# Array permutations

JavaScript, Array, Algorithm, Recursion · Dec 28, 2020

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.

```
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,
])
),
[]
);
};
```

```
permutations([1, 33, 5]);
// [ [1, 33, 5], [1, 5, 33], [33, 1, 5], [33, 5, 1], [5, 1, 33], [5, 33, 1] ]
```