Function determineAxisKey

packages/core/src/utils/variable.ts:17–47

(
	variableConfig: VariableAxisConfig,
)

 * Pick the default published axis key for a variable family.
 */
export const determineAxisKey = (
	variableConfig: VariableAxisConfig,
): VariableAxisKey => {
	// Filter out any inactive axes and `ital` since it's always included.
	const active = Object.keys(variableConfig).filter((axisKey) =>
		Boolean(variableConfig[axisKey]),
	);

	if (active.length === 0) {
		return 'wght'; // Default.
	}

	const standard = active.filter((axisKey) =>
		STANDARD_VARIABLE_AXES.has(axisKey),
	);
	const custom = active.filter(
		(axisKey) => !STANDARD_VARIABLE_AXES.has(axisKey),
	);

	// If there are any custom axes, we prefer those as the default entrypoint since they
	// are more specific. If there's more than one custom axis, we fall back to `full`.
	if (custom.length > 0) {
		return standard.length > 0 || custom.length > 1
			? 'full'
			: (custom[0] ?? '');
	}

	// If there are no custom axes, we prefer the standard axes.
	// If there's more than one, we fall back to `standard`.
	return standard.length > 1 ? 'standard' : (standard[0] ?? '');
};

/**
 * Return every published axis key from a variable font config.