Summary  
This chapter explains how to define dynamic route parameters in routing paths and access their values within request handlers.

General domain of usage  
RESTful web APIs


Sometimes you need dynamic routes that change based on input, such as an ID.

Express allows you to define route parameters using `:`.

```javascript id="n8xk3d"
app.get('/users/:id', (req, res) => {
  const userId = req.params.id;

  res.send(`User ID is ${userId}`);
});
```

Here, `:id` is a dynamic value. When a request comes in, Express extracts it and makes it available in `req.params`.

For example:

* `'/users/1'`: id = 1;
* `'/users/42'`: id = 42.

This is commonly used to get specific data, such as a user by ID or a product by ID.




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Route Parameters