Support No package.json

Previously the NodeJSMainModule function would return an error if a NodeJS
application did not have a package.json file.  This was incorrect as it is
possible to have a legal NodeJS application without one and the Paketo NodeJS
buildpack supports this style of application.

This change updates the detection logic to return server.js in cases where
there is no package.json or the package.json does not explicitly identify a
main, in line with the behavior of the Paketo NodeJS buildpack.

Signed-off-by: Ben Hale <bhale@vmware.com>

sherpa/nodejs.go

@@ -23,11 +23,15 @@ import (
 	"path/filepath"
 )
 
-// NodeJSMainModule returns the name of the main module as defined in <path>/package.json.
+// NodeJSMainModule returns the name of the main module as defined in <path>/package.json. If no package.json exists,
+// or the package.json does not include a main entry, value defaults to server.js in line with the behavior of the
+// Paketo NodeJS buildpack.
 func NodeJSMainModule(path string) (string, error) {
 	file := filepath.Join(path, "package.json")
 	in, err := os.Open(file)
-	if err != nil {
+	if os.IsNotExist(err) {
+		return "server.js", nil
+	} else if err != nil {
 		return "", fmt.Errorf("unable to open %s\n%w", file, err)
 	}
 	defer in.Close()
@@ -39,7 +43,7 @@ func NodeJSMainModule(path string) (string, error) {
 
 	m, ok := raw["main"].(string)
 	if !ok {
-		return "", fmt.Errorf("no main module defined in %s: %+v", file, raw)
+		return "server.js", nil
 	}
 
 	return m, nil

sherpa/nodejs_test.go

@@ -17,7 +17,6 @@
 package sherpa_test
 
 import (
-	"fmt"
 	"io/ioutil"
 	"os"
 	"path/filepath"
@@ -47,19 +46,20 @@ func testNodeJS(t *testing.T, context spec.G, it spec.S) {
 		Expect(os.RemoveAll(path)).To(Succeed())
 	})
 
-	context("NodeJSMainModule", func() {
-		it("returns main module", func() {
-			Expect(ioutil.WriteFile(filepath.Join(path, "package.json"), []byte(`{ "main": "test-main" }`), 0644)).
-				To(Succeed())
+	it("returns server.js if no package.json exists", func() {
+		Expect(sherpa.NodeJSMainModule(path)).To(Equal("server.js"))
+	})
+
+	it("returns server.js if package.json does not have a main entry", func() {
+		Expect(ioutil.WriteFile(filepath.Join(path, "package.json"), []byte(`{}`), 0644)).To(Succeed())
 
-			Expect(sherpa.NodeJSMainModule(path)).To(Equal("test-main"))
-		})
+		Expect(sherpa.NodeJSMainModule(path)).To(Equal("server.js"))
+	})
 
-		it("returns error if no main module defined", func() {
-			Expect(ioutil.WriteFile(filepath.Join(path, "package.json"), []byte(`{}`), 0644)).To(Succeed())
+	it("returns main module", func() {
+		Expect(ioutil.WriteFile(filepath.Join(path, "package.json"), []byte(`{ "main": "test-main" }`), 0644)).
+			To(Succeed())
 
-			_, err := sherpa.NodeJSMainModule(path)
-			Expect(err).To(MatchError(fmt.Errorf("no main module defined in %s: map[]", filepath.Join(path, "package.json"))))
-		})
+		Expect(sherpa.NodeJSMainModule(path)).To(Equal("test-main"))
 	})
 }