断言与非空断言

#非空断言 #断言

断言与非空断言的笔记

断言

// 类型断言
const el = document.getElementById("sam") as HTMLImageElement;

el.src = "url地址";

正常将大类型转成它的子类型

// 把宽泛的类型转成更窄的类型

class PersonAs {}

class Student extends PersonAs {
  studying() {
    console.log("study...");
  }
}

function sayHello(p: PersonAs) {
  (p as Student).studying();
}

const stu = new Student();
sayHello(stu);

非空断言

// msg? -> undefined | string
function printMsgLength(msg?: string) {
  console.log(msg!.length);
}