feat: show sample on how to package custom layer

README.md

@@ -27,7 +27,9 @@ in
 [this blog post](https://coderbyheart.com/how-i-package-typescript-lambdas-for-aws/).
 
 Also, have a look at the [test stack](./cdk/cloudformation.ts), which uses this
-library to publish a lambda.
+library to publish a lambda and especially the
+[`cdk/prepareResources.ts`](./cdk/prepareResources.ts) which shows how to
+package a lambda and a layer.
 
 ## Use with Yarn
 

cdk/prepareResources.ts

@@ -8,6 +8,7 @@ import {
 	WebpackMode,
 	LayeredLambdas,
 } from '../src'
+import { spawn } from 'child_process'
 
 export const prepareResources = async ({
 	stackName,
@@ -20,7 +21,7 @@ export const prepareResources = async ({
 	baseLayerZipFileName: string
 	lambdas: LayeredLambdas<TestStackLambdas>
 }> => {
-	// Pack the lambdas
+	// Prepare the output directory
 	const outDir = path.resolve(rootDir, 'dist', 'lambdas')
 	try {
 		await fs.stat(outDir)
@@ -30,11 +31,65 @@ export const prepareResources = async ({
 	const sourceCodeBucketName = await getLambdaSourceCodeBucketName({
 		stackName,
 	})
+
+	// Pack the baselayer, only with needed dependencies
+
+	// - This will contain the package.json to be used for the layer
+	const layerFolder = path.resolve(
+		rootDir,
+		'dist',
+		'lambdas',
+		'cloudFormationLayer',
+	)
+	try {
+		await fs.stat(layerFolder)
+	} catch (_) {
+		await fs.mkdir(layerFolder)
+	}
+
+	// - Pick relevant dependencies from the project's package.json
+	//   so it they have the right version
+	const { dependencies } = JSON.parse(
+		await fs.readFile(path.resolve(rootDir, 'package.json'), 'utf-8'),
+	)
+	const cdkLambdaDeps = {
+		'@aws-sdk/client-s3': dependencies['@aws-sdk/client-s3'],
+	}
+	if (Object.values(cdkLambdaDeps).find((v) => v === undefined) !== undefined) {
+		throw new Error(
+			`Could not resolve all dependencies in "${JSON.stringify(
+				cdkLambdaDeps,
+			)}"`!,
+		)
+	}
+	// - add them to the layers package.json
+	await fs.writeFile(
+		path.join(layerFolder, 'package.json'),
+		JSON.stringify({
+			dependencies: cdkLambdaDeps,
+		}),
+		'utf-8',
+	)
+	// - install them
+	await new Promise<void>((resolve, reject) => {
+		const p = spawn('npm', ['i', '--ignore-scripts', '--only=prod'], {
+			cwd: layerFolder,
+		})
+		p.on('close', (code) => {
+			if (code !== 0) {
+				const msg = `[CloudFormation Layer] npm i in ${layerFolder} exited with code ${code}.`
+				return reject(new Error(msg))
+			}
+			return resolve()
+		})
+	})
 	const baseLayerZipFileName = await packBaseLayer({
-		srcDir: rootDir,
+		srcDir: layerFolder,
 		outDir,
 		Bucket: sourceCodeBucketName,
 	})
+
+	// Pack the lambda
 	const lambdas = await packLayeredLambdas<TestStackLambdas>({
 		id: 'test-lambdas',
 		mode: WebpackMode.production,