Everyday TypeScript: Void

• What's the return type of a function that doesn't contain a return statement?

• In TypeScript, that function's return type is void. We can see that by causing a type error, then looking at the error message.

• (When a code example contains a type error, you can answer with type error.)

• >

  function f() {
  }
  const n: number = f();

  Result:

  type error: Type 'void' is not assignable to type 'number'.

• The void type indicates no value at all. In many other static languages, there are no exceptions: void truly can never have a value. But in TypeScript, weirdly, we can create a variable of type void. It can only hold the value undefined.

• >

  const aVoid: void = undefined;
  aVoid;

  Result:

  undefined

• (When TypeScript's "strictNullChecks" mode is off, a variable of type void can also hold the value null. In this course, "strictNullChecks" mode is always on.)

• This doesn't mean that we can then assign our void to a variable of type undefined. That would be a type error, because void isn't assignable to anything!

• >

  const aVoid: void = undefined;
  const u: undefined = aVoid;

  Result:

  type error: Type 'void' is not assignable to type 'undefined'.

• In practice, you'll probably never encounter a variable of type void. But you will encounter many void functions, because any function without a return is automatically inferred to be void.