util.inherits does not establish prototype chain

[silkentrance]

In inherits.js Object.setPrototypeOf is being called with the wrong parameters.

The reason for me assuming that the wrong parameters are being used is

(REPL)

> function MyError() {}
> util.inherits(MyError, Error);
> MyError
{ [Function: MyError]
  super_: 
   { [Function: Error]
     captureStackTrace: [Function: captureStackTrace],
     stackTraceLimit: 10 } }
> Error.isPrototypeOf(MyError);
false

While inheritance sort of works, the prototype chain is actually never established.

> myerr = new MyError()
[Error]
> myerr instanceof MyError
true
> myerr instanceof Error
true

Replacing the existing call to Object.setPrototypeOf() by

Object.setPrototypeOf(ctor, superCtor);

See https://github.com/nodejs/node/blob/master/lib/util.js#L805.

(REPL)

> function MyError() {}
> util.inherits(MyError, Error);
> MyError
[Function: OError]
> Error.isPrototypeOf(MyError);
true

Yet, instanceof will now fail

> myerr = new MyError()
[Error]
> myerr instanceof MyError
true
> myerr instanceof Error
false

When using the new class feature, everything works as expected, though

(REPL)

> class MyError extends Error {}
[Function: MyError]
> Error.isPrototypeOf(MyError)
true
> myerr = new MyError()
[Error]
> myerr instanceof MyError
true
> myerr instanceof Error
true

[silkentrance]

See also https://phabricator.babeljs.io/T6765

[silkentrance]

See also #3188

[bnoordhuis]

Object.setPrototypeOf(ctor, superCtor)

That's because inherits() calls Object.setPrototypeOf(ctor.prototype, superCtor.prototype). It goes all the way back to v0.3.2 so I don't think we can change that without breaking a lot of existing code.

Perhaps it's time for inherits() to go gently into that good night and for us to point people to ES6 extends.

[silkentrance]

Sigh perhaps making it @deprecated would suffice then...

[silkentrance]

Reopening as I deem it fit to at least document that shortcoming in the official documentation and deprecate util.inherits in favor of new ES2015/harmony features and also in favor of external libraries that get the job done properly.

[bnoordhuis]

We're open to pull requests that improve the documentation.

As to deprecation, IMO it should start as a documentation-only deprecation that steers people away from using it in new code. util.inherits() is used in a lot of existing code and is, for the most part, not broken. Printing deprecation warnings is arguably too aggressive / obnoxious.

[silkentrance]

Well I think that in the past, people did not care too much about isPrototypeOf, so I think it would be safe to fix util.inherits. Considering that most users would test recursively for _super. See also the vibejs-subclassof project, which BTW did not gain that much traction 😁

As such, I think that with the next release of node the behavior should just be changed and let users just fix their code if they ever managed to screw it up so badly as to rely on that falsely set up prototype chain.

[jasnell]

@nodejs/documentation ... not sure about the deprecation here but it would likely be good to include some documentation on the limitations of util.inherits with regards to the prototype chain.

[Knighton910]

+1

some documentation on the limitations of util.inherits