How can I use the Middy validator to validate pathParameters and queryStringParameters?

Question

I created a brand new project using the TypeScript template:

$ serverless create --template aws-nodejs-typescript --path demo

I edited tsconfig.json and enabled strictNullChecks as I prefer this feature to be turned on:

{
  "extends": "./tsconfig.paths.json",
  "compilerOptions": {
    "lib": ["ESNext"],
    "moduleResolution": "node",
    "noUnusedLocals": true,
    "noUnusedParameters": true,
    "removeComments": true,
    "strictNullChecks": true,
    "sourceMap": true,
    "target": "ES2020",
    "outDir": "lib"
  },
  "include": ["src/**/*.ts", "serverless.ts"],
  "exclude": [
    "node_modules/**/*",
    ".serverless/**/*",
    ".webpack/**/*",
    "_warmup/**/*",
    ".vscode/**/*"
  ],
  "ts-node": {
    "require": ["tsconfig-paths/register"]
  }
}

I edited the handler that is generated by the template a bit to validate that the request has a non-null path parameter named id:

import middy from '@middy/core'
import { APIGatewayProxyEvent, APIGatewayProxyResult } from 'aws-lambda'

async function lambdaHandler(event: APIGatewayProxyEvent): Promise<APIGatewayProxyResult> {
  if (event.pathParameters === null || event.pathParameters.id === null) {
    throw new Error("id path parameter is required but is null")
  }

  return {
    statusCode: 200,
    body: `Hello ${event.pathParameters.id} from ${event.path}`
  }
}
let handler = middy(lambdaHandler)

export default handler

How can I use Middy's validator middleware to perform this validation instead?

Answer 1

@middy/validator is a wrapper around ajv which uses JSON Schema.

Your eventSchema might look like this:

{
  "type":"object",
  "properties":{
    "pathParameters":{
      "type":"object",
      "properties":{
        "id": { "type":"string" }
      },
      "required":["id"]
    }
  },
  "required":["pathParameters"]
}