Initialization
const a = ImmutableMap([['x', 1], ['y', 2]]);
Extension
const b = a.set('z', 3);
Type
typeof ImmutableMap(); // 'map'
Value types are not objects and have no prototype. Member expressions are exposed on the Immutable Map's prototype which is unique for each realm, just like other value types. This prototype is exposed on the realm's global ImmutableMap.prototype.