TS allows typeof foo === undefined & typeof foo === null

[tommytroylin]

TypeScript Version: 3.2.2

Search Terms: typeof

Code

typeof window == null
typeof window == undefined

Expected behavior:
compiler error

This condition will always return 'false' since the types '"string" | "number" | "bigint" | "boolean" | "symbol" | "undefined" | "object" | "function"' and 'null' have no overlap.

Actual behavior:
no error

My opinion:

This behavior allows quick null/undefined checks regardless of type information.

but typeof foo === undefined needs to be considered as a typo much more.

[jack-williams]

Duplicate of #29200

[fatcerberus]

but typeof foo === undefined needs to be considered as a typo much more.

The compiler doesn't special-case typeof in an expression for the purpose of type checking. It looks the same to the compiler as, e.g., stringVar === undefined (which is deliberately allowed, as you've pointed out).

[tommytroylin]

thanks @jack-williams for references