When people refer to the "billion dollar mistake", they mean the language feature that every value could potentially be `null`, i.e. even if a function returns MyType, it could also return `null`.
TypeScript in strict mode (the default) still has `null` and `undefined`, but not the billion dollar mistake: if you want to be able to pass `null` to a function, you have to mark that parameter as being potentially `null`.
TypeScript in strict mode (the default) still has `null` and `undefined`, but not the billion dollar mistake: if you want to be able to pass `null` to a function, you have to mark that parameter as being potentially `null`.