Function setAt

src/Map.js:831–835

(array, idx, val, canEdit)

}

function setAt(array, idx, val, canEdit) {
  const newArray = canEdit ? array : arrCopy(array);
  newArray[idx] = val;
  return newArray;
}

function spliceIn(array, idx, val, canEdit) {
  const newLen = array.length + 1;