added test and fixed linter warnings

Author: terion-name

src/types/VariableType.ts

@@ -1,6 +1,6 @@
 class VariableType {
     constructor(public value: string) {}
-    
+
     toJSON() {
         return `$${this.value}`;
     }

src/types/__tests__/VariableType.tests.ts

@@ -43,4 +43,36 @@ describe('VariableType()', () => {
 }`);
     });
 
+    it('properly ads dollar sign on JSON.stringify', () => {
+        const query = {
+            query: {
+                __variables: {
+                    someString: 'String!',
+                    varWithDefault: 'String = "default_value"'
+                },
+                Posts: {
+                    __args: {
+                        arg1: 20,
+                        arg2: new VariableType('someString')
+                    },
+                    id: true,
+                    title: true,
+                    comments: {
+                        __args: {
+                            offensiveOnly: true
+                        },
+                        id: true,
+                        comment: true,
+                        user: true
+                    }
+                }
+            }
+        };
+        expect(JSON.stringify(query)).to.equal(
+            '{"query":{"__variables":{"someString":"String!","varWithDefault":"String = \\"default_value\\""},'
+            + '"Posts":{"__args":{"arg1":20,"arg2":"$someString"},"id":true,"title":true,"comments":'
+            + '{"__args":{"offensiveOnly":true},"id":true,"comment":true,"user":true}}}}'
+        );
+    });
+
 });