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Cheatsheet

Working with any

38. Use the Narrowest Possible Scope for any Types

  • Make your uses of any as narrowly scoped as possible to avoid undesired loss of type safety elsewhere in your code.
  • Never return an any type from a function. This will silently lead to the loss of type safety for any client calling the function.
  • Consider @ts-ignore as an alternative to any if you need to silence one error.

39. Prefer More Precise Variants of any to Plain any

  • When you use any, think about whether any JavaScript value is truly permissible.
  • Prefer more precise forms of any such as any[] or {[id: string]: any} or () => any if they more accurately model your data.

40. Hide Unsafe Type Assertions in Well-Typed Functions

  • Sometimes unsafe type assertions are necessary or expedient. When you need to use one, hide it inside a function with a correct signature.

41. Understand Evolving any

  • While TypeScript types typically only refine, implicit any and any[] types are allowed to evolve. You should be able to recognize and understand this construct where it occurs.
  • For better error checking, consider providing an explicit type annotation instead of using evolving any.

42. Use unknown Instead of any for Values with an Unknown Type

  • The unknown type is a type-safe alternative to any. Use it when you know you have a value but do not know what its type is.
  • Use unknown to force your users to use a type assertion or do type checking.
  • Understand the difference between , object, and unknown.

43. Prefer Type-Safe Approaches to Monkey Patching

  • Prefer structured code to storing data in globals or on the DOM.
  • If you must store data on built-in types, use one of the type-safe approaches (augmentation or asserting a custom interface).
  • Understand the scoping issues of augmentations.

44. Track Your Type Coverage to Prevent Regressions in Type Safety

  • Even with noImplicitAny set, any types can make their way into your code either through explicit anys or third-party type declarations (@types).
  • Consider tracking how well-typed your program is. This will encourage you to revisit decisions about using any and increase type safety over time.