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javascript/snippets/difference.md

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+---
+title: Array difference
+type: snippet
+tags: [array]
+cover: interior-3
+dateModified: 2020-10-19T18:51:03+03:00
+---
+
+Calculates the difference between two arrays, without filtering duplicate values.
+
+- Create a `Set` from `b` to get the unique values in `b`.
+- Use `Array.prototype.filter()` on `a` to only keep values not contained in `b`, using `Set.prototype.has()`.
+
+```js
+const difference = (a, b) => {
+  const s = new Set(b);
+  return a.filter(x => !s.has(x));
+};
+```
+
+```js
+difference([1, 2, 3, 3], [1, 2, 4]); // [3, 3]
+```