What is the difference between fs.readFileSync, fs.readFile and fs.promises.readFile?

Ordering a parcel: with the sync version you stand at the gate doing nothing until it arrives. With the callback version you go back to work and the guard calls you when it is here. The promise version is the same as the callback one, except you get a token you can await.

const fs = require("fs");
const fsp = require("fs/promises");

// 1. blocks everything until the file is read
const a = fs.readFileSync("notes.txt", "utf8");

// 2. non-blocking, error-first callback
fs.readFile("notes.txt", "utf8", (err, data) => {
  if (err) return console.error(err.code);
  console.log(data.length);
});

// 3. non-blocking, awaitable
async function load() {
  const data = await fsp.readFile("notes.txt", "utf8");
  return data;
}

When each one fits:

• Sync: start-up scripts, CLI tools, reading config once before the server listens.
• Callback or promise: anything inside a request handler or other hot path.

Without the encoding argument ("utf8") you receive a Buffer, not a string. All three read the entire file into memory, so for very large files use a stream.

• Using readFileSync inside an Express route.
• Forgetting the encoding and printing a Buffer.