use the LTS node when unspecified

This commit updates the nodejs.runtime buildpack to use 20.x.x as the default version specifier if the project does not include one. This pins Node.js to the LTS track by default instead of using the latest available version.

Fixes #416

PiperOrigin-RevId: 636948468
Change-Id: I31c9d7dda344ae899e5af4c79b860ca03296630a

pkg/nodejs/nodejs.go

@@ -43,6 +43,10 @@ const (
 
 	nodeVersionKey    = "node_version"
 	dependencyHashKey = "dependency_hash"
+	// defaultVersionConstraint is used if the project does not provide a Node.js version specifier in
+	// their package.json or via an env var. This pins them to the active LTS version, instead of the
+	// the latest available version.
+	defaultVersionConstraint = "20.*.*"
 )
 
 // semVer11 is the smallest possible semantic version with major version 11.
@@ -233,8 +237,8 @@ func RequestedNodejsVersion(ctx *gcp.Context, pjs *PackageJSON) (string, error)
 		ctx.Logf("Using runtime version from %s: %s", env.RuntimeVersion, version)
 		return version, nil
 	}
-	if pjs == nil {
-		return "", nil
+	if pjs == nil || pjs.Engines.Node == "" {
+		return defaultVersionConstraint, nil
 	}
 	return pjs.Engines.Node, nil
 }

pkg/nodejs/nodejs_test.go

@@ -228,7 +228,12 @@ func TestRequestedNodejsVersion(t *testing.T) {
 	}{
 		{
 			name: "default is empty",
-			want: "",
+			want: defaultVersionConstraint,
+		},
+		{
+			name:        "package.json without engines",
+			packageJSON: `{}`,
+			want:        defaultVersionConstraint,
 		},
 		{
 			name:    "GOOGLE_NODEJS_VERSION is set",