one line transpose for 2-dimensional Javascript arrays

transpose.js

function transpose(a)
{
  return a[0].map(function (_, c) { return a.map(function (r) { return r[c]; }); });
  // or in more modern dialect
  // return a[0].map((_, c) => a.map(r => r[c]));
}

@tatomyr empty arrays (or non-2-dimensional arrays) a legit concern for a library method, not sure if it's important for a one liner. Probably best dealt with something like this

function transpose(a)
{
  return a && a.length && a[0].map && a[0].map(function (_, c) { return a.map(function (r) { return r[c]; }); }) || [];
}