Flatten nested arrays

flatten.js

function flatten(arr) {
  return arr.reduce(
    (acc, c) => acc.concat(Array.isArray(c) ? flatten(c) : c),
    []
  );
}

flatten.test.js

// to run, execute: node flatten.test.js

'use strict';

const assert = require('assert');
const flatten = require('./flatten');
const tests = [
  [[1, 2], [1, 2]],
  [[1, [2]], [1, 2]],
  [[[1], 2], [1, 2]],
  [[[[[[[1], 2]]]]], [1, 2]],
  [[1, 2, 3, [[[[[4], 5]], 6], 7], 8], [1, 2, 3, 4, 5, 6, 7, 8]],
  [[], []],
  [[1], [1]],
  [[[1]], [1]],
  [[[[[[[[[1]]]]]]]], [1]]
];
const passedTests = tests.reduce(function(p, c, i) {
  try {
    assert.deepEqual(flatten(c[0]), c[1]);
  }
  catch(e) {
    console.error('Something broke in test %d', i + 1);
    throw e;
  }
  return ++p;
}, 0);

console.log('%d tests passed', passedTests);