[Bug]: Babel does not throw duplicated __proto__ when followed by a property whose value is an assignment expression

💻

• Would you like to work on a fix?

How are you using Babel?

Programmatic API (babel.transform, babel.parse)

Input code

({
  __proto__: a,
  __proto__: a,
  a: a = 1
})

REPL

Configuration file name

No response

Configuration

N. A.

Current and expected behavior

Currently Babel parses it without errors.

It should throw

Redefinition of __proto__ property.

just like Babel correctly throws in the following example:

({
  __proto__: a,
  __proto__: a,
})

Environment

REPL

Possible solution

This issue is very similar to https://github.com/babel/babel/issues/10262.

Like how we handle shorthandAssign

https://github.com/babel/babel/blob/7250d2562be881f9cdc24c6646468e0f3afcaf9d/packages/babel-parser/src/parser/expression.js#L317-L319

we should reset doubleProto only when it is used correctly.

https://github.com/babel/babel/blob/7250d2562be881f9cdc24c6646468e0f3afcaf9d/packages/babel-parser/src/parser/expression.js#L312

Additional context

No response

[The-x-Theorist]

@JLHwung Thank you for helping, I’m soon opening a PR.

[JLHwung]

A doubleProto assignment is an object literal with duplicate __proto__ key:

// invalid
({
  __proto__: a,
  __proto__: a,
})

However, the duplicate __proto__ key is allowed in destructuring:

// valid
({
  __proto__: a,
  __proto__: b
}) => {}

so we should track such errors in refExpressionErrors, and then throw later when we are sure it is a pattern. In this issue, the a = 1 tricks the parser to think __proto__: a, __proto__: a, has been in a pattern, so we should check how assignment expression is parsed.