Extend the Express Request Object with TypeScript

You will learn how TypeScript declaration merging extends Express's Request type, how middleware adds the value at runtime, and how to ensure the property is available before a route uses it.

Express request objects are ordinary JavaScript objects. A middleware can attach a new property at runtime:

req.property = setProperty();

However, TypeScript checks code against type definitions. Express's built-in Request type does not know that property exists, so TypeScript reports an error even though JavaScript would allow the assignment.

The solution is declaration merging. You add a type declaration that tells TypeScript: “Every Express request may include this property.” TypeScript combines your interface declaration with Express's existing Request interface.

This has two separate parts:

1. Type declaration: makes req.property valid to TypeScript.
2. Middleware assignment: actually puts a value on the request while the application runs.

The declaration does not create the property by itself. Middleware does.

Think of an Express request as a delivery box moving through a conveyor belt of middleware.

• Middleware can place a label or item in the box: req.property = value.
• TypeScript is the inventory system that tracks what a box is allowed to contain.
• Declaration merging updates the inventory system so it recognizes the new item.

A later route can safely use the item only if the middleware that adds it ran earlier on the conveyor belt.

Create a declaration file, for example src/types/express.d.ts:

export {};

declare global {
  namespace Express {
    interface Request {
      property?: string;
    }
  }
}

Then add the value in middleware:

import express from "express";

const app = express();

function setProperty(): string {
  return "value created by middleware";
}

app.use((req, res, next) => {
  req.property = setProperty();
  next();
});

app.get("/", (req, res) => {
  res.json({ property: req.property });
});

property?: string means the property is optional. This is usually accurate because not every request is guaranteed to pass through the middleware.

Consider this request flow:

app.use((req, res, next) => {
  req.requestId = "req-123";
  next();
});

app.get("/orders", (req, res) => {
  res.json({ id: req.requestId });
});

Assuming requestId was declared on Express.Request, this is what happens:

1. A client requests GET /orders.
2. Express creates a request object for this request.
3. The first middleware runs and assigns "req-123" to req.requestId.
4. next() passes the same request object to the next matching handler.
5. The /orders route receives that same object, including requestId.
6. The route returns { "id": "req-123" }.

If the route were registered before the middleware, or mounted outside the middleware's router path, req.requestId might not have been assigned yet.

Custom request properties are useful for data that middleware prepares for later handlers:

• Authentication: attach the authenticated user as req.user.
• Request tracing: attach a unique req.requestId for logs and error reports.
• Multi-tenant applications: attach the selected req.tenant after reading the host name or token.
• Locale selection: attach req.locale after parsing headers or cookies.
• Authorization context: attach permissions or roles after validating an access token.

For example, a request ID middleware can make all logs for one HTTP request easy to find:

app.use((req, res, next) => {
  req.requestId = crypto.randomUUID();
  next();
});

In production code, developers usually combine declaration merging with middleware ordering and defensive checks.

Authentication middleware

app.use((req, res, next) => {
  const user = findUserFromToken(req.headers.authorization);

  if (!user) {
    res.status(401).json({ error: "Unauthorized" });
    return;
  }

  req.user = user;
  next();
});

Protected route with a guard clause

Even when a type declares user as optional, a route can validate it before use:

app.get("/profile", (req, res) => {
  if (!req.user) {
    res.status(401).json({ error: "Unauthorized" });
    return;
  }

  res.json({ name: req.user.name });
});

Adding the value without extending the type

This runs in JavaScript but fails TypeScript checking:

app.use((req, res, next) => {
  req.property = "hello"; // Property 'property' does not exist on type 'Request'
  next();
});

Add a .d.ts declaration file rather than using as any.

Using as any to silence the error

(req as any).property = "hello";

This removes useful type checking. A typo such as req.proprety would no longer be caught. Declaration merging preserves dot notation and type safety.

Forgetting that the property may be missing

If the declaration uses property?: string, this is unsafe:

const upper = req.property.toUpperCase();

Use a guard or a fallback:

A local type can look like this:

type RequestWithProperty = . & {
  : ;
};

// src/types/express.d.ts
export {};

declare global {
  namespace Express {
    interface Request {
      requestId?: string;
    }
  }
}

// Add the runtime value before dependent routes
app.use((req, res, next) => {
  req.requestId = crypto.randomUUID();
  next();
});

// Safely read an optional property
if (req.requestId) {
  console.log(req.requestId);
}

Rules:

• A .d.ts file changes TypeScript's knowledge, not the runtime object.
• Middleware must assign the property before another handler reads it.
• Use ?: unless every relevant request is guaranteed to have the value.
• Keep request property names specific, such as user, tenant, or .

How do I add a property to Express Request in TypeScript?

Create a .d.ts file that augments Express.Request, then assign the property in middleware. Declaration merging enables typed dot notation such as req.user.

Why does TypeScript say a property does not exist on Request?

Express's original type definitions do not include your custom field. TypeScript needs a declaration merge before it accepts the property.

Does declaration merging add the property at runtime?

No. It only changes type information. Your middleware must assign the value during request handling.

Should my custom request property be optional?

Usually, yes. Make it required only when your routing structure guarantees that the assigning middleware always runs first.

Where should the Express declaration file go?

A common location is src/types/express.d.ts. Any location is fine as long as TypeScript includes it in the project.

Can I use req.property without bracket notation?

Yes. After augmenting Express.Request, TypeScript recognizes normal dot notation.

When should I use res.locals instead of extending req?

• TypeScript declaration merging — the mechanism used to add fields to existing interfaces.
• TypeScript module augmentation — an alternative way to extend types from installed packages.
• Express middleware — the request-processing pipeline where custom values are assigned.
• Express router middleware order — determines whether a route receives a property set by earlier middleware.
• Optional properties and optional chaining — useful when a request field may not exist.
• res.locals — another place to store values for the lifetime of an HTTP response.
• Authentication middleware — a common reason to attach a typed user to a request.