必填参
    参数一旦声明，就要求传递，且类型需符合，

    1. function greeting(person: string): string {
    2.   return "Hello, " + person;
    3. }
    5. greeting('tom')

    可选参数
    参数名后面加上问号，变成可选参数，

    1. function greeting(person: string, msg?: string): string {
    2.   return "Hello, " + person;
    3. }
    5. greeting('tom')
    6. greeting('tom', 'glad to see you')

    默认值
    跟可选参数相似，但是不传参时，会使用默认的参数，

    1. function greeting(person: string, msg = ""): string {
    2.   return "Hello, " + person;
    3. }
    5. greeting("tom");
    6. greeting("tom", "glad to see you");

    函数重载
    以参数数量或类型区分多个同名函数，

    2. // 重载1
    3. function watch(cb1: () => void): void;
    4. // 重载2
    5. function watch(cb1: () => void, cb2: (v1: any, v2: any) => void): void;
    6. // 实现
    7. function watch(cb1: () => void, cb2?: (v1: any, v2: any) => void) {
    8.   if (cb1 && cb2) {
    9.     console.log("执行watch重载2");
    10.   } else {
    11.     console.log("执行watch重载1");
    12.   }
    13. }
    15. // 执行watch重载1
    16. watch((): void => {
    17.   console.log(123);
    18. });
    19. // 执行watch重载2
    20. watch(
    21.   (): void => {
    22.     console.log(123);
    23.   },
    24.   (): void => {
    25.     console.log(321);
    26.   }
    27. );
    1. <template>
    2.   <div>
    3.     <input type="text" placeholder="输入新特性" @keyup.enter="addFeature" />
    4.   </div>
    5. </template>
    7. <script lang="ts">
    8. import { Component, Vue } from "vue-property-decorator";
    10. @Component
    11. export default class HelloWorld extends Vue {
    12.   addFeature(e: KeyboardEvent) {
    13.     const target = e.target as HTMLInputElement;
    14.     console.log(target.value);
    15.   }
    16. }
    17. </script>