`never` in TypeScript is an example of a bottom type

never.ts

/**
 * @source https://github.com/mike-works/typescript-fundamentals/blob/4e5782e00ed942ffe8770a694122190b6ed69b95/notes/6-guards-and-extreme-types.ts
 * A bottom type is a type case that won't match - this is `never` in TypeScript
 */
class UnreachableError extends Error {
  constructor(val: never, message: string) {
    super(`TypeScript thought we could never end up here\n${message}`);
  }
}

let y = 4 as string | number;

if (typeof y === "string") {
  // y is a string here
  y.split(", ");
} else if (typeof y === "number") {
  // y is a number here
  y.toFixed(2);
} else {
  // runtime type check in case weak types escape our checks
  throw new UnreachableError(y, "y should be a string or number");
}