Handle _schema types more carefully

* Use the isSchema predicate instead of instanceof checks
* Create a final `else` case for unexpected results of the _schema variable
* Add tests around AS Base constructor to validate previous and new behavior

packages/apollo-server-core/src/ApolloServer.ts

@@ -16,6 +16,7 @@ import {
   DocumentNode,
   isObjectType,
   isScalarType,
+  isSchema,
 } from 'graphql';
 import { GraphQLExtension } from 'graphql-extensions';
 import {
@@ -357,14 +358,16 @@ export class ApolloServerBase {
     // TODO: This is a bit nasty because the subscription server needs this.schema synchronously, for reasons of backwards compatibility.
     const _schema = this.initSchema();
 
-    if (_schema instanceof GraphQLSchema) {
+    if (isSchema(_schema)) {
       const derivedData = this.generateSchemaDerivedData(_schema);
       this.schema = derivedData.schema;
       this.schemaDerivedData = Promise.resolve(derivedData);
-    } else {
+    } else if (_schema.then) {
       this.schemaDerivedData = _schema.then(schema =>
         this.generateSchemaDerivedData(schema),
       );
+    } else {
+      throw new Error("ApolloServer encountered a programming error. For some reason, we were unable to resolve a valid GraphQLSchema. Please open an issue on the apollo-server repo with a reproduction of this error if possible.");
     }
   }
 

packages/apollo-server-core/src/__tests__/ApolloServerBase.test.ts

@@ -0,0 +1,50 @@
+import { ApolloServerBase } from '../ApolloServer';
+import { buildServiceDefinition } from '@apollographql/apollo-tools';
+import gql from 'graphql-tag';
+
+const typeDefs = gql`
+  type Query {
+    hello: String
+  }
+`;
+
+const resolvers = {
+  Query: {
+    hello() {
+      return 'world';
+    },
+  },
+};
+
+describe('ApolloServerBase construction', () => {
+  it('succeeds when a valid configuration options are provided to typeDefs and resolvers', () => {
+    expect(() => new ApolloServerBase({ typeDefs, resolvers })).not.toThrow();
+  });
+
+  it('succeeds when a valid GraphQLSchema is provided to the schema configuration option', () => {
+    expect(
+      () =>
+        new ApolloServerBase({
+          schema: buildServiceDefinition([{ typeDefs, resolvers }]).schema,
+        }),
+    ).not.toThrow();
+  });
+
+  it('throws when a GraphQLSchema is not provided to the schema configuration option', () => {
+    expect(() => {
+      new ApolloServerBase({
+        schema: {},
+      });
+    }).toThrowErrorMatchingInlineSnapshot(
+      `"ApolloServer encountered a programming error. For some reason, we were unable to resolve a valid GraphQLSchema. Please open an issue on the apollo-server repo with a reproduction of this error if possible."`,
+    );
+  });
+
+  it('throws when the no schema configuration option is provided', () => {
+    expect(() => {
+      new ApolloServerBase({});
+    }).toThrowErrorMatchingInlineSnapshot(
+      `"Apollo Server requires either an existing schema, modules or typeDefs"`,
+    );
+  });
+});