fix(stepfunctions): allow condition on array (#4340)

* fix(stepfunctions): allow condition on array

A Choice state after a Parallel state receives an array as input.

* use regex

Author: jogold

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

@@ -177,8 +177,8 @@ enum CompoundOperator {
 class VariableComparison extends Condition {
     constructor(private readonly variable: string, private readonly comparisonOperator: ComparisonOperator, private readonly value: any) {
         super();
-        if (!variable.startsWith('$.')) {
-            throw new Error(`Variable reference must start with '$.', got '${variable}'`);
+        if (!/^\$[.[]/.test(variable)) {
+            throw new Error(`Variable reference must start with '$.' or '$[', got '${variable}'`);
         }
     }
 

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

@@ -2,13 +2,27 @@ import { Test } from 'nodeunit';
 import stepfunctions = require('../lib');
 
 export = {
-    'Condition variables must start with $.'(test: Test) {
+    'Condition variables must start with $. or $['(test: Test) {
         test.throws(() => {
             stepfunctions.Condition.stringEquals('a', 'b');
         });
 
         test.done();
     },
+    'Condition variables can start with $.'(test: Test) {
+        test.doesNotThrow(() => {
+            stepfunctions.Condition.stringEquals('$.a', 'b');
+        });
+
+        test.done();
+    },
+    'Condition variables can start with $['(test: Test) {
+        test.doesNotThrow(() => {
+            stepfunctions.Condition.stringEquals('$[0]', 'a');
+        });
+
+        test.done();
+    },
     'NotConditon must render properly'(test: Test) {
         assertRendersTo(test,
             stepfunctions.Condition.not(stepfunctions.Condition.stringEquals('$.a', 'b')),