How to make “handler” lookup for “dispatcher” style function be typed correctly

by

Yeah, there’s no convenient and type-safe solution for you here. I’ve opened an issue at microsoft/TypeScript#30581 about this but I don’t expect it to be addressed.

I see two main ways forward. One is to just use a type assertion, since you legitimately know more than the compiler does here. It could be like this:

function handleInfo(info: TInfo) {
    // assert your way out.  Not type safe but convenient!
    (handlers[info.valueType] as (x: number | string)=>any)(info.value); 
}

Now there’s no error. It’s not type safe. But it’s convenient and doesn’t change the emitted JavaScript.

Or you could try to walk the compiler through the cases and prove to it that all is fine. This is complex, brittle, and has runtime effects:

const typeGuards: {
  [P in keyof TInfoTypeMap]: (x: TInfoTypeMap[keyof TInfoTypeMap])=>x is TInfoTypeMap[P];
} = {
    num: (x:any): x is number => typeof x === "number",
    str: (x:any): x is string => typeof x === "string"
}

function narrowTInfo<K extends keyof TAllPossibleTInfoMap>(
  x: TInfo, v: K): x is TAllPossibleTInfoMap[K] {
    return typeGuards[v](x.value);
} 

function handleInfo(info: TInfo) {
    if (narrowTInfo(info, "num")) {
        handlers[info.valueType](info.value); // okay
    } else {
        handlers[info.valueType](info.value); // okay
    }
}

That works but is obnoxious. So I’d recommend an assertion.

Hope that helps; good luck!

More Related Contents:

  1. What is “not assignable to parameter of type never” error in TypeScript?
  2. Expected 3 type arguments but got 1 but it should infer 2 types
  3. Import class in definition file (*d.ts)
  4. TypeScript union of string and string literals
  5. Are strongly-typed functions as parameters possible in TypeScript?
  6. What does the `is` keyword do in typescript?
  7. Why can I avoid excess property check in typescript just by passing a reference to an object to a function rather than the object in its literal form?
  8. TypeScript: Accept all Object keys that map to a specific type
  9. How to configure custom global interfaces (.d.ts files) for TypeScript?
  10. TypeScript – use correct version of setTimeout (node vs window)
  11. Is there a way to “extract” the type of TypeScript interface property?
  12. Using latest JavaScript features in TypeScript, such as ES2018
  13. How to define an opaque type in TypeScript?
  14. What is the Record type?
  15. Mapping tuple-typed value to different tuple-typed value without casts
  16. TypeScript function return type based on input parameter
  17. What does “homomorphic mapped type” mean?
  18. ‘tsc command not found’ in compiling typescript
  19. What does “all legal JavaScript is legal TypeScript” mean?
  20. Declare a type that allows all parts of all levels of another type
  21. Empty interface allow any object?
  22. extend and only specify known properties?
  23. Private “functions” in TypeScript
  24. RxJs get value from observable
  25. Implicitly create a tuple in Typescript
  26. How to import all modules from a directory in TypeScript?
  27. What means “ambient” in TypeScript
  28. Angular2 HTTP GET – Cast response into full object
  29. TypeScript keyof index type is too wide
  30. Can I force the TypeScript compiler to use nominal typing?

Leave a Comment