TypeScript Types

In TypeScript, undefined is a type with only one value: undefined. By default, it’s a subtype of all other types, meaning you can assign undefined to a variable of any type. However, with the strictNullChecks flag enabled, undefined is only assignable to any, unknown, and its own type.

Example of usage

let u: undefined = undefined;
let notAssigned: string; // "notAssigned" is undefined here

function doSomething(x: string | undefined) {
  if (x === undefined) {
    // Handle the undefined case
  }
}

Notes

• It’s a best practice to enable the strictNullChecks compiler option to avoid common bugs related to null and undefined values.