feat(bundler): use package.json "module" (es module format) over "main"

Improves compatibility with TypeScript generated code.
Try 1.browser > 2.module > 3.main.
The order is to target browser.
When aurelia-cli introduces multi-targets build in future, it probably should use different order for electron app:
For electron 1.module > 2.browser > 3.main

Author: 3cp

lib/build/package-analyzer.js

@@ -130,9 +130,18 @@ function determineLoaderConfig(project, description) {
   let metaMain;
 
   if (metadata) {
+    // try 1.browser > 2.module > 3.main
+    // the order is to target browser.
+    // when aurelia-cli introduces multi-targets build,
+    // it probably should use different order for electron app
+    // for electron 1.module > 2.browser > 3.main
     if (typeof metadata.browser === 'string')  {
-      // use package.json browser field if possible
+      // use package.json browser field if possible.
       metaMain = metadata.browser;
+    } else if (metadata.module) {
+      // prefer es module format over cjs, just like webpack.
+      // this improves compatibility with TypeScript.
+      metaMain = metadata.module;
     } else {
       metaMain = metadata.main;
     }

spec/lib/build/package-analyzer.spec.js

@@ -328,10 +328,10 @@ describe('The PackageAnalyzer', () => {
       .catch(e => done.fail(e));
   });
 
-  it('analyze() reads package.json as package metadata, respect browser field', done => {
+  it('analyze() reads package.json as package metadata, respect browser field over module/main', done => {
     // setup mock package.json
     const fsConfig = {};
-    fsConfig[path.join('node_modules/my-package', 'package.json')] = '{ "name": "my-package", "main": "index.js", "browser": "browser.js" }';
+    fsConfig[path.join('node_modules/my-package', 'package.json')] = '{ "name": "my-package", "main": "index.js", "browser": "browser.js", "module": "module.js" }';
     fsConfig[path.join('node_modules/my-package', 'index.js')] = 'some-content';
     fsConfig[path.join('node_modules/my-package', 'browser.js')] = 'some-content';
     fsConfig[project.paths.root] = {};
@@ -347,6 +347,26 @@ describe('The PackageAnalyzer', () => {
       .catch(e => done.fail(e));
   });
 
+  it('analyze() reads package.json as package metadata, respect module field over main', done => {
+    // setup mock package.json
+    const fsConfig = {};
+    fsConfig[path.join('node_modules/my-package', 'package.json')] = '{ "name": "my-package", "main": "index.js", "module": "module.js"}';
+    fsConfig[path.join('node_modules/my-package', 'index.js')] = 'some-content';
+    fsConfig[path.join('node_modules/my-package', 'browser.js')] = 'some-content';
+    fsConfig[path.join('node_modules/my-package', 'module.js')] = 'some-content';
+    fsConfig[project.paths.root] = {};
+    mockfs(fsConfig);
+
+    sut.analyze('my-package')
+      .then(description => {
+        expect(description.metadata.name).toBe('my-package');
+        expect(description.loaderConfig.path).toBe('../node_modules/my-package');
+        expect(description.loaderConfig.main).toBe('module');
+        done();
+      })
+      .catch(e => done.fail(e));
+  });
+
   it('analyze() supports parent node_modules folder', done => {
     // setup mock package.json
     const fsConfig = {};