RFC: Support __proto__ literal in object initializers

This is a new issue to specifically propose and elaborate on a feature I raised in this comment on #30587.

Search Terms

• __proto__
• prototype
• object literal
• object spread
• object initializer

Suggestion

While accessing or mutating an existing object via the Object.protptype.__proto__ getter/setter is deprecated, to the best of my knowledge defining the prototype of a new object via the object initializer __proto__ literal is very much encouraged.

The rules for specifying the prototype of a new object via these semantics are very well-specified and safe. See the relevant section of the spec here.

Basic support

Given the following:

const foo = {
  __proto__: { a: "a" },
  b: "b"
};

Typescript currently thinks that foo is the following shape:

type foo = {
  ["__proto__"]: { a: string };
  b: string;
};

when in reality, it is:

type foo = {
  a: string;
  b: string;
};

TypeScript should be able to correctly detect the type of this object initialization.

Strict validity checks

Additionally, TypeScript should prevent invalid __proto__ assignments that are “ignored” by the spec, and require all values to be null or an object. This should fail validation:

const invalid = { __proto__: "hello" }

Correct handling of computed properties

It’s important to note that per the spec, __proto__ literals are not the same as regular property assignments.

This object initialization, for example:

const foo = {
  __proto__: { a: "a" },
  b: "b",
  ["__proto__"]: { c: "c"},
};

creates an object of the following shape:

type foo = {
  a: string;
  b: string;
  ["__proto__"]: { c: "c"};
};

Given this, I would recommend that a __proto__ literal be forbidden in type/interface definitions, such that this is considered a syntax error:

type foo = {
  __proto__: { a: string }
}

while this is an allowable way to specify a property named __proto__ on the type foo.

type foo = {
  ["__proto__"]: string
}

Use-Cases & Examples

This feature allows TypeScript to correctly understand the shape of objects defined with standard JS semantics. While this pattern isn’t especially prevalent, it is an important feature of the language, and should be much more common in one particular use-case where TypeScript currently has a rather severe blind spot:

TypeScript currently PREVENTS the creation of safe indexed objects derived from existing indexed objects. For example:

Given this object, and the goal of “spreading” it into a new map:

// All safe map objects MUST have a `null` prototype.
const someMapObject: { [key: string]: boolean } = Object.create(null);

The following is UNSAFE, and probably the most common approach I see people using. TypeScript should catch this, and should issue a compile-time error. See bug #37963.

const unsafeSpreadMapObject: { [key: string]: boolean | undefined } = {
  ...someMapObject,
  foo: false
};

console.log(typeof unsafeSpreadMapObject["constructor"]);
// => function

console.log(typeof safeSpreadMapObject["foo"]);
// => boolean

The following is also UNSAFE. While using Object.assign and Object.create is a perfectly valid alternative, the any returned by Object.create propagates through the statement and breaks type safety. (Perhaps the result of Object.create should be unknown instead of any?)

const unsafeAssignMapObject: { [key: string]: boolean | undefined  } = Object.assign(
  Object.create(null),
  { foo: "this is not boolean" }
);

console.log(typeof safeSpreadMapObject["constructor"]);
// => undefined

console.log(typeof safeSpreadMapObject["foo"]);
// => string

This is the SAFE way to accomplish this while using object spreads, but TypeScript currently forbids it, since it lacks support for the __proto__ literal, and incorrectly believes a property of type null is being defined:

const safeSpreadMapObject: { [key: string]: boolean | undefined } = {
  __proto__: null,
  ...someMapObject,
  foo: false
};

console.log(typeof safeSpreadMapObject["constructor"]);
// => undefined

console.log(typeof safeSpreadMapObject["foo"]);
// => boolean

Checklist

My suggestion meets these guidelines:

• This wouldn’t be a breaking change in existing TypeScript/JavaScript code
• This wouldn’t change the runtime behavior of existing JavaScript code
• This could be implemented without emitting different JS based on the types of the expressions
• This isn’t a runtime feature (e.g. library functionality, non-ECMAScript syntax with JavaScript output, etc.)
• This feature would agree with the rest of TypeScript’s Design Goals.

References

• Object initialization: Prototype mutation (MDN)
• __proto__ Property Names in Object Initializers (ECMAScript Spec)

[Jack-Works]

supported in PR #42359, behavior:

__proto__ for a non null | object type is a compile error. {...Q, __proto__: T, ...W} is treated as {...T, ...Q, ...W} at the type level.

[bakkot]

Very concretely: { __proto__: null } for a dictionary isn’t just common, it’s also good; having a dictionary without __proto__: null is just incorrect. TypeScript considers let dict: Record<string, string> = { __proto__: null } to be a type error, which discourages the right thing and encourages the wrong thing. That’s bad. And fixing this particular thing doesn’t require getting the full semantics of prototypes into the type system; any of the three options mentioned in the previous comment would fix it, and would be a strict improvement on the current very-incorrect semantics.

As mentioned above, __proto__ in object literals is a standard part of ECMAScript. It is not deprecated or restricted to strict-only or any of that. TypeScript should not neglect it.