function find(arr, f, opt_obj) {
  const i = findIndex(arr, f, opt_obj);
  return i < 0 ? null : typeof arr === 'string' ? arr.charAt(i) : arr[i];
}
Search an array for the first element that satisfies a given condition and
return that element.
@param {IArrayLike<T>|string} arr Array or array
    like object over which to iterate.
@param {?function(this:S, T, number, ?) : boolean} f The function to call
    for every element. This function takes 3 arguments (the element, the
    index and the array) and should return a boolean.
@param {S=} opt_obj An optional "this" context for the function.
@return {T|null} The first array element that passes the test, or null if no
    element is found.
@template T,S