Typescript syntax for a lambda that is triggered from different events

Question

Using the serverless framework I have defined a lambda that can be either triggered every hour or via SNS

  ...
  functions: {
    fooAction: {
      handler: 'handler.fooAction',
      events: [
        {
          schedule: 'rate(1 hour)',
        },
        {
          sns: {
            topicName: 'fooTopic',
          },
        },
      ],
    },
   ...
  }
  ...

What is the correct typescript syntax when defining fooAction function?

I have tried

import { SNSHandler, ScheduledHandler} from 'aws-lambda';
...
export const fooAction: ScheduledHandler | SNSHandler = async (evt) => { ... };

but evt resolves to any.

Sandeep · Accepted Answer · 2021-02-28 22:28:45Z

18

There seems to be a type Handler in aws-lambda sdk, which is generic and can be used for situations like this,

import { SNSEvent, EventBridgeEvent, Handler } from 'aws-lambda';

const fooHandler: Handler<SNSEvent | EventBridgeEvent<any, any>> = (event) => {
    if ('Records' in event ) {
        // SNSEvent
        const records = event.Records;
        // so something
    } else {
        const { id, version, region, source } = event;
    }
};

You can also define your own type based on these two different function types.

type CustomEvent = (event: SNSEvent | EventBridgeEvent<"Scheduled Event", any>, context: Context, callback: Callback<void>) => Promise<void> | void

And then, use this new type with your lambda function,

const fooHandler: CustomEvent = (event) => {
    if ('Records' in event ) {
        // SNSEvent
        const records = event.Records;
        // so something
    } else {
        const { id, version, region, source } = event;
    }
};

edited Feb 28, 2021 at 22:28

answered Feb 28, 2021 at 21:58

Sandeep

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2 Comments

Seth Geoghegan Over a year ago

Just stumbled across this comment while looking into the same use-case. Consider creating a lambda function per event source to avoid the if/else logic within your Lambda function.

gamliela Over a year ago

I believe EventBridgeEvent<"Scheduled Event", any> should be EventBridgeEvent<"Scheduled Event", string>. EventBridge seems to JSON.stringify the details.

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