code: function index(a, fn) {
    let i, l;
    for (i = 0, l = a.length; i < l; i++) {
      if (fn(a[i]))
        return i;
    }
    return -1;
  }
docs: Function returning the index of the first element of a list matching the
  given predicate.
  
  @param  {array}     a  - The target array.
  @param  {function}  fn - The predicate function.
  @return {mixed}        - The index of the first matching item or -1.