sum-elements-nested.js

/**
 * Sum of all elements including nested arrays.
 *
 * by Dmitry Soshnikov <[email protected]>
 * MIT Style License
 */

[1, 2, [3, 4], 5, [6, [7, 8]]].reduce(function collect(previous, current) {
  return (
    (Array.isArray(previous) ? previous.reduce(collect) : previous) +
    (Array.isArray(current) ? current.reduce(collect) : current)
  );
}); // 36

You might also want to pass a second argument to the inner reduce call in order to handle nested arrays whose first element is an array:

var b = [[1], 2, [[3], 4], 5, [6, [7, 8]]].reduce(function collect(previous, current) {
  return previous +
    (Array.isArray(current) ? current.reduce(collect, 0) : current);
}, 0);

@stasm, yep, also a good observation. The initial solution BTW doesn't suffer from this too.