配列の中の値がユニークか調べる

 const array1 = [1,2,3,1];
 if (([...new Set(array1)]).length == array1.length) {
     console.log("unique");
 } else {
     console.log("not unique");
 }
 
 => not unique

 const array2 = [1,2,3];
 if (([...new Set(array2)]).length == array2.length) {
     console.log("unique");
 } else {
     console.log("not unique");
 }
 
 => unique

参考

https://stackoverflow.com/questions/9229645/remove-duplicate-values-from-js-array/9229821#9229821