如何根据Typescript中的作用域解决未定义的数据？

let myVariable: string;

function myFunction() {
    console.log(myVariable); // 这里不会报错，因为myVariable已经声明
}

function myFunction() {
    let localVar: string = "Hello, World!";
    console.log(localVar); // 正确访问局部变量
}

console.log(localVar); // 错误：localVar在此作用域中不可见

async function fetchData() {
    let data: any;
    try {
        const response = await fetch('https://api.example.com/data');
        data = await response.json();
    } catch (error) {
        console.error("Error fetching data:", error);
    }
    console.log(data); // 确保data在此处已定义
}

let myVariable: string | undefined;

function myFunction() {
    const value = myVariable ?? "Default Value";
    console.log(value); // 如果myVariable未定义，将输出"Default Value"
}

function isDefined<T>(value: T | undefined): value is T {
    return value !== undefined;
}

let myVariable: string | undefined;

function myFunction() {
    if (isDefined(myVariable)) {
        console.log(myVariable.toUpperCase()); // 确保myVariable在此处已定义
    } else {
        console.log("myVariable is undefined");
    }
}