_.unionBy([2.1], [1.2, 2.3], Math.floor); // => [2.1, 1.2]
// The `_.property` iteratee shorthand. _.unionBy([{ 'x': 1 }], [{ 'x': 2 }, { 'x': 1 }], 'x'); // => [{ 'x': 1 }, { 'x': 2 }]