Add Part Mode

add_part appends a new part to a polygon or multipolygon. Adding a part to a plain Polygon promotes it to a MultiPolygon. It is a selection-gated mode: select the target feature first (see Select Mode), then activate Add Part and draw the new part by clicking each vertex.

map.gm.enableMode('edit', 'add_part');
map.gm.disableMode('edit', 'add_part');
map.gm.toggleMode('edit', 'add_part');
map.gm.isModeEnabled('edit', 'add_part');

Programmatic API

// partRings: an array of rings ([[ [lng, lat], ... ]]) describing the new part
const result = await map.gm.edit.addPart(
  'subedit-polygon',
  [[[0, 50], [2, 50], [2, 52], [0, 52], [0, 50]]],
  { allowOverlap: false }, // optional; reject parts that overlap the existing geometry
);

if (result.ok) {
  console.log('Feature is now a multipolygon:', result.feature);
} else {
  console.warn('Rejected:', result.reason); // e.g. 'overlap'
}

The following methods are available on map.gm:

Method	Returns	Description
enableMode('edit', 'add_part')	Promise	Enables Add Part Mode.
disableMode('edit', 'add_part')	Promise	Disables Add Part Mode.
toggleMode('edit', 'add_part')	Promise	Toggles Add Part Mode.
isModeEnabled('edit', 'add_part')	Boolean	Returns true if Add Part Mode is enabled. false when disabled.

The following events are available on a map instance:

Event	Params	Description	Output
gm:add_part	event	Fired when a part is appended to a feature.	map, feature, shape
gm:operation_rejected	event	Fired when the operation is rejected.	map, mode, reason

map.on('gm:add_part', (event) => {
  console.log('Part added to:', event.feature?.id);
});

Live Add Part Example

Click the polygon to select it, activate Add Part, then draw a separate polygon next to it — the feature is promoted to a multipolygon.

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