TypeScript: `void` incompatible with optional/default parameter

Question

It seems bizarre to me that while undefined and null are compatible with void , the reverse is not true, considerably eroding the utility of optional parameters:

function getThing(): string | void {}
function checkThing(val: string = 'abcd') {}
checkThing(getThing())

t.ts(3,12): error TS2345: Argument of type 'string | void' is not assignable to parameter of type 'string'.
  Type 'void' is not assignable to type 'string'.

Specifying an optional arg without default value doesn't change the error, but specifying a union with void does (and generates that with default-initialize the parameter as expected). It seems rather onerous to insist on declaring a void union on all optional parameters that may at any point in time be the target of the result of a void function. What else would you do with a void parameter?

Answer 1

Your code sample:

function getThing(): string | void {}
function checkThing(val: string = 'abcd') {}
checkThing(getThing())

t.ts(3,12): error TS2345: Argument of type 'string | void' is not assignable to parameter of type 'string'.
  Type 'void' is not assignable to type 'string'.

Focus in on the error Type 'void' is not assignable to type 'string' . checkThing expects a string and is given something that you said could be void . Hence the error.

It seems rather onerous to insist on declaring a void union on all optional parameters that may at any point in time be the target of the result of a void function. What else would you do with a void parameter?

This is by design from the TypeScript team. You are entitled a different opinion and I can sympathise with your desires 🌹