Group array elements

group_by.js

function groupBy(arr, fn) {
  fn = fn || ((o) => o);
  let g = [];
  let v = [];
  for (let i = 0; i < arr.length; ++i) {
    let val = fn(arr[i]);
    let idx = g.indexOf(val);
    if (idx === -1) {
      g.push(val);
      v.push([arr[i]]);
    } else {
      v[idx].push(arr[i]);
    }
  }
  return v;
}