Hi, I'm struggling to make typescript happy for a simple EventBridgeHandler function .. I hope someone can point out where I'm going wrong:

Hmmmm, I can tell you how to fix the type errors, but I don't think you should be using

EventBridgeHandler

. Are you following a tutorial or something for this?

I’m not that familiar with EventBridge but looking at it the type def looks like this:

export type EventBridgeHandler<TDetailType extends string, TDetail, TResult> = Handler<
    EventBridgeEvent<TDetailType, TDetail>,
    TResult
>;

So you need to pass some type information in the definition so the event itself can be typed. e.g. if they were all strings it’d look like

export const evBridgeHandler: EventBridgeHandler<string, string, string> = (event) => {

}

Super quick code overview of how this looks:

interface DetailType {
    detailInfo: string;
}

interface Response {
    field: string;
}

const p : EventBridgeHandler<"some_detail_type", DetailType, Response> = async(evt) => {
    evt["detail-type"]; // "some_detail_type";
    evt.detail.detailInfo; // exists

    return {
        field: "some return value"
    };
}

@Chad (cysense) Yes, but it doesn't mention EventBridge only APIGateway and SNS. It look really straightforward https://serverlessfirst.com/aws-lambda-type-definitions/ I want to type a Lambda EB event handler

It shouldn’t be a lot more You just need to pass the appropriate types when defining your handler so TypeScript knows what to do with them. Without them, TS wouldn’t have any way to provide you any type checking since an eventbridge event could be anything

Looking at the docs for the API gateway handler, it doesn't have the same sort of generic signature:

/**
 * Works with Lambda Proxy Integration for Rest API or HTTP API integration Payload Format version 1.0
 * @see - <https://docs.aws.amazon.com/apigateway/latest/developerguide/http-api-develop-integrations-lambda.html>
 */
export type APIGatewayProxyHandler = Handler<APIGatewayProxyEvent, APIGatewayProxyResult>;

Which is why you need to tell the

EventBridgeHandler

about the generic arguments:

export type EventBridgeHandler<TDetailType extends string, TDetail, TResult> = Handler<

TL;DR - it's more code because then function signature requires more code.

Hmm .. Odd how the func signature is "more complex" than APIGateway. Typescript forces you do dig deeper doesn't it.

Well, it's just the types. If you look at the

Handler

that

APIGatewayProxyHandler

is has the more complex signature, it's just that the higher level signature is what you end up using. While the types themselves are a Typescript thing, this wouldn't necessarily be a Typescript-specific problem. It's just down to how they're intended to be used and the people that maintain them. You could run in to the same "problem" with any other typesafe language that supports generics. More than anything else it's largely just a design choice.

Interesting, thx for taking the time to explain. I'm TS noob learning ..

No worries, we were all there at some point 🙂