How to perform union or intersection on an array of arrays with Underscore.js

One can use apply to pass an arbitrary number of arguments to a method.

For union:

// Outputs [1, 4, 5, 6, 2, 3, 7]
var selectedUnion = _.union.apply(_, selected);

For intersection:

// Outputs [1]
var selectedIntersection = _.intersection.apply(_, selected);