TypeScript: void return type converted to any type?

by

This is by design (I’ll explain why it’s good design shortly). The spec says (in section 3.6.3, abridged for clarity):

A type S is assignable to a type T, and T is assignable from S, if one of the following is true…

  • S and T are object types and, for each member M in T, one of the following is true:

    • M is a call, construct or index signature and S contains a call, construct or index signature N where

      • the result type of M is Void, or the result type of N is assignable to that of M.

In this case, we’re testing if () => string is assignable to () => void. So either string has to be assignable to void (it isn’t), or void has to be void (it is).

In effect, the rule here is you are allowed to throw away the return value, which is consistent with how e.g. C++ treats void in template resolution.

function decrementWidgetHeight(w: Widget): number {
    // ... returns the new height of the widget
}

function applyToManyWidgets(w: Widget[], change: (x: Widget) => void): void {
    // for each widget in the array, apply 'change' to it
}

// Later...
applyToManyWidgets(widgetsToShorten, decrementWidgetHeight); // Should be allowed?

When we constrain the type of change to be (widget) => void, we’re making it so that you can pass decrementWidgetHeight as the second argument even though it has a return value, but still making sure that when we write the body of applyToManyWidgets, we don’t accidentally use the return value of change anywhere.

Note that void is still different than any because this is unallowed:

function f() { }
var x = f(); // Disallowed, f() is of type 'void'

More Related Contents:

  1. Transform union type to intersection type
  2. Type union not checking for excess properties
  3. TypeScript: Object.keys return string[]
  4. Does Typescript support mutually exclusive types?
  5. ‘any’ vs ‘Object’
  6. How to implement a typescript decorator?
  7. Get an object’s class name at runtime
  8. How to convert a string to number in TypeScript?
  9. Confusing “duplicate identifier” Typescript error message
  10. What does the “as const” mean in TypeScript and what is its use case?
  11. Typescript: Check “typeof” against custom type
  12. Angular2 – should private variables be accessible in the template?
  13. Typescript check object by type or interface at runtime with typeguards in 2020+
  14. How to use fetch in TypeScript
  15. How to declare a Fixed length Array in TypeScript
  16. Is it possible to restrict number to a certain range
  17. Higher-order type functions in TypeScript?
  18. Angular 6 View is not updated after changing a variable within subscribe
  19. ESLint – Configuring “no-unused-vars” for TypeScript
  20. Object destructuring with property names that are not valid variable names
  21. How to install Typescript typings for google maps
  22. How to use Object.values with typescript?
  23. is there a way to get all required properties of a typescript object
  24. What’s the meaning of “=>” in TypeScript? (Fat Arrow)
  25. How to use array.map with tuples in typescript?
  26. How to update TypeScript to latest version with npm?
  27. Typescript: Spread types may only be created from object types
  28. Typescript and filter Boolean
  29. Why TypeScript lets me add a string and a number? Can I prevent it?
  30. A way to mark arbitrary strings in Typescript Template Literals

Leave a Comment