node.js 如何正确使用异步/等待读取文件？
声明：本页面是StackOverFlow热门问题的中英对照翻译，遵循CC BY-SA 4.0协议，如果您需要使用它，必须同样遵循CC BY-SA许可，注明原文地址和作者信息，同时你必须将它归于原作者（不是我）：StackOverFlow
原文地址: http://stackoverflow.com/questions/46867517/
Warning: these are provided under cc-by-sa 4.0 license. You are free to use/share it, But you must attribute it to the original authors (not me):
StackOverFlow
How to read file with async/await properly?
提问by Jeremy Dicaire
I cannot figure out how async/awaitworks. I slightly understands it but I can't make it work.
我无法弄清楚async/是如何await工作的。我有点理解它，但我不能让它工作。
function loadMonoCounter() {
fs.readFileSync("monolitic.txt", "binary", async function(err, data) {
return await new Buffer( data);
});
}
module.exports.read = function() {
console.log(loadMonoCounter());
};
I know I could use readFileSync, but if I do, I know I'll never understand async/awaitand I'll just bury the issue.
我知道我可以使用readFileSync，但如果我这样做了，我知道我永远不会理解async/await并且我只会埋葬这个问题。
Goal: Call loadMonoCounter()and return the content of a file.
目标：调用loadMonoCounter()并返回文件的内容。
That file is incremented every time incrementMonoCounter()is called (every page load). The file contain the dump of a buffer in binary and is stored on a SSD.
每次incrementMonoCounter()调用（每次加载页面）时，该文件都会增加。该文件包含二进制缓冲区的转储，并存储在 SSD 上。
No matter what I do, I get an error or undefinedin the console.
无论我做什么，都会出现错误或undefined在控制台中。
回答by tadman
To use await/asyncyou need methods that return promises. The core API functions don't do that without wrappers like promisify:
要使用await/async您需要返回承诺的方法。如果没有包装器，核心 API 函数就不会这样做promisify：
const fs = require('fs');
const util = require('util');
// Convert fs.readFile into Promise version of same
const readFile = util.promisify(fs.readFile);
function getStuff() {
return readFile('test');
}
// Can't use `await` outside of an async function so you need to chain
// with then()
getStuff().then(data => {
console.log(data);
})
As a note, readFileSyncdoes not take a callback, it returns the data or throws an exception. You're not getting the value you want because that function you supply is ignored and you're not capturing the actual return value.
注意，readFileSync不接受回调，它返回数据或抛出异常。你没有得到你想要的值，因为你提供的函数被忽略了，你没有捕获实际的返回值。
回答by Joel
Since Node v11.0.0 fs promises are available natively without promisify:
由于 Node v11.0.0 fs 承诺在本机可用，而无需promisify：
const fs = require('fs').promises;
async function loadMonoCounter() {
const data = await fs.readFile("monolitic.txt", "binary");
return new Buffer(data);
}
回答by Shlomi Schwartz
You can easily wrap the readFile command with a promise like so:
您可以使用如下承诺轻松包装 readFile 命令：
async function readFile(path) {
return new Promise((resolve, reject) => {
fs.readFile(path, 'utf8', function (err, data) {
if (err) {
reject(err);
}
resolve(data);
});
});
}
then use:
然后使用：
await readFile("path/to/file");
回答by HKTonyLee
This is TypeScript version of @Joel's answer. It is usable after Node 11.0:
这是@Joel 答案的 TypeScript 版本。它在 Node 11.0 之后可用：
import { promises as fs } from 'fs';
async function loadMonoCounter() {
const data = await fs.readFile('monolitic.txt', 'binary');
return Buffer.from(data);
}
回答by arnaudjnn
You can use fs.promisesavailable natively since Node v11.0.0
fs.promises从 Node v11.0.0 开始，您可以使用本机可用
import fs from 'fs';
const readFile = async filePath => {
try {
const data = await fs.promises.readFile(filePath, 'utf8')
return data
}
catch(err) {
console.log(err)
}
}
