feat(lambda-nodejs): Allow setting mainFields for esbuild (#18569)

I’m trying to use the new format=esm bundling with NodejsFunction

I noticed that it bundles the “CJS” version of dependencies not the ESM version (if available). esbuild has some wack defaults - https://esbuild.github.io/api/#main-fields

This causes problems because the CJS versions usually have stuff like require() and they don’t work for tree-shaking.


----

*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

Author: revmischa

Diff for: packages/@aws-cdk/aws-lambda-nodejs/README.md

@@ -192,6 +192,7 @@ new lambda.NodejsFunction(this, 'my-handler', {
     footer: '/* comments */', // requires esbuild >= 0.9.0, defaults to none
     charset: lambda.Charset.UTF8, // do not escape non-ASCII characters, defaults to Charset.ASCII
     format: lambda.OutputFormat.ESM, // ECMAScript module output format, defaults to OutputFormat.CJS (OutputFormat.ESM requires Node.js 14.x)
+    mainFields: ['module', 'main'], // prefer ECMAScript versions of dependencies
   },
 });
 ```

Diff for: packages/@aws-cdk/aws-lambda-nodejs/lib/bundling.ts

@@ -211,6 +211,7 @@ export class Bundling implements cdk.BundlingOptions {
       ...this.props.banner ? [`--banner:js=${JSON.stringify(this.props.banner)}`] : [],
       ...this.props.footer ? [`--footer:js=${JSON.stringify(this.props.footer)}`] : [],
       ...this.props.charset ? [`--charset=${this.props.charset}`] : [],
+      ...this.props.mainFields ? [`--main-fields=${this.props.mainFields.join(',')}`] : [],
     ];
 
     let depsCommand = '';

Diff for: packages/@aws-cdk/aws-lambda-nodejs/lib/types.ts

@@ -270,6 +270,14 @@ export interface BundlingOptions {
    * @default OutputFormat.CJS
    */
   readonly format?: OutputFormat;
+
+  /**
+   * How to determine the entry point for modules.
+   * Try ['module', 'main'] to default to ES module versions.
+   *
+   * @default ['main', 'module']
+   */
+  readonly mainFields?: string[];
 }
 
 /**

Diff for: packages/@aws-cdk/aws-lambda-nodejs/test/bundling.test.ts

@@ -201,6 +201,7 @@ test('esbuild bundling with esbuild options', () => {
     footer: '/* comments */',
     charset: Charset.UTF8,
     forceDockerBundling: true,
+    mainFields: ['module', 'main'],
     define: {
       'process.env.KEY': JSON.stringify('VALUE'),
       'process.env.BOOL': 'true',
@@ -224,7 +225,7 @@ test('esbuild bundling with esbuild options', () => {
           defineInstructions,
           '--log-level=silent --keep-names --tsconfig=/asset-input/lib/custom-tsconfig.ts',
           '--metafile=/asset-output/index.meta.json --banner:js="/* comments */" --footer:js="/* comments */"',
-          '--charset=utf8',
+          '--charset=utf8 --main-fields=module,main',
         ].join(' '),
       ],
     }),