CODEWITHSUNDEEP
typescriptundefinedtypescript-generics
scand1sk
scand1sk

Reputation: 1124

Adaptable function that can return undefined if input is undefined

If have a TypeScript function f: A => B. However I in my applications the input may be undefined, so I always have to write, say, const b?: B = a && f(a);.

Can I change the function to include the check within the function (i.e., something like (a: A | undefined) => B | undefined = (a => a && …) but typed so that the return type does not contain undefined if a cannot be undefined, so that I can write either const b: B = f(a) if a cannot be undefined, and const b?: B = f(a) if a can be?

Upvotes: 0

Views: 801

Answers (1)

Aplet123
Aplet123

Reputation: 35502

This is doable with function overloads:

// these are the two overloads
function f(x: number): string;
function f(x: number | undefined): string | undefined;

// this is the implementation, its type signature is not visible
function f(x: number | undefined): string | undefined {
  return x?.toString();
}

Using the answers from this question, it can be extended to arrow functions:

const f: {
  // these are the two overloads
  (x: number): string;
  (x: number | undefined): string | undefined;
} = 
  // this is the implementation, its type signature is not visible
  // not sure why arrow functions make typescript dumber but "any" is now necessary as the return type
  (x: number | undefined): any => x?.toString();

Upvotes: 2

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