Use with TypeScript

Middy can be used with TypeScript with typings built in in every official package.

You may need to install additional types for AWS Lambda events.

npm i -D @types/aws-lambda

Here's an example of how you might be using Middy with TypeScript for a Lambda receiving events from API Gateway and fetching secrets from Secrets Manager:

import middy from '@middy/core'
import secretsManager from '@middy/secrets-manager'
import type { APIGatewayProxyEvent, APIGatewayProxyResult } from 'aws-lambda'

export const handler = middy<APIGatewayProxyEvent, APIGatewayProxyResult>()
  .use(
    secretsManager({
      fetchData: {
        apiToken: 'dev/api_token'
      },
      awsClientOptions: {
        region: 'us-east-1'
      },
      setToContext: true
    })
  )
  .handler(async (req, context) => {
    // The context type gets augmented here by the secretsManager middleware.
    // This is just an example, obviously don't ever log your secret in real life!
    console.log(context.apiToken)
    return {
      statusCode: 200,
      body: JSON.stringify({
        message: `Hello from ${req.path}`,
        req
      })
    }
  })

Note that when using TypeScript, you should use what we call the Middleware-first, Handler-last approach, which means that you should always call the handler method last, after you have attached all the middlewares you need.

This approach makes sure that, as you attach middlewares, the type system understands how the event and the context arguments are augmented by the various middlewares and inside your handler code you can have a nice type-checking and auto-completion experience.

You can also write custom middlewares with TypeScript.

This is an example tsconfig.json file that can be used for typescript projects

{
  "compilerOptions": {
    "incremental": true,
    "target": "es2020",
    "module": "es2020",
    "declaration": true,
    "sourceMap": true,
    "composite": true,
    "strict": true,
    "moduleResolution": "node",
    "esModuleInterop": true,
    "skipLibCheck": true,
    "forceConsistentCasingInFileNames": true,
    "preserveConstEnums": true,
    "resolveJsonModule": true,
    "allowJs": true,
    "rootDir": ".",
    "outDir": "lib"
  }
  "include": ["src/**/*", "tests/**/*"],
  "exclude": ["node_modules"]
}