Enums should have a null prototype

[matthieusieben]

TypeScript Version: 4.0.2

Search Terms:

• enum
• string
• prototype

Code

    const userInput = 'hasOwnProperty' as string

    enum Action {
        create,
        update
    }

    if (userInput in Action) {
      const a: Action = Action[foo as keyof typeof Action]

      console.log(typeof a) // => function
    }

Expected behavior: Transpiled enums should have a null prototype.

Actual behavior: Transpiled enums inherit from Object.prototype

Playground Link: https://www.typescriptlang.org/play?ssl=11&ssc=24&pln=9&pc=29#code/MYewdgzgLgBAZiEMC8MDkALAhhA8gdzAAUAnEABwFMSoBPNAWAChnKwBXAWxgEFgoAluBgBvZjBjASlLFEoAacTHbkAJrMrMAvs2YC4MABS4ARgCtK-AHTkyUEHSpXseQqQrU6V4FgA2vwz5BcHl4RABKcNElUEhYLAAuXn4hMBRk4LAAbQQkHBgAa0paEANHSlKM1IBdXSYJWIgQX0orXxAAc0Nyyqxw7SA

Related Issues:

[MartinJohns]

I believe it would be helpful if you explain your reasoning on why this should be the case, as this would be a breaking change.

[matthieusieben]

Well, I came upon this while trying to map user input into enum values without using a switch.

I figured the compiled version of enum was built using a regular object, and that the Enum type could actually be used as one in TS (e.g. using the in operator).

It felt kinda weired to have to check for own properties (using Object.prototype.hasOwnProperty.call(MyEnum, userInput, or MyEnum.hasOwnProperty(k)), so I used in and realized it was not safe.

Maybe there is something I am missing something but using a null prototype will definitely feel safer.

[matthieusieben]

A search on google on "cast string to enum typescript" yields this stack overflow link in which all answers allow hasOwnPropery to be cast into the Enum type. This is also the case for the other first google resuts.

[matthieusieben]

Maybe using a null prototype is not the answer but TS should provide a safe and convenient way to check if a value can be cast into an enum.