Solving the AWS Lambda payload-might-be-undefined problem in Typescript with interface overloads

a Typescript Lambda problem.md

I ran in to some trouble when I wanted to use more explicit types than the APIGatewayProxyEventQueryStringParameters or APIGatewayProxyEventPathParameters ones tied to the base event, so I figured I'd give it a shot and jot it down for my own future reference.

Please leave a comment if you've got a cleaner idea 💌

my-lambda-utility-library.ts

export const getEvent = (): AWSLambda.APIGatewayProxyEvent => {
  return req.event
}

interface QueryStringParams {
  (): AWSLambda.APIGatewayProxyEventQueryStringParameters
  <Type>(): Type // An overload that we can use to explicitly type the returned value
}
export const getQueryStringParams: QueryStringParams = () => {
  return getEvent().queryStringParameters ?? {}
}

problem.ts

import { getQueryStringParams } from './my-lambda-utility-library.ts'

// This allows us to do things like:
interface DatabaseListQuery {
  user: string
  potato: string
}
// Because we define which type we expect in the result,
// we wont get the error "query.user is not part of APIGatewayProxyEventQueryStringParameters"
const query = getQueryStringParams<DatabaseListQuery>()
console.log(query.user.toUpperCase())

// If we dont, we get the default errorneous behavior:
const brokenQuery = getQueryStringParams()
console.log(brokenQuery.user.toUpperCase()) // <- Object is possibly 'undefined'.ts(2532)

solvedHandler.ts

import { setEvent, getQueryStringParams } from './my-lambda-utility-library.ts'

// More realistically it might look like this

interface DatabaseListQuery {
  user: string
  potato: string
}

const getDataFromDatabase = (userid: string): string => 'Hey world'

export const myAwesomeHandler = (event: AWSLambda.APIGatewayProxyEvent, context: AWSLambda.Context): AWSLambda.APIGatewayProxyResult => {
  setEvent(event, context)

  const query = getQueryStringParams<DatabaseListQuery>()
    
  // This would normally fail with APIGatewayProxyEventQueryStringParameters because string is required, undefined not allowed
  const data = getDataFromDatabase(query.user)

  return { statusCode: 200, body: JSON.stringify(data) }
}