Random strings generator

I was converting a ECMAScript 3 function to ES6's "fat-arrow" format. It certainly abbreviates.

function randomString(c) {
  for (var a = [], b = 0; b < c; b++) {
    a.push(Math.floor(36 * Math.random()).toString(36));
  }
  return a.join("");
}

was how it was originally defined. Now it looks like this:

const randomString = c => Array(c).fill(0).map(function (itm, idx) {
    return Math.floor(36 * Math.random()).toString(36);
  }).join("");

The ES3 form is slightly faster than the ES6 version but but only by about 40 nanoseconds. The fill and map probably have overheads that the for doesn't.

(a few minutes later)

Have made the ES6 version a bit more terse:

const ES6randomString = c => Array(c)
  .fill(0)
  .map(() => Math.floor(36 * Math.random()).toString(36))
  .join("");

But sadly the ES3 version is still faster. Any suggestions?