refactor: execution methods into Executor class

src/README.md

@@ -20,4 +20,3 @@ Each sub directory within is a sub-module of graphql-js:
 - [`graphql/error`](error/README.md): Creating and formatting GraphQL errors.
 - [`graphql/utilities`](utilities/README.md): Common useful computations upon
   the GraphQL language and type objects.
-- [`graphql/subscription`](subscription/README.md): Subscribe to data updates.

src/error/GraphQLAggregateError.ts

@@ -0,0 +1,23 @@
+import type { GraphQLError } from './GraphQLError';
+
+/**
+ * A GraphQLAggregateError contains a collection of GraphQLError objects.
+ * See documentation for the GraphQLError class for further details.
+ *
+ * @internal
+ *
+ * TODO: Consider exposing this class for returning multiple errors from
+ * resolvers.
+ */
+export class GraphQLAggregateError extends Error {
+  /**
+   * An array of GraphQLError objects.
+   *
+   */
+  readonly errors: ReadonlyArray<GraphQLError>;
+
+  constructor(errors: ReadonlyArray<GraphQLError>) {
+    super();
+    this.errors = errors;
+  }
+}

src/subscription/__tests__/subscribe-test.ts → src/execution/__tests__/subscribe-test.ts

@@ -6,13 +6,14 @@ import { resolveOnNextTick } from '../../__testUtils__/resolveOnNextTick';
 import { invariant } from '../../jsutils/invariant';
 import { isAsyncIterable } from '../../jsutils/isAsyncIterable';
 
+import { Kind } from '../../language/kinds';
 import { parse } from '../../language/parser';
 
 import { GraphQLSchema } from '../../type/schema';
 import { GraphQLList, GraphQLObjectType } from '../../type/definition';
 import { GraphQLInt, GraphQLString, GraphQLBoolean } from '../../type/scalars';
 
-import { createSourceEventStream, subscribe } from '../subscribe';
+import { subscribe } from '../subscribe';
 
 import { SimplePubSub } from './simplePubSub';
 
@@ -406,10 +407,6 @@ describe('Subscription Initialization Phase', () => {
       });
       const document = parse('subscription { foo }');
       const result = await subscribe({ schema, document });
-
-      expect(await createSourceEventStream(schema, document)).to.deep.equal(
-        result,
-      );
       return result;
     }
 
@@ -446,6 +443,32 @@ describe('Subscription Initialization Phase', () => {
     ).to.deep.equal(expectedResult);
   });
 
+  it('resolves to an error if no operation is provided', async () => {
+    const schema = new GraphQLSchema({
+      query: DummyQueryType,
+      subscription: new GraphQLObjectType({
+        name: 'Subscription',
+        fields: {
+          foo: { type: GraphQLString },
+        },
+      }),
+    });
+
+    // If we receive variables that cannot be coerced correctly, subscribe() will
+    // resolve to an ExecutionResult that contains an informative error description.
+    const result = await subscribe({
+      schema,
+      document: { kind: Kind.DOCUMENT, definitions: [] },
+    });
+    expect(result).to.deep.equal({
+      errors: [
+        {
+          message: 'Must provide an operation.',
+        },
+      ],
+    });
+  });
+
   it('resolves to an error if variables were wrong type', async () => {
     const schema = new GraphQLSchema({
       query: DummyQueryType,