code: function findIndex( array, condition ) {
  
      for ( let i = 0; i < array.length; ++i ) {
  
          if ( condition( array[ i ], i ) ) return i;
  
      }
  
      return -1;
  
  }
docs: The `findIndex()` method returns the index of the first element in the array that satisfies the
  provided testing function, otherwise `-1` is returned.