Null and Undefined

Null and Undefined

In TypeScript, both undefined and null actually have their own types named undefined and null respectively. Much like void, they’re not extremely useful on their own:

// Not much else we can assign to these variables!
let u: undefined = undefined;
let n: null = null;

By default null and undefined are subtypes of all other types. That means you can assign null and undefined to something like number.

However, when using the --strictNullChecks flag, null and undefined are only assignable to void and their respective types. This helps avoid many common errors. In cases where you want to pass in either a string or null or undefined, you can use the union type string | null | undefined. Once again, more on union types later on.

As a note: we encourage the use of --strictNullChecks when possible, but for the purposes of this handbook, we will assume it is turned off.