void in TypeScript. Why do I get the result from the console.log?

Question

let foo: void;
foo = 2;

I didn't get, explain to me please. When we type void with LET - ok, I got.

But, when we type for example here:

function test(message): void {
  console.log(message);
}

test("hi");

anyway I will get "hi". Why ?

Answer 1

The last type on function is used to define type of its return.

And you might asking, why it does work?

function test(msg: string): void {
  console.log(msg);
}

Because it doesn't return anything.

let myVar: void = test("hello!");
console.log(myVar);

When you try it, myVar will return undefined, because the function itself return nothing. It is the same as this Typescript example.

let unusable: void = undefined;

---

So what return means? Return is data that returned from function. Example:

function idk(): string {
   return "Hello World!";
}

Then, when I execute it, it does return string.

let myVar: string = idk();
console.log(myVar);

Have a look to this return documentation by MDN.

Answer 2

I'm a bit confused trying to understand

I didn't get, explain to me please. When we type void with LET - ok, I got

but if I'm parsing it correctly in my head, the second half of the question shows void as the return type of a function (in that case, nothing) whereas the first half of the question shows a void variable, to which Typescript only allows null or undefined as assignments.

Second half works because the function doesn't try to return anything. It does not accept "void" as a parameter. message type is untyped, so effectively any object would be allowed in.