__proto__

Deprecated: This feature is no longer recommended. Though some browsers might still support it, it may have already been removed from the relevant web standards, may be in the process of being dropped, or may only be kept for compatibility purposes. Avoid using it, and update existing code if possible; see the compatibility table at the bottom of this page to guide your decision. Be aware that this feature may cease to work at any time.

Warning: Changing the [[Prototype]] of an object is, by the nature of how modern JavaScript engines optimize property accesses, currently a very slow operation in every browser and JavaScript engine. In addition, the effects of altering inheritance are subtle and far-flung, and are not limited to the time spent in the obj.__proto__ = ... statement, but may extend to any code that has access to any object whose [[Prototype]] has been altered. You can read more in JavaScript engine fundamentals: optimizing prototypes.

Note: The use of __proto__ is controversial and discouraged. Its existence and exact behavior have only been standardized as a legacy feature to ensure web compatibility, while it presents several security issues and footguns. For better support, prefer Object.getPrototypeOf()/Reflect.getPrototypeOf() and Object.setPrototypeOf()/Reflect.setPrototypeOf() instead.

The __proto__ accessor property of Object instances exposes the [[Prototype]] (either an object or null) of this object.

The __proto__ property can also be used in an object literal definition to set the object [[Prototype]] on creation, as an alternative to Object.create(). See: object initializer / literal syntax. That syntax is standard and optimized for in implementations, and quite different from Object.prototype.__proto__.