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javascript/snippets/filter-unique-by.md

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+---
+title: Filter unique array values based on function
+type: snippet
+tags: [array]
+cover: washed-ashore
+dateModified: 2020-11-02T19:41:07+02:00
+---
+
+Creates an array with the unique values filtered out, based on a provided comparator function.
+
+- Use `Array.prototype.filter()` and `Array.prototype.every()` to create an array containing only the non-unique values, based on the comparator function, `fn`.
+- The comparator function takes four arguments: the values of the two elements being compared and their indexes.
+
+```js
+const filterUniqueBy = (arr, fn) =>
+  arr.filter((v, i) => arr.some((x, j) => (i !== j) === fn(v, x, i, j)));
+```
+
+```js
+filterUniqueBy(
+  [
+    { id: 0, value: 'a' },
+    { id: 1, value: 'b' },
+    { id: 2, value: 'c' },
+    { id: 3, value: 'd' },
+    { id: 0, value: 'e' }
+  ],
+  (a, b) => a.id == b.id
+); // [ { id: 0, value: 'a' }, { id: 0, value: 'e' } ]
+```