enum with bigger integer than available is allowed

[donaldpipowitch]

Code

export enum ModelType {
  Repositories,
  Gists
};

function test(modelType: ModelType) {

}

test(2);

Example
Expected behavior:

Throw error, because 2 shouldn't be available in ModelType.

Actual behavior:

No error is thrown.

[RyanCavanaugh]

We don't distinguish between flag and non-flag enums, so a number that is above any given enum member isn't necessarily invalid. For example

enum Flags {
  Neat = 1,
  Cool = 2,
  Great = 4
}
// like saying Neat | Cool | Great
var x: Flags = 7;

[donaldpipowitch]

Can I opt-out from that? This is unexpected for me and for my use cases not useful. Or can I use strings as values and reference them like ModelType.Gists (instead of using 'foo' | 'bar' and use strings directly).

[RyanCavanaugh]

There's no opt-out setting. Can you show an example of what you mean about using strings?

[donaldpipowitch]

Say the function test only accepts 'foo' or 'bar' as possible types. I could do it that way:

type Value = 'foo' | 'bar';

function test(value: Value) {

}

test('foo');

However I need to pass 'foo'``directly which can be troublesome for refactoring. I'd rather pass something likeValue.fooas a reference to'foo'` (which also reads nicer in my opinion). Is this possible somehow? The best thing I can come up with is this beast:

type Value = 'foo' | 'bar';

type IValueMap = {
  foo: 'foo',
  bar: 'bar'
};

const ValueMap: IValueMap = {
  foo: 'foo',
  bar: 'bar'
};

function test(value: Value) {

}

test(ValueMap.foo);

(You need IValueMap or you'll get Argument of type 'string' is not assignable to to parameter of type '"foo" | "bar"'.)

Going back to my original example I probably would like something like enum, but as a fixed dictionary (which would compile to the same output as my last example with IValueMap):

export dictionary ModelType {
  Repositories: 'repos',
  Gists: 'gists
};

function test(modelType: ModelType) {

}

test(ModelType.Repositories);
test('repos'); // this would work, too

---

Conclusion

Maybe I should rather write a dictionary proposal which works exactly like this?:

export dictionary ModelType {
  Repositories: 'repos',
  Gists: 'gists'
};

function test(modelType: ModelType.Value) {

}

compiles to

type ModelTypeValue = 'repos' | 'gists';

type IModelType = {
  Repositories: 'repos',
  Gists: 'gists'
};

const ModelType: IModelType = {
  Repositories: 'repos',
  Gists: 'gists'
};

export ModelType;

function test(modelType: ModelTypeValue) {

}

[Elephant-Vessel]

I also think that the flag-behavior of enums would have been nice to have separated into another keyword like flag since that behavior is really different from the common usage of enums as a a simple set of named constant values.

However I'm unsure and curious about what you want to achieve here, why don't you just reference the enum value on the ModelType directly like test(ModelType.Gists) instead of sending in wild integers if you want more safety and improving refactoring?

[donaldpipowitch]

However I'm unsure and curious about what you want to achieve here, why don't you just reference the enum value on the ModelType directly like test(ModelType.Gists) instead of sending in wild integers if you want more safety and improving refactoring?

Oh, I don't want the send integers directly. I do use the reference like ModelType.Gists. I found that by accident and thought it was a bug.

[Elephant-Vessel]

@donaldpipowitch So that stuff about strings, how would that improve the language?

[donaldpipowitch]

You could reference the string instead of passing it directly. Or what do you mean?

[Elephant-Vessel]

I mean, this is still about how to work with enums right? Or have this issue changed to be about the general possibility to reference values in string literal types like we do with enums?

[donaldpipowitch]

I mean, this is still about how to work with enums right?

Yeah. I'd definitely like to see an enum alternative to not allow things like that:

export enum ModelType {
  Repositories,
  Gists
};

function test(modelType: ModelType) {

}

test(2);

Or have this issue changed to be about the general possibility to reference values in string literal types like we do with enums?

That would probably a second issue. However this could both be solved with the same feature, if I could define an enum with limited custom values (which could be just 0 and 1 or 'gists' and 'repos').

[mhegazy]

The issue referenced in OP is by design as noted in #8020 (comment)

The rest of the discussion is about non-number based enums, and this is already tracked by #1206.