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javascript/snippets/permutations.md

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+---
+title: Array permutations
+type: snippet
+tags: [array,algorithm,recursion]
+cover: body-of-water
+dateModified: 2020-12-28T13:49:24+02:00
+---
+
+Generates all permutations of an array's elements (contains duplicates).
+
+- Use recursion.
+- For each element in the given array, create all the partial permutations for the rest of its elements.
+- Use `Array.prototype.map()` to combine the element with each partial permutation, then `Array.prototype.reduce()` to combine all permutations in one array.
+- Base cases are for `Array.prototype.length` equal to `2` or `1`.
+- ⚠️ **WARNING**: This function's execution time increases exponentially with each array element. Anything more than 8 to 10 entries may cause your browser to hang as it tries to solve all the different combinations.
+
+```js
+const permutations = arr => {
+  if (arr.length <= 2) return arr.length === 2 ? [arr, [arr[1], arr[0]]] : arr;
+  return arr.reduce(
+    (acc, item, i) =>
+      acc.concat(
+        permutations([...arr.slice(0, i), ...arr.slice(i + 1)]).map(val => [
+          item,
+          ...val,
+        ])
+      ),
+    []
+  );
+};
+```
+
+```js
+permutations([1, 33, 5]);
+// [ [1, 33, 5], [1, 5, 33], [33, 1, 5], [33, 5, 1], [5, 1, 33], [5, 33, 1] ]
+```