Since Node.JS 11.0.0 (stable), and version 10.0.0 (experimental), you have access to file system methods that are already promisify'd and you can use them with try catch
exception handling rather than checking if the callback's returned value contains an error.
The API is very clean and elegant! Simply use .promises
member of fs
object:
import fs from 'fs';
const fsPromises = fs.promises;
async function listDir() {
try {
return fsPromises.readdir('path/to/dir');
} catch (err) {
console.error('Error occured while reading directory!', err);
}
}
listDir();
Starting with node 8.0.0, you can use this:
const fs = require('fs');
const util = require('util');
const readdir = util.promisify(fs.readdir);
async function myF() {
let names;
try {
names = await readdir('path/to/dir');
} catch (err) {
console.log(err);
}
if (names === undefined) {
console.log('undefined');
} else {
console.log('First Name', names[0]);
}
}
myF();
See https://nodejs.org/dist/latest-v8.x/docs/api/util.html#util_util_promisify_original
From this version, you can use native Node.js function from util library.
const fs = require('fs')
const { promisify } = require('util')
const readFileAsync = promisify(fs.readFile)
const writeFileAsync = promisify(fs.writeFile)
const run = async () => {
const res = await readFileAsync('./data.json')
console.log(res)
}
run()
const fs = require('fs')
const readFile = (path, opts = 'utf8') =>
new Promise((resolve, reject) => {
fs.readFile(path, opts, (err, data) => {
if (err) reject(err)
else resolve(data)
})
})
const writeFile = (path, data, opts = 'utf8') =>
new Promise((resolve, reject) => {
fs.writeFile(path, data, opts, (err) => {
if (err) reject(err)
else resolve()
})
})
module.exports = {
readFile,
writeFile
}
...
// in some file, with imported functions above
// in async block
const run = async () => {
const res = await readFile('./data.json')
console.log(res)
}
run()
Always use try..catch
for await blocks, if you don't want to rethrow exception upper.
fs.Promises
readdir
const { promises: fs } = require("fs");
async function myF() {
let names;
try {
names = await fs.readdir("path/to/dir");
} catch (e) {
console.log("e", e);
}
if (names === undefined) {
console.log("undefined");
} else {
console.log("First Name", names[0]);
}
}
myF();
readFile
const { promises: fs } = require("fs");
async function getContent(filePath, encoding = "utf-8") {
if (!filePath) {
throw new Error("filePath required");
}
return fs.readFile(filePath, { encoding });
}
(async () => {
const content = await getContent("./package.json");
console.log(content);
})();
You might produce the wrong behavior because the File-Api fs.readdir
does not return a promise. It only takes a callback. If you want to go with the async-await syntax you could 'promisify' the function like this:
function readdirAsync(path) {
return new Promise(function (resolve, reject) {
fs.readdir(path, function (error, result) {
if (error) {
reject(error);
} else {
resolve(result);
}
});
});
}
and call it instead:
names = await readdirAsync('path/to/dir');
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