If you prefer the "or throw" approach to handling null values, you can write a little helper to get something like it in TypeScript: function assert<T>(v:

If you prefer the "or throw" approach to handling null values, you can write a little helper to get something like it in TypeScript: function assert<T>(v: T | null | undefined, message?: string): asserts v is T { if (v === null || v === undefined) { throw new Error(message); } } function parseFile(file: File | null) { assert(file, 'File must exist'); // TS now infers type of file as File } Check out the TypeScript 3.7 release notes for more on assertion functions: https://www.typescriptlang.org/docs/handbook/release-notes/typescript-3-7.html#assertion-functions https://www.typescriptlang.org/docs/handbook/release-notes/t...