Cannot use the enum[member] notation in types

[pushkine]

Bug Report

🕗 Version & Regression Information

ts stable & nightly

⏯ Playground Link

Playground Link

💻 Code

const enum CharCode {
    "(" = 40,
    ")" = 41,
    "a" = 97,
    "b" = 98,
    "c" = 99,
}

const lowerA = CharCode.a;
type LowerA = CharCode.a;

const parensLeft = CharCode["("];
type ParensLeft = CharCode["("];
//                         ^^^ Error: Property '(' does not exist on type 'CharCode'. (2339)

type ParensLeft2 = typeof CharCode["("]; // No Error

🙁 Actual behavior

The enum[member] notation does not work in types, rendering members that are not valid identifiers inaccessible.
Using typeof is required to make it work.

🙂 Expected behavior

The enum[member] notation works in types.

[DanielRosenwasser]

This is working as intended - that syntax is an indexed access type. If you want to grab the type of "(", you need to write (typeof CharCode)["("].

[DanielRosenwasser]

Also, to be explicit, typeof CharCode["("] is (typeof CharCode)["("] and not typeof (CharCode["("]). That's why that last alias works.

#19707 discusses this further.

[pushkine]

This is working as intended

I don't understand how that is intended, or why typeof is necessary. Using const enum means CharCode is not an object in runtime, referring to it using typeof as if it were a runtime object makes no sense.

[fatcerberus]

Because CharCode is the type of a value of the enum, a number, and number["("] doesn’t make sense. So if typeof didn’t work, you couldn’t do this at all.

Also, const enum can have runtime representation in certain cases, IIRC.

[pushkine]

@fatcerberus I'm not sure what you're trying to say, directly accessing properties enum.member always works fine without typeof both as a value and as a type

Please explain why CharCode.a means number but CharCode["a"] means number["a"]

[fatcerberus]

let x: CharCode = CharCode.a;
console.log(x["("]);  // type error

You now have a number value stored in x. Foo["bar"], as a type, means that given a value of type Foo, what type do I get if I index that value with ["bar"]. If Foo is number or an enum type, then x.bar makes no sense.

As for your last question: CharCode.a happens in value space. CharCode["a"] in your example happens in type space. That's the difference.

[typescript-bot]

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