fix(stepfunctions): valid reference path '$' fails with an error (#6483)

'$' is a valid reference path that indicates that the entire object
should be selected.

'$$' is a valid selection of the context object that indicates that the
entire context object should be selected.

fixes #6388

Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>

packages/@aws-cdk/aws-stepfunctions/lib/fields.ts

@@ -129,13 +129,13 @@ export class FieldUtils {
 }
 
 function validateDataPath(path: string) {
-  if (!path.startsWith('$.')) {
-    throw new Error("Data JSON path values must start with '$.'");
+  if (path !== '$' && !path.startsWith('$.')) {
+    throw new Error("Data JSON path values must either be exactly equal to '$' or start with '$.'");
   }
 }
 
 function validateContextPath(path: string) {
-  if (!path.startsWith('$$.')) {
-    throw new Error("Context JSON path values must start with '$$.'");
+  if (path !== '$$' && !path.startsWith('$$.')) {
+    throw new Error("Context JSON path values must either be exactly equal to '$$' or start with '$$.'");
   }
 }

packages/@aws-cdk/aws-stepfunctions/test/test.fields.ts

@@ -80,17 +80,29 @@ export = {
   },
 
   'datafield path must be correct'(test: Test) {
+    test.ok(Data.stringAt('$'));
+
+    test.throws(() => {
+      Data.stringAt('$hello');
+    }, /exactly equal to '\$' or start with '\$.'/);
+
     test.throws(() => {
       Data.stringAt('hello');
-    }, /must start with '\$.'/);
+    }, /exactly equal to '\$' or start with '\$.'/);
 
     test.done();
   },
 
   'context path must be correct'(test: Test) {
+    test.ok(Context.stringAt('$$'));
+
+    test.throws(() => {
+      Context.stringAt('$$hello');
+    }, /exactly equal to '\$\$' or start with '\$\$.'/);
+
     test.throws(() => {
       Context.stringAt('hello');
-    }, /must start with '\$\$.'/);
+    }, /exactly equal to '\$\$' or start with '\$\$.'/);
 
     test.done();
   },