How to solve "Cannot redefine property: handler" on AWS Lambda

#aws #tutorial #debugging #javascript

Hey, there!
Today I spent a good few hours trying to solve a problem on AWS Lambda and given how deep the problem was, I thought it would be a good idea to share the solution.

✏️ The Context

I am using Jovo Framework to develop a voice app for both Alexa and Google Assistant and one of their guides teach how to deploy the code to an AWS Lambda function using serverless CLI.

🛑 The Problem

After setting everything up and having the code deployed successfully, when trying to run the lambda function the following error was popping up in the cloud logs



{
  "errorType": "TypeError",
  "errorMessage": "Cannot redefine property: handler",
  "trace": [
    "TypeError: Cannot redefine property: handler",
    "    at Function.defineProperty (<anonymous>)",
    "    at defineProperty (/opt/otel-extension/node_modules/shimmer/index.js:14:10)",
    "    at AwsLambdaInstrumentation.wrap [as _wrap] (/opt/otel-extension/node_modules/shimmer/index.js:56:3)",
    "    at InstrumentationNodeModuleFile.patch (/opt/otel-extension/node_modules/@opentelemetry/instrumentation-aws-lambda/src/instrumentation.ts:124:20)",
    "    at AwsLambdaInstrumentation._onRequire (/opt/otel-extension/node_modules/@opentelemetry/instrumentation/src/platform/node/instrumentation.ts:109:23)",
    "    at /opt/otel-extension/node_modules/@opentelemetry/instrumentation/src/platform/node/instrumentation.ts:143:25",
    "    at Module.Hook._require.Module.require (/opt/otel-extension/node_modules/require-in-the-middle/index.js:154:32)",
    "    at Module.Hook._require.Module.require (/opt/otel-extension/node_modules/require-in-the-middle/index.js:80:39)",
    "    at Module.Hook._require.Module.require (/opt/otel-extension/node_modules/require-in-the-middle/index.js:80:39)",
    "    at Module.Hook._require.Module.require (/opt/otel-extension/node_modules/require-in-the-middle/index.js:80:39)"
  ]
}

Note: in the image above the error shows as "Cannot redefine property: myHandler" because I've changed the function name for debugging purposes.

The easiest way to reproduce the error was using the Test feature from AWS Lambda with an Alexa Start Session template.

🤕 The culprit

It took me a while to notice that, according to the stack trace, the error was happening in a dependency of otel-extension: shimmer



at defineProperty (/opt/otel-extension/node_modules/shimmer/index.js:14:10)

Shimmer is a monkeypatching library that a telemetry extension for AWS Lambda was using.

According to this issue, the problem is using esbuild bundle generator - that Jovo uses - with this telemetry extension.

The responsible for actually adding this telemetry extensions as an AWS Lambda layer was serverless cli.

🔧 The solution

As suggested in both opentelemetry-js-contrib and aws-otel-lambda issues, the solution is changing ES6 export to CommonJS module.exports.

Instead of



export const handler = () => {
  // removed for brevity
};

Do this



const handler = () => {
  // removed for brevity
};
module.exports = { handler }

But, since I'm using Jovo with AWS Lambda, I actually had to change the app.prod.ts file (where prod can be other stage name you defined while running jovo new:stage <stage>).



// app.prod.ts

import { app } from './app'

import { handler } from './server.lambda'

app.configure({

  //...

})

module.exports = { handler }

🙏 The Conclusion

I was sure that I was doing something wrong in some configuration step, since this setup involves several services: Jovo, Serveless, AWS Lambda and AWS API Gateway.

It was kinda relieving to know that the problem was actually a compatibility issue between esbuild and open-telemetry.

Nonetheless, I was able to learn a lot about these services as usual with any deep debugging problem we have to deal as developers.

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