Debugging a regression: https://github.com/kellnerd/musicbrainz-scripts/issues/34

batchCreateRecordingRels.js

/**
 * Fetches the core entity with the given MBID from the internal API ws/js.
 * @param {string} gid MBID of the entity.
 * @param {string[]} inc Include parameters for ws/js.
 * @returns {Promise<CoreEntityT>}
 */
async function fetchCoreEntity(gid, inc = []) {
	const query = new URLSearchParams({ inc: inc.join(' ') });
	const result = await fetch(`/ws/js/entity/${gid}?${query}`);
	return result.json();
}

/**
 * @param {CoreEntityTypeT} sourceType 
 * @param {CoreEntityTypeT} targetType 
 */
function isRelBackward(sourceType, targetType) {
	return sourceType > targetType;
}

// Taken from root/static/scripts/relationship-editor/hooks/useRelationshipDialogContent.js
const RELATIONSHIP_DEFAULTS = {
	_lineage: [],
	_original: null,
	_status: 1, // add relationship
	attributes: null,
	begin_date: null,
	editsPending: false,
	end_date: null,
	ended: false,
	entity0_credit: '',
	entity1_credit: '',
	id: null,
	linkOrder: 0,
	linkTypeID: null,
};

/**
 * Creates a relationship between the given source and target entity.
 * @param {Partial<RelationshipT> & { source?: CoreEntityT, target: CoreEntityT, batchSelectionCount?: number }} options
 * @param {CoreEntityT} [options.source] Source entity, defaults to the currently edited entity.
 * @param {CoreEntityT} options.target Target entity.
 * @param {number} [options.batchSelectionCount] Batch-edit all selected entities which have the same type as the source.
 * The source entity only acts as a placeholder in this case.
 * @param {Partial<RelationshipT>} props Relationship properties.
 */
function createRelationship({
	source = MB.relationshipEditor.state.entity,
	target,
	batchSelectionCount = null,
	...props
}) {
	const backward = isRelBackward(source.entityType, target.entityType);

	MB.relationshipEditor.dispatch({
		type: 'update-relationship-state',
		sourceEntity: source,
		batchSelectionCount,
		creditsToChangeForSource: '',
		creditsToChangeForTarget: '',
		newRelationshipState: {
			...RELATIONSHIP_DEFAULTS,
			entity0: backward ? target : source,
			entity1: backward ? source : target,
			id: MB.relationshipEditor.getRelationshipStateId(),
			...props,
		},
		oldRelationshipState: null,
	});
}

/**
 * Creates the same relationship between each of the selected source entities and the given target entity.
 * @param {import('weight-balanced-tree').ImmutableTree<CoreEntityT>} sourceSelection Selected source entities.
 * @param {CoreEntityT} target Target entity.
 * @param {RelationshipProps} props Relationship properties.
 */
function batchCreateRelationships(sourceSelection, target, props) {
	return createRelationship({
		source: sourceSelection.value, // use the root node entity as a placeholder
		target,
		batchSelectionCount: sourceSelection.size,
		...props,
	});
}


/*
 * Run the following code on any release relationship editor page to add the example label
 * as phongraphic copyright holder of all selected recordings.
 */

let targetLabel = await fetchCoreEntity('e5d95d7c-efec-47f6-8644-aedcc661d03e');
let selectedRecordings = MB.relationshipEditor.state.selectedRecordings ;

batchCreateRelationships(selectedRecordings, targetLabel, {
	linkTypeID: 867,
	entity0_credit: 'Test',
});

Without selecting any recordings I get at least an error message (as expected): Uncaught (in promise) TypeError: sourceSelection is null
But once I have selected recordings, the code silently fails and tries to perform a sentry request (which I've blocked after the first time, because the contained error message e is undefined is useless).