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.d.ts files strip private property types, which can be used to distinguish instance types #38953
Description
Activity
Removing the types is necessary for compatibility/implementation-hiding reasons. The correct fix would be for us to emit the variance measurement of
Tin the.d.ts; see #1394A workaround is to use
protectedinstead ofprivate; it will preserve the type in the .d.ts, while still keeping it nominal.Reacted by Jérôme Beau, Killian Guibout-Chatelain and German JablonskiWhat if the compiler outputted
(variance: any) => any,(variance: any) => T,(variance: T) => any, or(variance: T) => T(multiple generics can be handled with intersections, i.e.(variance: T & U) => U), based on the variance. When #1394 lands, it could be changed to the corresponding types, which wouldn't be a breaking change, as the types of private properties are not relevant for anything.This leads to confusing behavior where vscode (or your editor of choice) reports errors that the compiler doesn't catch when using project references.
E.g. in this playground the editor can infer the type of
xeven though it's private. If you've set up vscode with project references, the types will be read from the.tsfiles rather than the.d.tsfiles, which means the editor can get the type ofx.However, if the type is imported via a .d.ts file (i.e. at compile time), the type of
xisany! This leads to a long path of debugging where you can't figure out why vscode doesn't match tsc.Given that it seems we can't annotate private members with their types, it would probably make sense to explicitly forbid using types of private members imported from .d.ts files.
2 years later and ran exactly into this issue
However, if the type is imported via a .d.ts file (i.e. at compile time), the type of x is any! This leads to a long path of debugging where you can't figure out why vscode doesn't match tsc.
Would like to clarifications inside docs on
protectedvsprivateand/or remove the ability to get types from private methods entirely.Reacted by Killian Guibout-Chatelain, Ainias, Karl Stoney and Zefir2 years later, exactly the same rabbithole and just stumbled across this 🤣
Reacted by Zefir
TypeScript Version: 4.0.0-dev.20200604
Search Terms:
private class property type declaration
Expected behavior: .d.ts files preserve the types of private properties, as these determine if an instance type is assignable to another
Actual behavior: .d.ts files strip private property type, creating an error
Related Issues: N/A
Code
Obviously, this is a contrived example, but this was an issue I ran into.
Output
Compiler Options
{ "compilerOptions": { "noImplicitAny": true, "strictNullChecks": true, "strictFunctionTypes": true, "strictPropertyInitialization": true, "strictBindCallApply": true, "noImplicitThis": true, "noImplicitReturns": true, "alwaysStrict": true, "esModuleInterop": true, "declaration": true, "experimentalDecorators": true, "emitDecoratorMetadata": true, "moduleResolution": 2, "target": "ES2017", "jsx": "React", "module": "ESNext" } }Playground Link: Provided