Adding an interface to a type is now a dangerous change.

src/utilities/__tests__/findBreakingChanges-test.js

@@ -34,6 +34,7 @@ import {
   findValuesAddedToEnums,
   findArgChanges,
   findInterfacesRemovedFromObjectTypes,
+  findInterfacesAddedToObjectTypes,
 } from '../findBreakingChanges';
 
 describe('findBreakingChanges', () => {
@@ -1427,6 +1428,60 @@ describe('findDangerousChanges', () => {
     );
   });
 
+  it('should detect interfaces added to types', () => {
+    const interface1 = new GraphQLInterfaceType({
+      name: 'Interface1',
+      fields: {
+        field1: { type: GraphQLString },
+      },
+      resolveType: () => null,
+    });
+    const oldType = new GraphQLObjectType({
+      name: 'Type1',
+      interfaces: [],
+      fields: {
+        field1: {
+          type: GraphQLString,
+        },
+      },
+    });
+
+    const newType = new GraphQLObjectType({
+      name: 'Type1',
+      interfaces: [
+        interface1
+      ],
+      fields: {
+        field1: {
+          type: GraphQLString,
+        },
+      },
+    });
+
+    const oldSchema = new GraphQLSchema({
+      query: queryType,
+      types: [
+        oldType,
+      ]
+    });
+
+    const newSchema = new GraphQLSchema({
+      query: queryType,
+      types: [
+        newType,
+      ]
+    });
+
+    expect(
+      findInterfacesAddedToObjectTypes(oldSchema, newSchema)
+    ).to.eql([
+      {
+        description: 'Interface1 added to interfaces implemented by Type1.',
+        type: DangerousChangeType.INTERFACE_ADDED_TO_OBJECT
+      }
+    ]);
+  });
+
   it('should detect if a type was added to a union type', () => {
     const type1 = new GraphQLObjectType({
       name: 'Type1',
@@ -1551,11 +1606,42 @@ describe('findDangerousChanges', () => {
       },
     });
 
+    const interface1 = new GraphQLInterfaceType({
+      name: 'Interface1',
+      fields: {
+        field1: { type: GraphQLString },
+      },
+      resolveType: () => null,
+    });
+
+    const typeThatGainsInterfaceOld = new GraphQLObjectType({
+      name: 'TypeThatGainsInterface1',
+      interfaces: [],
+      fields: {
+        field1: {
+          type: GraphQLString,
+        },
+      },
+    });
+
+    const typeThaGainsInterfaceNew = new GraphQLObjectType({
+      name: 'TypeThatGainsInterface1',
+      interfaces: [
+        interface1
+      ],
+      fields: {
+        field1: {
+          type: GraphQLString,
+        },
+      },
+    });
+
     const oldSchema = new GraphQLSchema({
       query: queryType,
       types: [
         oldType,
         enumThatGainsAValueOld,
+        typeThatGainsInterfaceOld,
         unionTypeThatGainsATypeOld
       ]
     });
@@ -1565,6 +1651,7 @@ describe('findDangerousChanges', () => {
       types: [
         newType,
         enumThatGainsAValueNew,
+        typeThaGainsInterfaceNew,
         unionTypeThatGainsATypeNew
       ]
     });
@@ -1578,6 +1665,11 @@ describe('findDangerousChanges', () => {
         description: 'VALUE2 was added to enum type EnumType1.',
         type: 'VALUE_ADDED_TO_ENUM',
       },
+      {
+        description: 'Interface1 added to interfaces implemented ' +
+        'by TypeThatGainsInterface1.',
+        type: DangerousChangeType.INTERFACE_ADDED_TO_OBJECT
+      },
       {
         type: DangerousChangeType.TYPE_ADDED_TO_UNION,
         description: 'TypeInUnion2 was added to union type ' +

src/utilities/findBreakingChanges.js

@@ -44,6 +44,7 @@ export const BreakingChangeType = {
 export const DangerousChangeType = {
   ARG_DEFAULT_VALUE_CHANGE: 'ARG_DEFAULT_VALUE_CHANGE',
   VALUE_ADDED_TO_ENUM: 'VALUE_ADDED_TO_ENUM',
+  INTERFACE_ADDED_TO_OBJECT: 'INTERFACE_ADDED_TO_OBJECT',
   TYPE_ADDED_TO_UNION: 'TYPE_ADDED_TO_UNION',
 };
 
@@ -87,7 +88,8 @@ export function findDangerousChanges(
   return [
     ...findArgChanges(oldSchema, newSchema).dangerousChanges,
     ...findValuesAddedToEnums(oldSchema, newSchema),
-    ...findTypesAddedToUnions(oldSchema, newSchema)
+    ...findInterfacesAddedToObjectTypes(oldSchema, newSchema),
+    ...findTypesAddedToUnions(oldSchema, newSchema),
   ];
 }
 
@@ -647,3 +649,36 @@ export function findInterfacesRemovedFromObjectTypes(
   });
   return breakingChanges;
 }
+
+export function findInterfacesAddedToObjectTypes(
+  oldSchema: GraphQLSchema,
+  newSchema: GraphQLSchema
+): Array<DangerousChange> {
+  const oldTypeMap = oldSchema.getTypeMap();
+  const newTypeMap = newSchema.getTypeMap();
+  const interfacesAddedToObjectTypes = [];
+
+  Object.keys(newTypeMap).forEach(typeName => {
+    const oldType = oldTypeMap[typeName];
+    const newType = newTypeMap[typeName];
+    if (
+      !(oldType instanceof GraphQLObjectType) ||
+      !(newType instanceof GraphQLObjectType)
+    ) {
+      return;
+    }
+
+    const oldInterfaces = oldType.getInterfaces();
+    const newInterfaces = newType.getInterfaces();
+    newInterfaces.forEach(newInterface => {
+      if (!oldInterfaces.some(int => int.name === newInterface.name)) {
+        interfacesAddedToObjectTypes.push({
+          type: DangerousChangeType.INTERFACE_ADDED_TO_OBJECT,
+          description: `${newInterface.name} added to interfaces implemented ` +
+          `by ${typeName}.`
+        });
+      }
+    });
+  });
+  return interfacesAddedToObjectTypes;
+}