What is the difference between route parameters and query strings in Express?

Think of an IRCTC ticket. The PNR number identifies exactly one booking: that is a route parameter. Filters such as "AC only" or "sort by price" change how a list is shown without identifying a single thing: those are query parameters.

const express = require("express");
const app = express();

// GET /students/42
app.get("/students/:id", (req, res) => {
  res.json({ id: req.params.id });          // "42" (a string)
});

// GET /students?branch=cse&page=2
app.get("/students", (req, res) => {
  const { branch = "all", page = "1" } = req.query;
  res.json({ branch, page: Number(page) });
});

// GET /courses/web/lessons/3
app.get("/courses/:course/lessons/:n", (req, res) => {
  res.json(req.params);   // {"course":"web","n":"3"}
});

Rules of thumb:

• Use params for identity: which student, which order.
• Use query strings for optional filtering, sorting and pagination.
• Both arrive as strings, so convert with Number() and validate.
• Query values can also be arrays or objects (?tag=a&tag=b), so never assume a string.

• Using req.params.id as a number without conversion, so "2" + 1 becomes "21".
• Putting secrets in query strings, which end up in logs.