What is this Typescript `0` type?

user_4185992 0
Viewed 43

Why does this code throw a Typescript error?

const foo: number | undefined = 22

function bar(x: Date | undefined) {
    !!x && console.log(x) || console.log('x is undefined')
}

bar(foo && new Date())
    ^^^^^^^^^^^^^^^^^
TS2345: Argument of type '0 | Date' is not assignable to parameter of type 'Date | undefined'.   Type 'number' is not assignable to type 'Date'

Seems like there are two cases:

  1. foo is a number → foo && new Date() is of type Date
  2. foo is undefined → foo && new Date() is of type undefined

Where does the 0 type come from here?

typescript
2 Answers

The only case where foo && new Date() is false is when foo is 0. 0 is a falsy value so Logical AND (&&) will return the first falsy value that makes the condition false which is 0.

If foo was any number apart from zero it would be truthy and the Date object, the last truthy value would instead be returned.

This means foo && new Date can be only ever either 0, a Date or undefined whereas your function is typed only for a Date or undefined.

user_148617370

So there is a case when foo is falsy. Since it is a number, only falsy value possible is 0.

Here is a quote from the && docs:

More generally, the operator returns the value of the first falsy operand encountered when evaluating from left to right, or the value of the last operand if they are all truthy.

That's why the below two statements return 0.

console.log(0 && 1);
console.log(0 && new Date());

The reason you do not get any error with regards to undefined is because undefined type is assignable to anything.

user_101401240
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