CDK NodejsFunction with Lambda Layer

Question

I want to import node_modules in Lambda Layer in a Lambda function defined in CDK `NodejsFunction`.  
Pass the deployed Lambda Layer's arn to `NodejsFunction` and `execute cdk synth --no-staging` before `sam local invoke`.  
At this time, if the module you are importing in the Lambda function is not installed in the CDK node_modules, you will get a `Could not resolve ` error.  
So, if we install the module in the CDK node_modules (`yarn add `), the sam local invoke will seem to refer to the CDK node_modules instead of the layer.  
How can I make the Lambda function reference the node_modules in the deployed layer instead of the node_modules in the CDK?

lambda-stack.ts
```javascript
new lambda.NodejsFunction(this, 'ExportQuotationPDF', {
  runtime: Runtime.NODEJS_14_X,
  entry: 'lambda/export-quotation-pdf/index.ts',
  layers: [
    LayerVersion.fromLayerVersionArn(
      this,
      'layer:datefns',
      'arn:aws:lambda:ap-northeast-1:1234567890:layer:datefns:1
    )
  ]
})
```

lambda/sample-function/index.ts
```javascript
// When I run cdk synth without running `yarn add data-fns`, it returns the following error
// Could not resolve "date-fns"
// But if I do `yarn add data-fns`,
// this function appears to reference the node_modules in the CDK, not the layers.
import { format } from 'date-fns'

export const handler = async () => {
  try {
    console.log(format(new Date(), "'Today is a' eeee"))
  } catch (error) {
    console.log(error)
  }
}
```

Answer

The concept of lambda layers will come into the picture only in AWS environment.

When you're testing locally, you can refer to the local package.  However, **you need to mention this package as external modules when bundling the lambda** so that your lambda function will not bundle that package when deploying the lambda.

You would do something like this

```
const nodeJsFnProps: NodejsFunctionProps = {
      bundling: {
        externalModules: [
          'aws-sdk', // Use the 'aws-sdk' available in the Lambda runtime
         'date-fns',
        ],
      },
      runtime: Runtime.NODEJS_16_X,
      timeout: Duration.minutes(3),
      memorySize: 256,
    };

const lambdaWithLayer = new NodejsFunction(this, 'lambdaWithLayer', {
      entry: path.join(__dirname, '../src/lambdas', 'lambda.ts'),
      ...nodeJsFnProps,
      functionName: 'lambdaWithLayer',
      handler: 'handler',
      layers: [logicLayer, utilsLayer],
    });
```
I've written a detailed step-by-step guide on lambda layers here - https://www.cloudtechsimplified.com/aws-lambda-layers/

CDK NodejsFunction with Lambda Layer

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