6.联合类型和类型断言

let str = "str";
// 联合类型union types
let numberOrString: number | string = "Good Morning!";
numberOrString = 123;
// 类型断言type assertion
function getLength(input: string | number): number {
  const str = input as string;
  if (str.length) {
    return str.length;
  } else {
    const num = input as number;
    return num.toString().length;
  }
}