It's a poor design carried over from legacy JavaScript, but there are clear differences at the language level. While not the only one, the biggest one to me is

It's a poor design carried over from legacy JavaScript, but there are clear differences at the language level. While not the only one, the biggest one to me is this kind of code: const x = {}; x.foo // returns undefined const y = { foo: undefined }; y.foo // also undefined Basically a member not being provided will return undefined, whereas `null` had to actually be set. Hence I think the tendency for checking for the absence of something via `undefined`, while expressing the deletion of something with `null`. Sure, you can assert more specific types in TypeScript to step around that, or you can do something less intuitive like checking for `keyof` to detect absence; but this is both straightforward and presents less issues when interfacing with inputs from non-typescript-using users.