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filterWithKey

Note

This is a point-in-time snapshot of the API documentation from January 2026. Going forward, we will not be maintaining a public copy of these references, and recommend users to refer to the built-in signature helpers available in the Hack LSP instead for complete and up-to-date information.

Returns a Map containing the values of the current Map that meet a supplied condition applied to its keys and values

public function filterWithKey(
  (function(Tk, Tv): bool) $callback,
): Map<Tk, Tv>;

Only keys and values that meet a certain criteria are affected by a call to filterWithKey(), while all values are affected by a call to mapWithKey().

The keys associated with the current Map remain unchanged in the returned Map; the keys will be used in the filtering process only.

Parameters​

  • (function(Tk, Tv): bool) $callback

Returns​

  • Map<Tk,Tv> - a Map containing the values after a user-specified condition is applied to the keys and values of the current Map.

Examples​

This example shows how filterWithKey allows you to use an element's value and corresponding key to determine whether to include it in the filtered Map.

$m = Map {
  'red' => '#ff0000',
  'green' => '#00ff00',
  'blue' => '#0000ff',
  'yellow' => '#ffff00',
  'purple' => '#663399',
};

$primary_colors = Set {'red', 'green', 'blue'};

// Filter non-primary colors with a 100% red component
$non_primary_red_100 = $m->filterWithKey(
  ($color, $hex_code) ==> {
    $is_primary = $primary_colors->contains($color);
    $has_full_red_component = \strpos($hex_code, '#ff') === 0;
    return $has_full_red_component && !$is_primary;
  },
);

\var_dump($non_primary_red_100);