Bad error message for enum initializers

[orzFly]

TypeScript Version: typescript@2.9.0-dev.20180516

Search Terms: enum infinity, TS2553, enum undefined

---

Code 1

enum TestEnum {
  "undefined" = undefined,
}

Expected behavior: No string valued member at all. Maybe the error message is wrong.

Actual behavior:

xxx.ts:2:17 - error TS2553: Computed values are not permitted in an enum with string valued members.

2   "undefined" = undefined,
                   ~~~~~~~~~

What does the 0 mean?

var TestEnum;
(function (TestEnum) {
    TestEnum[TestEnum["undefined"] = 0] = "undefined";
})(TestEnum || (TestEnum = {}));

Playground Link: here

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Code 2

enum TestEnum {
  "undefined" = undefined,
  "NaN" = NaN,
  "Infinity" = Infinity,
  "-Infinity" = -Infinity,
}

Expected behavior: Maybe?

var TestEnum;
(function (TestEnum) {
    TestEnum[TestEnum["undefined"] = undefined] = "undefined";
    TestEnum[TestEnum["NaN"] = NaN] = "NaN";
    TestEnum[TestEnum["Infinity"] = Infinity] = "Infinity";
    TestEnum[TestEnum["-Infinity"] = -Infinity] = "-Infinity";
})(TestEnum || (TestEnum = {}));

Actual behavior: No error at all!
So what's TestEnum.undefined, TestEnum.NaN, TestEnum.Infinity, -TestEnum.Infinity?

var TestEnum;
(function (TestEnum) {
    TestEnum[TestEnum["undefined"] = TestEnum.undefined] = "undefined";
    TestEnum[TestEnum["NaN"] = TestEnum.NaN] = "NaN";
    TestEnum[TestEnum["Infinity"] = TestEnum.Infinity] = "Infinity";
    TestEnum[TestEnum["-Infinity"] = -TestEnum.Infinity] = "-Infinity";
})(TestEnum || (TestEnum = {}));

Playground Link: here

---

Related Issues:
Issue #21959: "It is a compile time error for constant enum expressions to be evaluated to NaN or Infinity."
PR #22022: "disallow nan and infinity in enum member" is made for the previous issue

I think this is not duplicated as #21959 and #22022 are talking about making undefined/NaN/Infinity an error in const enum instead of making them generate correct code in non-const enum.

We only want this error for const enums (enums with a const modifier, I mean), not all enums.

• disallow nan and infinity in enum member #22022 (comment)

[DanielRosenwasser]

Yeah this is a terrible and broad error message when complex non-numeric expressions are used in initializers and it needs to be fixed.

[orzFly]

@DanielRosenwasser Note in the second code snippet generated by the compiler:

TestEnum[TestEnum["undefined"] = TestEnum.undefined] = "undefined";

TestEnum.undefined is wrong. It should be undefined

[g0rducci]

Hi! Its fixed yet?

[aminpaks]

Yeah this is a terrible and broad error message when complex non-numeric expressions are used in initializers and it needs to be fixed.

What should the fix look like?
I fixed it by making a new error as in "'null' or 'undefined' cannot be an enum value member", is that the behaviour that we would want?

[jakebailey]

After #50528 in TS 5.0, this bad error message is no longer present; the only error on the first code sample is:

Property '"undefined"' is used before being assigned.(2565)

Which makes sense, given it's self-referencing and the hover shows that.

TS 3.9 fixed the second example case; the correct error now appears.