Node.js中文件I/O的执行优先级：理解同步与异步操作

const fs = require('fs');

async function loadData() {
    fs.readFile('cfg.json', 'utf8', (err, data) => {
        if (err) {
            console.error(err);
            return;
        }
        const map = JSON.parse(data);
        console.log("1: " + serverAddr); // 预期在此处看到旧值
        serverAddr = map.serverAddr;
        console.log("2: " + serverAddr); // 预期在此处看到新值
    });
    console.log("3: " + serverAddr);
    console.log("4: " + serverAddr);
}

var serverAddr = "NOT INIT";
console.log("5: " + serverAddr);
loadData();
console.log("6: " + serverAddr);

{
  "serverAddr": "https://google.com/"
}

5: NOT INIT
3: NOT INIT
4: NOT INIT
6: NOT INIT
1: NOT INIT
2: https://google.com/

const fs = require('fs');

function loadDataSync() { // 更名为loadDataSync以示区分
    try {
        const data = fs.readFileSync('cfg.json', 'utf8');
        const map = JSON.parse(data);
        console.log("1: " + serverAddr); // 此时serverAddr仍是旧值
        serverAddr = map.serverAddr;
        console.log("2: " + serverAddr); // 此时serverAddr已更新
    } catch (err) {
        console.error("Error reading config file:", err);
    }
}

var serverAddr = "NOT INIT";
console.log("5: " + serverAddr);
loadDataSync(); // 调用同步加载函数
console.log("3: " + serverAddr); // 此时serverAddr已是新值
console.log("4: " + serverAddr); // 此时serverAddr已是新值
console.log("6: " + serverAddr); // 此时serverAddr已是新值

5: NOT INIT
1: NOT INIT
2: https://google.com/
3: https://google.com/
4: https://google.com/
6: https://google.com/

const fs = require('fs').promises; // 导入fs模块的Promise版本

async function loadDataAsync() {
    try {
        const data = await fs.readFile('cfg.json', 'utf8'); // 使用fs.promises.readFile
        const map = JSON.parse(data);
        console.log("1: " + serverAddr);
        serverAddr = map.serverAddr;
        console.log("2: " + serverAddr);
    } catch (err) {
        console.error("Error reading config file:", err);
    }
}

var serverAddr = "NOT INIT";
console.log("5: " + serverAddr);

// 由于loadDataAsync是异步函数，它会返回一个Promise
// 如果希望在loadDataAsync完成后才执行后续代码，需要await它
async function main() {
    console.log("Before loadDataAsync call: " + serverAddr);
    await loadDataAsync(); // 等待文件读取完成
    console.log("3: " + serverAddr);
    console.log("4: " + serverAddr);
    console.log("After loadDataAsync call: " + serverAddr);
}

main(); // 调用主异步函数
console.log("6: " + serverAddr); // 这行代码会先执行，因为main()也是异步的，它不会阻塞最外层代码

5: NOT INIT
Before loadDataAsync call: NOT INIT
6: NOT INIT
1: NOT INIT
2: https://google.com/
3: https://google.com/
4: https://google.com/
After loadDataAsync call: https://google.com/

const fs = require('fs').promises;

let serverAddr = "NOT INIT"; // 使用let更符合现代JS实践

async function initializeConfig() {
    try {
        console.log("5: " + serverAddr); // 初始值
        const data = await fs.readFile('cfg.json', 'utf8');
        const map = JSON.parse(data);
        console.log("1: " + serverAddr); // 读取前
        serverAddr = map.serverAddr;
        console.log("2: " + serverAddr); // 读取后
        console.log("3: " + serverAddr);
        console.log("4: " + serverAddr);
        return true; // 表示初始化成功
    } catch (err) {
        console.error("Failed to initialize config:", err);
        return false; // 表示初始化失败
    }
}

// 立即执行的异步函数表达式 (IIFE) 来处理启动逻辑
(async () => {
    console.log("Program start.");
    const success = await initializeConfig();
    if (success) {
        console.log("Config loaded successfully. Current serverAddr: " + serverAddr);
        // 可以在这里开始应用程序的其他部分，因为serverAddr已经准备好
    } else {
        console.error("Application cannot start without config.");
        process.exit(1); // 退出程序
    }
    console.log("6: " + serverAddr); // 确保在所有异步操作完成后执行
})();