Don't emit `const enum`s with reverse mapping (under `preserveConstEnum`)

Search Terms

preserveConstEnums, const enum, enum

Suggestion

By default all const enums are preserved from output. One of the key differences of const enums is disallowing to looking up the reversed value:

enum Enum {
  Zero,
  One,
}

console.log(Enum[1]); // allowed, result is 'One'

const enum ConstEnum {
  Zero,
  One,
}

console.log(ConstEnum[1]); // Error

This means that we can't access const enum's "reversed" member even if preserveConstEnums flag is enabled.

JS code of const enum with enabled preserveConstEnums is pretty similar to just enums right now and contains string literals (even they couldn't be accessed from the code):

var ConstEnum;
(function (ConstEnum) {
    ConstEnum[ConstEnum["Zero"] = 0] = "Zero";
    ConstEnum[ConstEnum["One"] = 1] = "One";
})(ConstEnum || (ConstEnum = {}));

My feature request is change output of const enums when preserveConstEnums is enabled and strip "reversed" values from it:

var ConstEnum;
(function (ConstEnum) {
    ConstEnum["Zero"] = 0;
    ConstEnum["One"] = 1;
})(ConstEnum || (ConstEnum = {}));

Note: actually tsc already has similar behaviour if you specify string constant value for every const enum's member, but the values are strings, not numbers - it is kind of workaround if your const enum is used to declare string constants.

Use Cases

Reversed values for const enums are useless and cannot be accessed in the TS code, so why we should emit them? 🤔 I guess in this case the behaviour of const enums from types and from execution (JS) purposes will be the same.

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.

この issue ではファイルもテストも指定されていません。まず、preserveConstEnums が有効なときに const enums を出力するコンパイラーのエントリーポイントを特定し、既存の文字列値の const enums の動作と比較して、関連する emitter テストを追跡してください。数値の const enums が reverse mappings なしで出力され、通常の enum が現在の出力を維持すれば完了です。