Declaration emit widens literal types in readonly properties

[evmar]

TypeScript Version: 2.3.1
Code

$ cat > foo.ts
export const FOO = 'FOO';
export class Bar { readonly type = FOO; }
$ tsc --declaration foo.ts
$ cat foo.d.ts
export declare const FOO = "FOO";
export declare class Bar {
    readonly type: string;
}

EDIT: added readonly to clarify bug report

Expected behavior:
Bar.type is of type "FOO"

Actual behavior:
The type falls back to string

[mhegazy]

This is not specific to declaration emit.

The general idea is that a property declaration (or any other mutable declaration e.g. let or var) has no use of a literal type. you would not declare a variable with only one possible value usually. in these cases you would use a const. similarly for property declarations, you would use readonly.

export const FOO = "FOO";
export class Bar {
     readonly type = FOO
}

for non-const, non-readonly declaration the type is widened to its base type, in this case string.

You can always put a type annotation to tell the compiler what type you want the property to be.

export class Bar {
    type: typeof FOO = FOO
}

[alex-okrushko]

@mhegazy

Readonly is also widened:

cat > foo.ts
export const FOO = 'FOO'; 
export class Bar {readonly type = FOO;}

$ tsc --declaration foo.ts
$ cat foo.d.ts 

export declare const FOO = "FOO";
export declare class Bar {
    readonly type: string;
}

[evmar]

@mhegazy sorry I changed the report to add readonly

[mhegazy]

thanks for pointing that. it is indeed a bug in the declaration emitter.

[evmar]

Workaround: whenever you export a class, write properties with an extra typeof.

export class Bar {
  readonly type: typeof FOO = FOO;
}