Node.js is a very popular JavaScript runtime environment that can be used to build high-performance web applications, command line tools, web crawlers, etc. In Node.js applications, we often need to perform some operations before Node.js is closed, such as releasing resources, saving state, etc. However, before Node.js is shut down, we need to ensure that all asynchronous operations have been completed, otherwise data loss or inconsistency may result. So, how do you wait for execution before Node.js closes? This article will introduce you to several commonly used methods.

Method 1: Use process.exitCode

The process object process of Node.js provides an exitCode attribute, which is used to specify the exit code of the application and can also be executed before the application exits. some operations. For example, we can encapsulate the asynchronous operation in a Promise object, and then wait for the completion of the Promise object in the process.on('beforeExit') event. If the Promise object is resolved, we can set process.exitCode to 0, otherwise set to 1.

const fs = require('fs'); function readFile(path) { return new Promise((resolve, reject) => { fs.readFile(path, (err, data) => { if (err) { reject(err); } else { resolve(data); } }); }); } process.on('beforeExit', async () => { try { const data = await readFile('data.txt'); console.log(data.toString()); process.exitCode = 0; } catch (err) { console.error(err); process.exitCode = 1; } });

Method 2: Use Promise.all and setTimeout

Another commonly used method is to use Promise.all and setTimeout to wait for the asynchronous operation to complete. Promise.all is used to execute a set of asynchronous operations in parallel. When all asynchronous operations are completed, Promise.all will return a Promise object, and then we can wait for a period of time in setTimeout, and finally exit the application in the setTimeout callback function. . For example:

async function myApp() { const promises = [promise1(), promise2(), promise3()]; await Promise.all(promises); setTimeout(() => { process.exit(); }, 1000); } myApp();

In this example, we first define a set of asynchronous operations promise1, promise2 and promise3, and then use Promise.all to wait for all operations to complete. Then, we wait for 1 second in setTimeout, and finally exit the application through the process.exit() method in the setTimeout callback function.

Method 3: Using async_hooks

Finally, we introduce an advanced method: using async_hooks. This is a Node.js built-in module for tracking the life cycle of asynchronous operations. We can create an asynchronous resource through async_hooks and track its life cycle. Specifically, we can use the async_hooks.createHook() method to create an async hook, and then define before, after, and destroy functions in the hook. These functions are called at different stages of the asynchronous operation. For example:

const async_hooks = require('async_hooks'); const resources = new Map(); const hook = async_hooks.createHook({ init(asyncId, type, triggerAsyncId) { if (type === 'Promise') { resources.set(asyncId, 'Promise'); } }, destroy(asyncId) { resources.delete(asyncId); }, }); hook.enable(); async function myApp() { const promise = new Promise((resolve, reject) => { setTimeout(() => { resolve(); }, 1000); }); await promise; } myApp(); process.on('beforeExit', () => { if (resources.size > 0) { console.warn('There are some unhandled promises!'); } else { console.log('Everything is OK!'); } });

In this example, we first create an async hook and track the Promise resource in the hook's init function. Next, in the myApp function we create a Promise object and wait for it to complete after 1 second. Finally, in the process.on('beforeExit') event, we check whether there are unprocessed Promise resources. If so, output a warning message, otherwise output a normal message.

Summary

This article introduces several ways to wait for asynchronous operations to complete in Node.js. They include using process.exitCode, Promise.all and setTimeout, and async_hooks. In actual development, we need to choose the appropriate method according to the needs of the application and ensure that all asynchronous operations are handled correctly to ensure data integrity and correctness.

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