Express.js Mastery / Introduction To Node.js And NPM

Reading and Writing Files Asynchronously

🎯 Goal of This Note

Learn how to read and write files using the asynchronous (non-blocking) versions of fs functions, and understand how to chain multiple dependent async steps using callbacks.

📥 Step 1: Reading a File Asynchronously

  • Add this Code block

fs.readFile('./txt/start.txt','utf8',(err, data)=>{
   console.log(data);
});

  • In Terminal Type: node index.js

  • Output

The avocado 🥑 is popular in vegetarian cuisine as a substitute for meats in sandwiches and salads because of its high fat content 😄
File written!
read-this

  • We still use the fs module, but this time the function is readFile (not readFileSync).

  • Arguments passed to readFile:

    1. 📁 Path to the file — e.g. ./txt/start.txt

    2. 🔤 Character encoding — e.g. 'utf8' (recommended to include, to be safe)

    3. 🔁 Callback function — runs once the file has finished reading

🔄 The Callback Function Pattern

(err, data)=>{...}

  • The callback receives two arguments:

    1. ❌ err — the error, if one occurred

    2. 📄 data — the actual file content

  • This is called the "error-first callback" style — extremely common in Node.js.

  • The error is always the first parameter, and the actual result (data) comes second.

🚀 Step 2: Proving It's Non-Blocking

To demonstrate that readFile doesn't block execution, add logs before and after:

fs.readFile('./txt/start.txt','utf8',(err, data)=>{
  console.log('Read this!');
});

console.log('Will read file!');

What do you think will be printed first❓

  • Type node index.js in terminal

✅ Output

Will read file!
Read this!

🔗 Step 3: Chaining Multiple Async Steps (Dependent Reads)

Sometimes one file read depends on the result of a previous one. Example scenario:

  1. 📄 Read start.txt → its content tells you the name of the next file to read.

  2. 📖 Read that file → its content needs to be combined with another file's content.

  3. ✍️ Write the final combined result into a new output file.

💻 Code

fs.readFile('./txt/start.txt', 'utf8', (err, data1) => {
  fs.readFile(`./txt/${data1}.txt`, 'utf8', (err, data2) => {
    console.log(data2);

    fs.readFile('./txt/append.txt', 'utf8', (err, data3) => {
      console.log(data3);

      fs.writeFile('./txt/final.txt', `${data2}\n${data3}`, 'utf8', (err) => {
        console.log('Your file has been written 🎉');
      });
    });
  });
});

console.log('Will read file!');

✅ Terminal Output

✅ final.txt Output

  1. 📄 First read: start.txt is read → result stored as data1

    data1 contains the name of another file (e.g., read-this)

  2. 📖 Second read: Using data1, Node reads ./txt/read-this.txt
    (built dynamically using a template string) → result stored as data2.

  3. 📑 Third read: Reads append.txt → result stored as data3.

  4. ✍️ Write step: Combines data2 and data3 (with a newline \n between them) and writes it to final.txt.

📌 Important Notes

  • Each step is nested inside the callback of the previous step — this is necessary because each step depends on data from the one before it.

  • This nested pattern is called "callback hell" (mentioned in the previous note) — it works, but becomes hard to read as more steps are added.

📤 Step 4: Writing a File Asynchronously

💻 Code

fs.writeFile('./txt/final.txt', textToWrite,'utf8',(err)=>{
  console.log('Your file has been written 🎉');
});

  • Just like readFile, we use the asynchronous version: writeFile (not writeFileSync).

  • Arguments:

    1. 📁 Path to the file to write — ./txt/final.txt

    2. 📝 The content to write into the file

    3. 🔤 Encoding — 'utf8'

    4. 🔁 Callback function — runs once writing is complete

⚖️ Difference from readFile's Callback

  • writeFile's callback only receives one argument: err (the error, if any).

  • There's no data to receive here, since we're writing, not reading.

⚠️ Step 5: Handling Errors

It's good practice to check for errors inside callbacks:

fs.readFile('./txt/start.txt', 'utf8', (err, data1) => {
  if (err) {
    console.log('ERROR 💥');
    return;
  }
  ......

Let’s simulate an error:

  • Rename start.txt to starttttt.txt.

  • Run the application again.

  • Observe the error output in the terminal.

✅ Terminal Output

💡 Why This Matters

  • ❌ If the file doesn't exist (or another error occurs), err will contain error details.

  • 🛑 Using return after logging the error stops the rest of the callback from running — preventing your code from trying to use data1 when it doesn't actually exist.

  • 🧪 Test this by temporarily giving a wrong file name — you'll see the error log instead of a crash, and the rest of the code will be skipped safely.

✅ Full Code

const fs = require("fs");

// Blocking, Synchronous way
// const textIn = fs.readFileSync("./txt/input.txt", "utf8");
// console.log(textIn);

// const textOut = `This is what we know about the avocado:${textIn}. Created on${Date.now()}`;
// fs.writeFileSync("./txt/output.txt", textOut);
// console.log("File written!");

// Non blocking, Asynchronous way
fs.readFile("./txt/start.txt", "utf8", (err, data1) => {
  fs.readFile(`./txt/${data1}.txt`, "utf8", (err, data2) => {
    console.log(data2);

    fs.readFile("./txt/append.txt", "utf8", (err, data3) => {
      console.log(data3);

      fs.writeFile("./txt/final.txt", `${data2}\n${data3}`, "utf8", (err) => {
        console.log("Your file has been written 🎉");
      });
    });
  });
});
console.log("Will read file!");
const fs = require("fs");

// Blocking, Synchronous way
// const textIn = fs.readFileSync("./txt/input.txt", "utf8");
// console.log(textIn);

// const textOut = `This is what we know about the avocado:${textIn}. Created on${Date.now()}`;
// fs.writeFileSync("./txt/output.txt", textOut);
// console.log("File written!");

// Non blocking, Asynchronous way
fs.readFile("./txt/starttttttt.txt", "utf8", (err, data1) => {
  if (err) return console.log("ERROR! ❤️💥");

  fs.readFile(`./txt/${data1}.txt`, "utf8", (err, data2) => {
    console.log(data2);

    fs.readFile("./txt/append.txt", "utf8", (err, data3) => {
      console.log(data3);

      fs.writeFile("./txt/final.txt", `${data2}\n${data3}`, "utf8", (err) => {
        console.log("Your file has been written 🎉");
      });
    });
  });
});
console.log("Will read file!");