Flatten nested array using reduce

main.js

/**
 * Takes a nested array and ftattens it
 *
 * @param  {Array} val They input array
 * @return {Array} val Flatten array
 */
function flatten(val) {

    /*
    * Recursively walk over the array and concat all elements back to the first level array
    * after making sure they are all integers
     */
    return val
        .map(a => !Number.isInteger(a) && !Array.isArray(a) ? (() =>{throw new Error('Non integer element found')})() : a)
        .reduce((a, b) => a.concat(Array.isArray(b) ? flatten(b) : b), []);

}

test.js

const assert = require('assert');

// It should handle one level nested arrays
assert.deepEqual([1, 2, 3], flatten([1, [2, 3]]));
// It should handle two level nested arrays
assert.deepEqual([1, 2, 1, 2, 3], flatten([1, [2, [1, 2, 3]]]));
// Throws error when an element is not an integer
assert.throws(() => flatten([1, ['string', 2]]), /Non integer element found/, 'Non integer element found');
// Throws error when an element is not an integer
assert.throws(() => flatten(['string', [1, 2, 3]]), /Non integer element found/, 'Non integer element found');