The same is true for the relationship of T to T | null. So, whereas T and T | undefined are considered synonymous in regular type checking mode (because undefined is considered a subtype of any T), they are different types in strict type checking mode, and only T | undefined permits undefined values. In strict null checking mode, the null and undefined values are not in the domain of every type and are only assignable to themselves and any (the one exception being that undefined is also assignable to void).

StrictNullChecks switches to a new strict null checking mode. The type checker previously considered null and undefined assignable to anything.Įffectively, null and undefined were valid values of every type and it wasn’t possible to specifically exclude them (and therefore not possible to detect erroneous use of them). Previously it was not possible to explicitly name these types, but null and undefined may now be used as type names regardless of type checking mode.

TypeScript has two special types, Null and Undefined, that have the values null and undefined respectively.

Html compiler 2.0