typescript 类型别名与类型断言

//type aliases

type PlusType = (x: number, y: number) => number;

function sum(x: number, y: number): number {
  return x + y;
}

const sum2: PlusType = sum;

type NameResolver = () => string;
type NameOrResolver = string | NameResolver;

function getName(n: NameOrResolver): string {
  if (typeof n === "string") {
    return n;
  } else {
    return n();
  }
}

//type assertion

function getLength(input: string | number): number {
  // const str = input as String;
  // if (str.length) {
  //   return str.length;
  // } else {
  //   const number = input as Number;
  //   return number.toString().length;
  // }

  if ((<string>input).length) {
    return (<string>input).length;
  } else {
    return input.toString().length;
  }
}