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readFileSync() method is an inbuilt application programming interface of fs module which is used to read the file and return its content. In fs. readFile() method, we can read a file in a non-blocking asynchronous way, but in fs. readFileSync() method, we can read files in a synchronous way, i.e. we are telling node.
“An async function can contain an await expression that pauses the execution of the async function and waits for the passed Promise 's resolution, and then resumes the async function's execution and returns the resolved value. Remember, the await keyword is only valid inside async functions.”
Fetch returns a promise which resolves to the request data or an error. Async await is the modern syntax to handle promises. Using async await, the code looks like synchronous code, which makes it easier for people to read and understand the code.
readFileSync() is synchronous and blocks execution until finished. These return their results as return values. readFile() are asynchronous and return immediately while they function in the background. You pass a callback function which gets called when they finish.
Since Node v11.0.0 fs promises are available natively without promisify:
const fs = require('fs').promises;
async function loadMonoCounter() {
const data = await fs.readFile("monolitic.txt", "binary");
return new Buffer(data);
}
To use await/async you need methods that return promises. The core API functions don't do that without wrappers like 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, readFileSync does 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.
This is TypeScript version of @Joel's answer. It is usable after Node 11.0:
import { promises as fs } from 'fs';
async function loadMonoCounter() {
const data = await fs.readFile('monolitic.txt', 'binary');
return Buffer.from(data);
}
You can easily wrap the readFile command with a promise like so:
async function readFile(path) {
return new Promise((resolve, reject) => {
fs.readFile(path, 'utf8', function (err, data) {
if (err) {
reject(err);
}
resolve(data);
});
});
}
then use:
await readFile("path/to/file");
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