It's usual to have places where you only can set a group of strings, for example names of events or types of media, and don't want to allow others. At this moment the only way to document them is creating a Javascript object and set it as an enum, but this forces to create an object for this enum that in other case is unnecesary. I propose three non-mutually incompatible solutions:

• allow to use ad-hoc objects and reference them in a similar way as with callbacks
• allow to set quoted strings as a type

/**
 * @param {('MP4'|'WEBM')} mediaType - Type of the media
 */
function foo(mediaType){};

/**
 * @param {MediaType} mediaType - Type of the media
 */
function foo(mediaType){};
/**
 * @type {('MP4'|'WEBM')} MediaType - Allowed media types
 */

• allow to set quoted strings on the enum statement