Why can’t I return a generic ‘T’ to satisfy a Partial?

by

When thinking about writing a generic function, there’s an important rule to remember

The Caller Chooses the Type Parameter

The contract you’ve provided for getThing …

function getThing<T extends Point>(p: T): Partial<T>

… implies legal invocations like this one, where T is a subtype of Point:

const p: Partial<Point3D> = getThing<Point3D>({x: 1, y: 2, z: 3});

Of course, { x: 10 } is a legal Partial<Point3D>.

But the ability to subtype doesn’t just apply to adding additional properties — subtyping can include choosing a more restricted set of the domain of the properties themselves. You might have a type like this:

type UnitPoint = { x: 0 | 1, y: 0 | 1 };

Now when you write

const p: UnitPoint = getThing<UnitPoint>({ x: 0, y: 1});

p.x has the value 10, which is not a legal UnitPoint.

If you find yourself in a situation like this, odds are good that your return type is not actually generic. A more accurate function signature would be

function getThing<T extends Point>(p: T): Partial<Point> {

More Related Contents:

  1. Generic type to get enum keys as union string in typescript?
  2. Create a new object from type parameter in generic class
  3. TypeScript how to create a generic type alias for a generic function?
  4. Typescript: infer type of generic after optional first generic
  5. How to create a Partial-like that requires a single property to be set
  6. Typescript, merge object types?
  7. In TypeScript, how to get the keys of an object type whose values are of a given type?
  8. Parameters generic of overloaded function doesn’t contain all options
  9. Higher-order type functions in TypeScript?
  10. ‘compile-time’ way to get all property names defined interface
  11. Typescript ReturnType of generic function
  12. Is it possible to generate string literal combinations with template literal in TypeScript?
  13. Iterable cannot confirm generic T in function
  14. Getting the return type of a function which uses generics
  15. typescript difference between placement of generics arguments
  16. Create a global variable in TypeScript
  17. Is it possible to restrict number to a certain range
  18. ESLint – Configuring “no-unused-vars” for TypeScript
  19. Object destructuring with property names that are not valid variable names
  20. How to use Object.values with typescript?
  21. is there a way to get all required properties of a typescript object
  22. Typescript: ReturnType of overloaded function
  23. What TypeScript version is Visual Studio Code using? How to update it?
  24. How to use array.map with tuples in typescript?
  25. how to remove properties via mapped type in TypeScript
  26. “An index signature parameter type cannot be a union type.” – what should I use instead?
  27. Typescript: type narrowing not working for `in` when key is stored in a variable
  28. How do I decide whether @types/* goes into `dependencies` or `devDependencies`?
  29. How to define an interface for objects with dynamic keys?
  30. Types from both keys and values of object in Typescript

Leave a Comment