Promises, async await in Node context

Now let’s cover Promises and async/await in Node.js, which are essential for handling asynchronous operations cleanly. We’ll go step by step and tie it to the Node environment.

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1️⃣ What Are Promises?

• Promise = an object representing the eventual completion or failure of an asynchronous operation.

• Avoids “callback hell” by chaining .then() and .catch().

States of a Promise

1. Pending → initial state

2. Fulfilled → operation completed successfully

3. Rejected → operation failed

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2️⃣ Basic Promise Example in Node.js

const fs = require("fs").promises; // fs.promises for async file ops
 
fs.readFile("example.txt", "utf8")
  .then((data) => {
    console.log("File contents:", data);
  })
  .catch((err) => {
    console.error("Error reading file:", err);
  });

• fs.promises.readFile returns a Promise

• .then() handles success, .catch() handles errors

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3️⃣ async/await Syntax

• Cleaner syntax for Promises

• Allows writing asynchronous code as if it were synchronous

const readFileAsync = async () => {
  try {
    const data = await fs.readFile("example.txt", "utf8");
    console.log("File contents:", data);
  } catch (err) {
    console.error("Error reading file:", err);
  }
};
 
readFileAsync();

• async → marks function as asynchronous

• await → pauses function execution until the Promise resolves/rejects

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4️⃣ Multiple Async Operations

Sequential Execution

const sequential = async () => {
  const data1 = await fs.readFile("file1.txt", "utf8");
  const data2 = await fs.readFile("file2.txt", "utf8");
  console.log(data1, data2);
};

Parallel Execution

const parallel = async () => {
  const [data1, data2] = await Promise.all([
    fs.readFile("file1.txt", "utf8"),
    fs.readFile("file2.txt", "utf8")
  ]);
  console.log(data1, data2);
};

• Use Promise.all for parallel async operations → faster than sequential

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5️⃣ Error Handling

const fetchData = async () => {
  try {
    const data = await someAsyncFunction();
    console.log(data);
  } catch (err) {
    console.error("Something went wrong:", err);
  }
};

• Always wrap await calls in try/catch or .catch() to prevent unhandled rejections

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6️⃣ Node-Specific Examples

1. Database Queries (Mongoose / Sequelize)

const getUsers = async () => {
  const users = await User.find(); // Mongoose
  // or await User.findAll(); // Sequelize
  return users;
};

2. HTTP Requests (Axios)

const axios = require("axios");
 
const fetchData = async () => {
  try {
    const response = await axios.get("https://jsonplaceholder.typicode.com/posts");
    console.log(response.data);
  } catch (err) {
    console.error(err);
  }
};

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7️⃣ Key Takeaways

• Promises → manage async operations, chain .then()/.catch()

• async/await → cleaner syntax, easier to read, reduces callback nesting

• Use Promise.all for parallel operations

• Always handle errors with try/catch or .catch()

• Works seamlessly with Node.js I/O, DB calls, APIs

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