配列からユニークな値を取り出す

Set()を使う方法

 const objects =  [
 {id: 1, num: 1},
 {id: 2, num: 2},
 {id: 3, num: 3},
 {id: 4, num: 2},
 {id: 5, num: 1},
 ];
 
 const unique_nums = [...new Set(objects.map(({ num }) => num))];
   => [ 1, 2, 3 ]

参考

https://stackoverflow.com/questions/1960473/get-all-unique-values-in-a-javascript-array-remove-duplicates