Incorrect TS2790 error when removing property of the instance, that is defined in prototype

[mnaoumov]

🔎 Search Terms

TS2790

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about _________

⏯ Playground Link

https://www.typescriptlang.org/play?#code/MYGwhgzhAECyCeBhcVoG8BQ1oDMCuAdsABQCU6W20ATgKYAue1B0ALAEwDclAvhnxmAB7AhHrQAlqPpgitaAF5oBWgHc4SFBDLdh0ydNnBa7RcrUbkkbaV0iIQkLQB0IIQHNiAIilijL-CIyLwAaAz85Z0CSUltBe0cXN08fQzl2KMIY0PCZdMyg2LtRRNcPbwB5ACMAK1pgemd3BgAFaiF6DvgAB1oKnGJfPONSAuyw6rqGptb2zvoevoGh-1HosjiMFcjoszJFAD5oAEZ2AGZihycylO3jMeCwu4CsjcvS5O9njPXSHO+HkV4iVrp8vJN6o1mvQ2h0ur1+oM0iNATkIdNobD5otEc81q8gRgAPRE6AASQIADcwCAJAATaC0ajtajQegQdgAdgAnAAGYmkgAqAAt5EJetRZAyhDhoGBoAByOm0Jz0WgK6DiplgTqsgC2eDE0CqYu69AkIhpzgwytV8meY04QA

💻 Code

class MyClass {
  func() {
    return 42;
  }
}

const instance = new MyClass();
const instance2 = new MyClass();
console.log("instance.func()", instance.func());
console.log("instance2.func()", instance2.func());
console.log("Object.getPrototypeOf(instance).func()", Object.getPrototypeOf(instance).func());

instance.func = () => 123;
console.log("instance.func()", instance.func());
console.log("instance2.func()", instance2.func());
console.log("Object.getPrototypeOf(instance).func()", Object.getPrototypeOf(instance).func());

// Invalid error ts2790
// The operand of a 'delete' operator must be optional.
delete instance.func;

🙁 Actual behavior

Error ts2790: The operand of a 'delete' operator must be optional.

It is wrong, because property func is defined in the instance prototype, not the instance itself and we set to the instance manually. Therefore we should be able to remove it to rollback to the prototype behavior

🙂 Expected behavior

No compiler error and the property is used again from the prototype as before

Additional information about the issue

No response

[fatcerberus]

The type system doesn't make a distinction between prototype properties and instance properties

[jcalz]

depends on #51073 --> #9726

also #20922 (comment)

[typescript-bot]

This issue has been marked as "Duplicate" and has seen no recent activity. It has been automatically closed for house-keeping purposes.