docs: function nanrange( arr, clbk ) {
  	if ( !isArray( arr ) ) {
  		throw new TypeError( 'range()::invalid input argument. Must provide an array. Value: `' + arr + '`.' );
  	}
  	if ( arguments.length > 1 && typeof clbk !== 'function' ) {
  		throw new TypeError( 'range()::invalid input argument. Accessor must be a function. Value: `' + clbk + '`.' );
  	}
  	var len = arr.length,
  		min = null,
  		max = min,
  		flg = true,
  		x;
  
  	for ( var i = 0; i < len; i++ ) {
  		x = ( clbk ) ? clbk( arr[i] ) : arr[ i ];
  		if ( !isNumber( x ) ) {
  			continue;
  		}
  		if ( flg ) {
  			min = x;
  			max = x;
  			flg = false;
  			continue;
  		}
  		if ( x < min ) {
  			min = x;
  		} else if ( x > max ) {
  			max = x;
  		}
  	}
  	return ( flg ) ? null : [ min, max ];
  }