---
title: Empty
slug: /types/empty
---

The `empty` type has no values. It is the [subtype of all other types](../../lang/type-hierarchy) (i.e. the [bottom type](https://en.wikipedia.org/wiki/Bottom_type)).
In this way it is the opposite of [`unknown`](../unknown), which is the supertype of all types.

It is not common to annotate your code using `empty`. However, there are a couple of situations that it might be useful:

If you have a function that always throws, you can annotate the return as `empty`, as the function never returns:

```js flow-check
function throwIt(msg: string): empty {
  throw new Error(msg);
}
```

You can use a cast to `empty` to assert that you have refined away all members of a union:

```js flow-check
function f(x: 'a' | 'b'): number {
  switch (x) {
    case 'a':
      return 1;
    case 'b':
      return 2;
    default:
      return (x: empty);
  }
}
```

If you had not checked for all members of the union (for example, changed `x` to be of type `'a' | 'b' | 'c'`),
then `x` would no longer be `empty` in the `default`, and Flow would error.

> Note: If you want exhaustively checked enums by default, without having to cast to `empty`,
> you could enable and use [Flow Enums](../../enums) in your project.

Since `empty` is the subtype of all types, all operations are permitted on something that has the `empty` type.
However since no values can be `empty`, this is "safe", unlike with [`any`](../any).

```js flow-check
const str = "hello";

if (typeof str === "string") {
  (str: string); // Yes it's a string
} else {
  // Works! Since we will never enter this branch
  (str: empty);
  const n: number = str + 1;
}
```

We put "safe" in quotes above, as due type safety holes in your code or bugs within Flow itself,
it is possible to get values which are `empty` typed.

You can use the [coverage](../../cli/coverage/) command to identify code typed as `empty`.
