Always guard against undefined type in fieldAvailableOnType (#247)

Previously, only the first half of the check inside
`fieldAvailableOnType` guarded against `type` being
`undefined`.

To prevent this from accidentally happening again if
we ever have to add another check to this logic,
I moved the check for `!type` up into it's own guard,
with an early return.

Author: vitorbal

CHANGELOG.md

@@ -2,6 +2,8 @@
 
 ### vNEXT
 
+- Fix an issue that caused `graphql/required-fields` to throw on non-existent field references. [PR #231](https://github.com/apollographql/eslint-plugin-graphql/pull/231) by [Vitor Balocco](https://github.com/vitorbal)
+
 ### v3.0.3
 
 - chore: Update dependency `graphql-tools` to `v4.0.4`. [PR #210](https://github.com/apollographql/eslint-plugin-graphql/pull/210)

src/customGraphQLValidationRules.js

@@ -19,8 +19,12 @@ function getFieldWasRequestedOnNode(node, field) {
 }
 
 function fieldAvailableOnType(type, field) {
+  if (!type) {
+    return false;
+  }
+
   return (
-    (type && type._fields && type._fields[field]) ||
+    (type._fields && type._fields[field]) ||
     (type.ofType && fieldAvailableOnType(type.ofType, field))
   );
 }

test/validationRules/required-fields.js

@@ -11,6 +11,7 @@ const requiredFieldsTestCases = {
     "const x = gql`query { greetings { id, hello, foo } }`",
     "const x = gql`query { greetings { id ... on Greetings { hello } } }`",
     "const x = gql`fragment Name on Greetings { id hello }`",
+    "const x = gql`fragment Foo on FooBar { id, hello, foo }`",
     "const x = gql`fragment Id on Node { id ... on NodeA { fieldA } }`",
     "const x = gql`query { nodes { id ... on NodeA { fieldA } } }`",
   ],