When building APIs in Express.js, you’ll constantly see URLs like these:

/users/42

and:

/products?category=mobile&sort=price

Both send data through the URL.

But they solve completely different problems.

Understanding the difference between:

• URL parameters

• query strings

is important because it affects:

• route design

• API readability

• filtering logic

• resource identification

Let’s break them down properly.

---

What Are URL Parameters?

URL parameters are dynamic values inside the route path itself.

Example:

/users/42

Here:

• 42 is a URL parameter

• it usually represents a specific resource identifier

---

Why URL Parameters Exist

Applications often need routes that work dynamically.

Instead of creating:

/users/1
/users/2
/users/3

Express allows placeholders.

Example:

app.get("/users/:id", (req, res) => {
  res.send(req.params.id);
});

Now:

• /users/1

• /users/42

• /users/999

all match the same route.

---

Accessing Params in Express

Express stores route parameters inside:

req.params

Example:

app.get("/users/:id", (req, res) => {
  console.log(req.params);
});

Request:

/users/42

Output:

{
  id: "42"
}

---

URL Params Represent Identity

This is the most important mental model.

URL params usually identify:

• a specific user

• a specific product

• a specific post

• a specific resource

Examples:

/users/42
/products/10
/posts/99

These routes point to exact resources.

---

What Are Query Strings?

Query strings are extra key-value pairs added after ? in a URL.

Example:

/products?category=mobile&sort=price

Here:

• category=mobile

• sort=price

are query parameters.

---

Why Query Strings Exist

Sometimes we don’t want a specific resource.

Instead, we want to:

• filter data

• sort results

• paginate

• search

• modify behavior

Query strings solve this.

---

Accessing Query Strings in Express

Express stores query parameters inside:

req.query

Example:

app.get("/products", (req, res) => {
  console.log(req.query);
});

Request:

/products?category=mobile&sort=price

Output:

{
  category: "mobile",
  sort: "price"
}

---

Query Strings Represent Filters or Modifiers

This is the key difference.

Query strings usually describe:

• how data should be filtered

• how results should be sorted

• optional behavior changes

Examples:

/products?category=laptop
/users?page=2
/search?q=react

These are not identifying a single resource.

They are modifying the request behavior.

---

URL Structure Breakdown

Example:

/users/42/orders?status=completed&page=2

Breakdown:

Part	Meaning
/users	resource
42	URL parameter
/orders	nested resource
status=completed	query string
page=2	query string

---

URL Params vs Query Strings

Feature	URL Params	Query Strings
Position	Inside route path	After ?
Purpose	Identify resource	Filter/modify request
Required Usually?	Often yes	Usually optional
Accessed Using	req.params	req.query
Example	/users/42	/users?page=2

---

Real-World Examples

---

Example 1: User Profile

/users/42

Why param?

Because:

• user 42 is a specific identity

• route points to one exact resource

---

Example 2: Product Search

/products?category=shoes&sort=price

Why query?

Because:

• category is a filter

• sorting changes response behavior

• results may vary

---

Example 3: Pagination

/posts?page=3&limit=10

These values modify:

• which results appear

• how many results are returned

Not resource identity.

---

Combining Params and Query Strings

Both can be used together.

Example:

/users/42/posts?published=true

Here:

• 42 identifies the user

• published=true filters posts

This is very common in REST APIs.

---

Important API Design Insight

A good rule:

Use Params For Identity

Examples:

• user ID

• post ID

• product ID

---

Use Query Strings For Optional Behavior

Examples:

• search

• sorting

• filtering

• pagination

---

Common Mistake

Bad API design:

/getUser?id=42

Better:

/users/42

Because:

• the resource identity belongs in the path

• filters/modifiers belong in query strings

Cleaner URLs improve API readability.

---

Simple Mental Model

URL Params

“Which exact resource do you want?”

---

Query Strings

“How should the result be modified?”

---

URL parameters and query strings may look similar, but they serve very different purposes.

• URL params identify resources

• Query strings modify requests

In Express:

• req.params handles route identifiers

• req.query handles filters and modifiers