19. nullish-coalescing-operator19. nullish-coalescing-operatora ?? b // 相当于 (a !== null && a !== void 0) ? a : b 举个例子: var foo = object.foo ?? "default"; // 相当于 var foo = (object.foo != null) ? object.foo : "default"; 需要 @babel/plugin-proposal-nullish-coalescing-operator 插件支持