Array.flat polyfill

Array.prototype.flat.js

export const flat = arr => {
    // Create a copy of the original array
    const result = [...arr];

    for (let i = 0; i < result.length; i++) {
        while (Array.isArray(result[i])) {
            // insert in the current position a copy of the original item
            result.splice(i, 1, ...result[i]);
        }
    }

    return result;
}

export default flat;

tests.js

import test from 'ava';
import flat from '../src/array-flat';

test('should not mutate the input array', t => {
    const a = [[[1]], 2, 3, [4, 5], 6];
    const b = flat(a);

    t.not(a, b);
    t.deepEqual(a, [[[1]], 2, 3, [4, 5], 6]);
})

test('flattens single level array', t => {
    t.deepEqual(flat([[1], [2, 3], 4, [5]]), [1, 2, 3, 4, 5]);
    t.deepEqual(flat([1, [2], [3], 4, 5]), [1, 2, 3, 4, 5]);
});

test('flattens deep level array', t => {
    t.deepEqual(
        flat([
            [[1]],
            {},
            [[[3]], [4, {}, [1, 2, [1, [1, 2, 3, 4], 3, 4], 4], 5], 6],
            1,
            {},
            [3, [4, {}, [1, 2, [1, [1, 2, 3, 4], 3, 4], 4], 5], 6]
        ]),
        [
            1,
            {},
            3,
            4,
            {},
            1,
            2,
            1,
            1,
            2,
            3,
            4,
            3,
            4,
            4,
            5,
            6,
            1,
            {},
            3,
            4,
            {},
            1,
            2,
            1,
            1,
            2,
            3,
            4,
            3,
            4,
            4,
            5,
            6
        ]
    );
});