Express.js Request Handling

In Express.js, handling requests and responses is the core of your application logic. Every route handler receives a Request (req) and Response (res) object, which provide the methods and properties needed to process data and talk back to the client.

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The Request Object (req)

The req object represents the HTTP request and has properties for the request query string, parameters, body, HTTP headers, and so on.

1. Request Body (req.body)

Contains key-value pairs of data submitted in the request body. By default, it is undefined and is populated when you use body-parsing middleware such as express.json() or express.urlencoded().

app.use(express.json()); // Middleware to parse JSON bodies

app.post("/api/users", (req, res) => {
  const { name, email } = req.body;
  console.log(`Creating user: ${name} (${email})`);
  res.status(201).send("User created");
});

2. Route Parameters (req.params)

Used to capture values from the URL path. These are defined with a colon (:) in the route path.

// URL: /users/42
app.get("/users/:id", (req, res) => {
  const userId = req.params.id;
  res.send(`Fetching data for user ${userId}`);
});

3. Query Strings (req.query)

Contains the result of the parsed URL query string (everything after the ?).

// URL: /search?term=javascript&limit=10
app.get("/search", (req, res) => {
  const { term, limit } = req.query;
  res.send(`Searching for "${term}" with a limit of ${limit}`);
});

4. Headers (req.headers or req.get())

You can access request headers to check things like Content-Type, Authorization, or custom headers.

app.get("/admin", (req, res) => {
  const apiKey = req.get("x-api-key");
  if (!apiKey) {
    return res.status(401).send("API Key missing");
  }
  res.send("Welcome to the admin panel");
});

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The Response Object (res)

The res object represents the HTTP response that an Express app sends when it gets an HTTP request.

1. Sending Content

• res.send(): Sends a basic response (string, object, or buffer).
• res.json(): Sends a JSON response. This is the preferred way for APIs.
• res.end(): Ends the response process without sending any data.

app.get("/api/info", (req, res) => {
  res.json({
    version: "1.0.0",
    status: "online",
  });
});

2. Setting Status Codes

You can chain .status() before sending a response to set the HTTP status code.

app.get("/not-found", (req, res) => {
  res.status(404).send("Page not found");
});

3. Redirecting

Use res.redirect() to send a client to a different URL.

app.get("/old-path", (req, res) => {
  res.redirect(301, "/new-path"); // 301 is Permanent Redirect
});

4. Sending Files

• res.sendFile(): Sends a file at the given path.
• res.download(): Prompts the client to download a file.

import path from "path";

app.get("/report", (req, res) => {
  const filePath = path.resolve("docs/report.pdf");
  res.download(filePath, "monthly-report.pdf");
});

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Common Response Methods Summary

Method	Description
res.download()	Prompt a file to be downloaded.
res.end()	End the response process.
res.json()	Send a JSON response.
res.redirect()	Redirect a request.
res.render()	Render a view template.
res.send()	Send a response of various types.
res.sendFile()	Send a file as an octet stream.
res.status()	Set the response status code.

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[!IMPORTANT] You can only send one response for each client request. If you attempt to call res.send() or res.json() multiple times in a single handler, you will encounter the “Headers already sent” error. Always use return to exit the handler after sending a response.

app.get("/check", (req, res) => {
  if (condition) {
    return res.send("Condition met"); // Correct: returns and stops execution
  }
  res.send("Condition not met");
});

Now you have a solid understanding of how to manage data flow between the client and your Express.js server!