getFieldDef: accept FieldNode instead of field name (#3084)

Author: IvanGoncharov

src/execution/execute.js

@@ -589,10 +589,7 @@ function resolveField(
   fieldNodes: $ReadOnlyArray<FieldNode>,
   path: Path,
 ): PromiseOrValue<mixed> {
-  const fieldNode = fieldNodes[0];
-  const fieldName = fieldNode.name.value;
-
-  const fieldDef = getFieldDef(exeContext.schema, parentType, fieldName);
+  const fieldDef = getFieldDef(exeContext.schema, parentType, fieldNodes[0]);
   if (!fieldDef) {
     return;
   }
@@ -1171,8 +1168,10 @@ export const defaultFieldResolver: GraphQLFieldResolver<
 export function getFieldDef(
   schema: GraphQLSchema,
   parentType: GraphQLObjectType,
-  fieldName: string,
+  fieldNode: FieldNode,
 ): ?GraphQLField<mixed, mixed> {
+  const fieldName = fieldNode.name.value;
+
   if (
     fieldName === SchemaMetaFieldDef.name &&
     schema.getQueryType() === parentType

src/subscription/subscribe.js

@@ -199,10 +199,10 @@ async function executeSubscription(
     new Set(),
   );
   const [responseName, fieldNodes] = [...fields.entries()][0];
-  const fieldName = fieldNodes[0].name.value;
-  const fieldDef = getFieldDef(schema, type, fieldName);
+  const fieldDef = getFieldDef(schema, type, fieldNodes[0]);
 
   if (!fieldDef) {
+    const fieldName = fieldNodes[0].name.value;
     throw new GraphQLError(
       `The subscription field "${fieldName}" is not defined.`,
       fieldNodes,