---
lang: en
title: 'API docs: express.middleware'
keywords: LoopBack 4.0, LoopBack 4, Node.js, TypeScript, OpenAPI
sidebar: lb4_sidebar
editurl: https://github.com/loopbackio/loopback-next/tree/master/packages/express
permalink: /doc/en/lb4/apidocs.express.middleware.html
---

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[Home](./index.md) &gt; [@loopback/express](./express.md) &gt; [Middleware](./express.middleware.md)

## Middleware interface

Interface LoopBack 4 middleware to be executed within sequence of actions. A middleware for LoopBack is basically a generic interceptor that uses `MiddlewareContext`<!-- -->.

<b>Signature:</b>

```typescript
export interface Middleware extends GenericInterceptor<MiddlewareContext> 
```
<b>Extends:</b> [GenericInterceptor](./context.genericinterceptor.md)<!-- -->&lt;[MiddlewareContext](./express.middlewarecontext.md)<!-- -->&gt;

## Remarks

The middleware function is responsible for processing HTTP requests and responses. It typically includes the following logic.

1. Process the request with one of the following outcome - Reject the request by throwing an error if request is invalid, such as validation or authentication failures - Produce a response by itself, such as from the cache - Proceed by calling `await next()` to invoke downstream middleware. When `await next()` returns, it goes to step 2. If an error thrown from `await next()`<!-- -->, step 3 handles the error.

2. Process the response with one the following outcome - Reject the response by throwing an error - Replace the response with its own value - Return the response to upstream middleware

3. Catch the error thrown from `await next()`<!-- -->. If the `catch` block does not exist, the error will be bubbled up to upstream middleware

The signature of a middleware function is described at [Middleware](https://loopback.io/doc/en/lb4/apidocs.express.middleware.html)<!-- -->. It's very much the same as [Koa middleware](https://github.com/koajs/koa/blob/master/docs/guide.md#writing-middleware)<!-- -->.

## Example


```ts
const log: Middleware = async (requestCtx, next) => {
  const {request} = requestCtx;
  console.log('Request: %s %s', request.method, request.originalUrl);
  try {
    // Proceed with next middleware
    await next();
    console.log('Response received for %s %s', request.method, request.originalUrl);
  } catch(err) {
    console.error('Error received for %s %s', request.method, request.originalUrl);
    throw err;
  }
}
```


