什么是类型收窄以及编译器如何执行它？

typeof x === "string"          // typeof guard — for primitives
x instanceof Date              // instanceof — for classes
"role" in obj                  // in operator — property existence
Array.isArray(x)               // built-in guards
if (x) { ... }                  // truthiness narrows out null/undefined/0/""
if (x === null) { ... }         // equality narrowing

type Shape =
  | { kind: "circle"; r: number }
  | { kind: "square"; side: number };

function area(s: Shape) {
  switch (s.kind) {            // the discriminant
    case "circle": return Math.PI * s.r ** 2;  // s is the circle variant
    case "square": return s.side ** 2;          // s is the square variant
  }
}

function isUser(x: unknown): x is User { return !!x && typeof (x as any).name === "string"; }