Joins two arrays while removing duplicate items

union.js

Array.prototype.union = ( array ) => {
	const concatenated = this.concat( array );
	return concatenated.filter( ( item, pos ) => concatenated.indexOf( item ) === pos );
};

// example
[ 'a', 'b', 'c' ].union([ 'b', 'c', 'd', 'e' ]); // [ 'a', 'b', 'c', 'd', 'e' ]