Get permutations of arrays and strings recursively

src/permutations/permutations.js

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+const permutations = (input) => {
+    const isString = typeof input === 'string';
+
+    if (isString) {
+        if (input.length < 2) {
+            return [input];
+        }
+
+        input = input.split('');
+    } else if (!Array.isArray(input) || input.length < 2) {
+        return [input];
+    }
+
+    const output = [];
+
+    const perms = permutations(input.slice(1));
+    const current = input[0];
+
+    for (let i = 0; i < perms.length; i++) {
+        const perm = perms[i];
+
+        for (let position = 0; position < input.length; position++) {
+            const add = [...perm.slice(0, position), current, ...perm.slice(position, perm.length)];
+
+            output.push(add);
+        }
+    }
+
+    if (isString) {
+        for (let i = 0; i < output.length; i++) {
+            output[i] = output[i].join('');
+        }
+    }
+
+    return output;
+};
+
+export default permutations;

src/permutations/permutations.test.js

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+import permutations from './permutations';
+
+describe('permutations', () => {
+    const cases = [
+        ['a', ['a']],
+        [[1], [[1]]],
+        [[0, 1], [[0, 1], [1, 0]]],
+        ['01', ['01', '10']],
+        [[0, 1, 2], [[0, 1, 2], [0, 2, 1], [1, 0, 2], [1, 2, 0], [2, 0, 1], [2, 1, 0]]],
+        [null, [null]],
+        [undefined, [undefined]],
+        [{}, [{}]],
+        [123, [123]],
+        [false, [false]]
+    ];
+
+    for (let i = 0; i < cases.length; i++) {
+        const [ input, output ] = cases[i];
+        it(`${JSON.stringify(input)} returns ${JSON.stringify(output)}`, () => {
+            expect(permutations(input)).toEqual(expect.arrayContaining(output));
+        });
+    }
+});