new Set()
and the spread operator (...
) to create an array of the unique values in arr
.Array.prototype.filter()
to create an array containing only the non-unique values.const filterUnique = arr =>
[...new Set(arr)].filter(i => arr.indexOf(i) !== arr.lastIndexOf(i));
filterUnique([1, 2, 2, 3, 4, 4, 5]); // [2, 4]
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