Unions and narrowing

A discriminated union plus a check on the discriminant is the workhorse pattern of typed application code. Narrowing is how the compiler follows your control flow.

What this lesson covers

• Model alternatives as a discriminated union
• Narrow with typeof, in, and discriminant checks
• Make a switch exhaustive with a never assertion

This lesson sits in How the Type System Thinks, part of TypeScript for Application Developers. It assumes what came before it and leads directly into the next lesson in the module.