}
	, { id: 6, title: 'new'}
	, { id: 7, title: 'neu'}
]

var ids_one = _.uniq(_.map(posts_one, function(p) {
	return p.id
}))

var ids_two = _.uniq(_.map(posts_two, function(p) {
	return p.id
}))

var ids_diff = _.difference(ids_two, ids_one)

var posts_diff = _.filter(posts_two, function(p) {
	if(_.contains(ids_diff, p.id)) {
		return p
	}
})

console.log('then', ids_one)
console.log('now', ids_two)
console.log('diff', ids_diff)
console.log('posts', posts_diff)