Function union

Set.union() creates a new immutable Set that is the union of a collection of other sets.

import { Set } from 'immutable'
const unioned = Set.union([
  Set([ 'a', 'b', 'c' ])
  Set([ 'c', 'a', 't' ])
])
// Set [ "a", "b", "c", "t" ]
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