What are TypeScript’s any, void, never, undefined and null types

Unlike intuitive types such as string or number, the types any, void, never, undefined and null might cause a confusion for a newbie TypeScript developer. In this post, I will share what I have learned about these types.


The simplest one is any. From the spec:

The Any type is used to represent any JavaScript value. The Any type is a supertype of all types and is assignable to and from all types. In general, in places where a type is not explicitly provided and TypeScript cannot infer one, the Any type is assumed.

So, the any is the way how so-called optional typing is implemented in TypeScript. With any you effectively switch off the type checking and working in “JavaScript mode”. It is very useful when you gradually migrate from JavaScript and have not yet figured out all types. This is one of the core TypeScript’s value propositions.

undefined, null and –strictNullChecks

From the spec:

The Null type corresponds to the similarly named JavaScript primitive type and is the type of the null literal. The Undefined type corresponds to the similarly named JavaScript primitive type and is the type of the undefined literal. The Null type is a subtype of all types, except the Undefined type. The undefined type is a subtype of all types.

So from the spec, these types are the types whose domains consist of only one value and undefined is a specialization or subtype of null. However, despite of having this specialization relationship you still can assign null to the variable of type undefined and vice versa, hence more specifically the domains of both null and undefined are two values null and undefined:

The compiler option --strictNullChecks enables strict null checking mode, which removes the null and undefined values from the domain of every type and is only assignable to themselves and any. I believe this is much simpler to understand and predict how TypeScript will behave in different situations. Strict null checks provide us with the tool to increase the code reliability and therefore I recommend to always have it switched on unless you are dealing with a huge legacy system which does not compile because of this compiler switch.


From the spec:

The Void type, referenced by the void keyword, represents the absence of a value and is used as the return type of functions with no return value. The only possible values for the Void type are null and undefined. The Void type is a subtype of the Any type and a supertype of the Null and Undefined types.

This seems to be the pretty simple type with the domain of two values null and undefined. But as we learned earlier, the same domain is defined for null and undefined types. While the need for later types is dictated by Javascript, why we need yet another type with the same domain? Well, first, with --strictNullChecks the domains become not the same and second, we want to logically differentiate between function which might return undefined in some cases and function which does not expect to return any value at all:


The never type represents the type of values that never occur. It is a bit weird, but it perfectly makes sense in cases when function never returns a value or when you write a code in the code branch which will never execute:


Leave a Reply

Your email address will not be published.