Compiler allows 'number' to be used where enum is expected

[AbakumovAlexandr]

Bug Report

Compiler shouldn't allow number to be used where enum is expected.

🔎 Search Terms

number where enum expected

🕗 Version & Regression Information

• This is a crash
• This changed between versions ______ and _______
• This is the behavior in every version I tried, and I reviewed the FAQ for entries about enums_and_function_params
• I was unable to test this on prior versions because _______

⏯ Playground Link

Playground link with relevant code

💻 Code

export enum StatusEnum {
    Cancelled = 4,
    Paid = 6
}

export interface Summary {
    readonly status: number;
}

export class TripSummary {
    private readonly status: StatusEnum;

    public constructor(dto: Summary, status: number) {
        this.status = status; // status is number, but allowed to be assigned to an enum field
        
        this.setFullStatus(dto.status); // dto.status is number, but allowed to be passed as an enum param
    }

    private setFullStatus(status: StatusEnum): void {
        console.log(status);
    }
}

🙁 Actual behavior

Compiler allows number to be used where enum is expected. It's incorrect because narrowing conversion is done implicitly while the compiler cannot make sure that a value passed as the enum param is within a range of the enum values.

🙂 Expected behavior

Implicit conversions from number to enum should generate an error requiring an explicit conversion to be performed (for exampe, using as)

[fatcerberus]

This is intentional because numeric enums are often used as bitfields, e.g. FooEnum.Foo | FooEnum.Bar is typed as number because it’s not explicitly part of the enum. You can use string-valued enums for stricter checks.

[AbakumovAlexandr]

This is intentional because numeric enums are often used as bitfields, e.g. FooEnum.Foo | FooEnum.Bar is typed as number because it’s not explicitly part of the enum. You can use string-valued enums for stricter checks.

Can you provide a full line of code to demostrate what you've meant? I'm not sure how this is related to allowing implicitly assigning numbers into enum typed members.

[AbakumovAlexandr]

@RyanCavanaugh What is the justification for 'Working as Intended'? Especially in 'strict' mode?

[fatcerberus]

let computedFlags = FlagsEnum.One | FlagsEnum.Two;
obj.flagsEnum = computedFlags;

computedFlags should ideally be assignable back to the enum type, if obj is using it as a bitfield. However it’s currently typed as number because its actual value is not (necessarily) part of FlagsEnum. Thus why this type hole is present.

[AbakumovAlexandr]

let computedFlags = FlagsEnum.One | FlagsEnum.Two;
obj.flagsEnum = computedFlags;

computedFlags should ideally be assignable back to the enum type, if obj is using it as a bitfield. However it’s currently typed as number because its actual value is not (necessarily) part of FlagsEnum. Thus why this type hole is present.

If you needed this type 'abuse' (in my opinion, sorry if too harsh :) ), you always could shut up the compiler explicitly:

let computedFlags: FlagsEnum = (FlagsEnum.One | FlagsEnum.Two) as FlagsEnum;
obj.flagsEnum = computedFlags;

So, fixing this TS compiler bug doesn't affect your use case and can't be the justification for the missing type safety related to enums in 'strict' mode.

[RyanCavanaugh]

@AbakumovAlexandr because we did this behavior on purpose

[fatcerberus]

I’m not making any value judgments here, just telling you why the behavior is the way it is, and why it’s intentional. It’s also unlikely to change—at least, not without an additional opt-in compiler flag—as it would be pretty disruptive to a lot of code in the wild.

[AbakumovAlexandr]

@AbakumovAlexandr because we did this behavior on purpose

Can you elaborate: what is the purpose?

[fatcerberus]

Can you elaborate: what is the purpose?

🤦‍♂️

[RyanCavanaugh]

So that you can use bitflag-style enums without getting an error, as described above.

[AbakumovAlexandr]

So that you can use bitflag-style enums without getting an error, as described above.

So, what should I do to ensure that the value in an enum-typed member is actually the one I've defined in my enum without any runtime code?

[RyanCavanaugh]

There currently isn't a way to do that. The suggestion tracking adding a way to do so is #32690

[fatcerberus]

Use a string-valued enum.

[RyanCavanaugh]

You could also use a const-style enum

const StatusEnum = {
    Cancelled: 4,
    Paid: 6
} as const;
type StatusEnum = (typeof StatusEnum)[keyof typeof StatusEnum];

from there the code sample works as desired