CODEWITHSUNDEEP
typescript
Qwertiy
Qwertiy

Reputation: 21400

Why can I use type assertions with literals such as `false`?

In TypeScript I can successfully compile following code:

const x = true as false;

So I have a constant x with value true and type false. I expected that such direct assertions should be invalid, but surprisingly it is valid. For example, for the similar code

const x = 0 as false;

there is a compilation error

Conversion of type 'number' to type 'false' may be a mistake because neither type sufficiently overlaps with the other. If this was intentional, convert the expression to 'unknown' first.

I expected a similar error to occur with true and false.

Why there is no error and is there a way (like some set of compiler options) to fix it?

Upvotes: 3

Views: 222

Answers (1)

Arthur Bruel
Arthur Bruel

Reputation: 673

It's because typescript expands the types themselves. true and false do overlap, they are both boolean types. 0 and false don't overlap, one is a number, the other a boolean.

Upvotes: 4

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