默认参数

在 ES6 中，我们允许给函数的参数添加默认值，TypeScript 会将添加了默认值的参数识别为可选参数：

function buildName(firstName: string, lastName: string = "Cat") {
  return firstName + " " + lastName;
}
let tomcat = buildName("Tom", "Cat");
let tom = buildName("Tom");

注意：此时就不受「可选参数必须接在必需参数后面」的限制了！

function buildName(firstName: string = "Tom", lastName: string) {
  return firstName + " " + lastName;
}
let tomcat = buildName("Tom", "Cat");
let cat = buildName(undefined, "Cat");

参考

• 可选和默认参数