Code to flatten an array of arbitrarily nested arrays of integers into a flat array of integers.

flatten.js

// NodeJS
const assert = require('assert');

/**
 * Flatten an array of arbitrarily nested arrays of integers into a flat array of integers.
 * e.g. [[1,2,[3]],4] -> [1,2,3,4]. 
 * 
 * @name flatten
 * @param {Array} arr An array to flatten, nested or otherwise
 * @returns {Array} the flattened array
 * @throws {Error}
 */
function flatten(arr) {
    if (Array.isArray(arr)) {
        return arr.reduce(function(acc, element) {
            const items = Array.isArray(element) ? flatten(element) : [element];
            return acc.concat(items);
        }, []);
    }
    throw Error('Argument passed is not an array');
}


// Tests
function testFlatten() {
    const testCase1 = flatten([[1, 2,], [3, 4], [5, 6]]);
    const testCase2 = flatten([[1], 2, [3, 4], 5, 6]);
    const testCase3 = flatten([[1, [2, [3, 4, [5, [6]]]]]]);
    
    assert.deepEqual(testCase1, [1,2,3,4,5,6]);
    assert.deepEqual(testCase2, [1,2,3,4,5,6]);
    assert.deepEqual(testCase3, [1,2,3,4,5,6]);
}

testFlatten();