flatten.js

function flatten (arr) {
    if (!(arr instanceof Array)) {
        return arr;
    }

    return arr.reduce((acc, val) => acc.concat(flatten(val)), []);
}

module.exports = flatten;

test.js

const assert = require('assert');
const flatten = require('./flatten');

assert.equal(flatten("foo"), "foo");
assert.deepEqual(flatten({foo: 'bar'}), {foo: 'bar'})
assert.deepEqual(flatten([1]), [1]);
assert.deepEqual(flatten([1, {foo: 'bar'}]), [1, {foo: 'bar'}]);
assert.deepEqual(flatten([1, "foo"]), [1, "foo"]);
assert.deepEqual(flatten([1, [2]]), [1, 2]);
assert.deepEqual(flatten([1, [2], 3]), [1, 2, 3]);
assert.deepEqual(flatten([1, [2], [3, [4, 5]]]), [1, 2, 3, 4, 5]);
assert.deepEqual(
    flatten([1, [2], 3, [[[4], [5, [6, 7]], [8, 9, [10]]]], 11]), 
    [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11]
);