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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.

Add a key/value pair to the end of the current Map

public function add(
  Pair<Tk, Tv> $val,
): Map<Tk, Tv>;

This method is equivalent to Map::set(). If the key in the Pair exists in the Map, the value associated with it is overwritten.

$map->add($p) is equivalent to both $map[$k] = $v and $map[] = Pair {$k, $v} (except that add() returns the Map).

Future changes made to the current Map ARE reflected in the returned Map, and vice-versa.

Parameters​

Returns​

Examples​

The following example adds a single key-value pair to the Map $m and also adds multiple key-value pairs to $m through chaining. Since Map::add() returns a shallow copy of $m itself, you can chain a bunch of add() calls together, and that will add all those values to $m.

$m = Map {};

$m->add(Pair {'red', '#ff0000'});
\var_dump($m);

// Map::add returns the Map so it can be chained
$m->add(Pair {'green', '#00ff00'})
  ->add(Pair {'blue', '#0000ff'})
  ->add(Pair {'yellow', '#ffff00'});
\var_dump($m);