Union and Literal Types

Part of the TypeScript course

Some values are not one thing. A function that looks up a user either finds one or does not. A form field holds either a number the user typed or nothing yet. A request is either pending, or successful, or failed, and never anything else.

The types you have written so far cannot say any of that. string means every possible string. number means every possible number. There is no way to say "one of these two types" and no way to say "this exact value and no other". Both gaps are filled by the same idea, which is the most useful thing in TypeScript that has no equivalent in most other languages. ...