Function selectVariableAxisKey

packages/core/src/utils/variable.ts:139–190

(
	variableConfig: VariableAxisConfig,
	selectedAxes: Iterable<string>,
)

 * - anything involving multiple custom axes -> `full`
 */
export const selectVariableAxisKey = (
	variableConfig: VariableAxisConfig,
	selectedAxes: Iterable<string>,
): VariableAxisKey => {
	const publishedAxisKeys = getVariableAxisKeys(variableConfig);
	const defaultKey = determineAxisKey(variableConfig);

	// Normalize all axes for comparison.
	const activeAxes = Array.from(selectedAxes)
		.map((axis) => axis.trim().toLowerCase())
		.filter(Boolean)
		.filter((axis) => axis !== 'ital');

	// Deduplicate after normalization.
	const uniqueAxes = [...new Set(activeAxes)];

	if (uniqueAxes.length === 0) {
		return defaultKey;
	}

	// Single axis should go for a direct lookup.
	if (uniqueAxes.length === 1) {
		return findAxisKey(publishedAxisKeys, uniqueAxes[0]) ?? defaultKey;
	}

	// Two axes where one is `wght` should still try for a direct lookup of the other axis as `wght` is implicit in most bundles.
	if (uniqueAxes.length === 2 && uniqueAxes.includes('wght')) {
		const directAxis = uniqueAxes.find((axis) => axis !== 'wght');

		if (directAxis) {
			const directAxisKey = findAxisKey(publishedAxisKeys, directAxis);
			if (directAxisKey) {
				return directAxisKey;
			}
		}
	}

	// If every requested axis is a standard axis, try the `standard` bundle.
	const allStandard = uniqueAxes.every((axis) =>
		STANDARD_VARIABLE_AXES.has(axis),
	);

	if (allStandard) {
		const standardAxisKey = findAxisKey(publishedAxisKeys, 'standard');
		if (standardAxisKey) {
			return standardAxisKey;
		}
	}

	// Fall back to the `full` bundle which contains every axis.
	return findAxisKey(publishedAxisKeys, 'full') ?? defaultKey;
};

/**
 * Return the axis keys a caller actually wants to emit. Providing an empty list defaults to all