Add addField method for object type

src/type/__tests__/definition-test.js

@@ -410,6 +410,40 @@ describe('Type System: Objects', () => {
       'AnotherObject must provide "isTypeOf" as a function, but got: {}.',
     );
   });
+
+  describe('Add field correctly', () => {
+    it('normal fields', () => {
+      const objType = new GraphQLObjectType({
+        name: 'SomeObject',
+        fields: {},
+      });
+      // $DisableFlowOnNegativeTest
+      objType.addField('add', { name: 'add', type: ScalarType });
+
+      expect(objType.getFields()).to.deep.equal({
+        add: {
+          name: 'add',
+          type: ScalarType,
+        },
+      });
+    });
+
+    it('thunk fields', () => {
+      const objType = new GraphQLObjectType({
+        name: 'SomeObject',
+        fields: () => ({}),
+      });
+      // $DisableFlowOnNegativeTest
+      objType.addField('add', { name: 'add', type: ScalarType });
+
+      expect(objType.getFields()).to.deep.equal({
+        add: {
+          name: 'add',
+          type: ScalarType,
+        },
+      });
+    });
+  });
 });
 
 describe('Type System: Interfaces', () => {
@@ -699,6 +733,38 @@ describe('Type System: Input Objects', () => {
       );
     });
   });
+
+  describe('Add field correctly', () => {
+    it('normal fields', () => {
+      const objType = new GraphQLInputObjectType({
+        name: 'SomeObject',
+        fields: {},
+      });
+      objType.addField('add', { name: 'add', type: ScalarType });
+
+      expect(objType.getFields()).to.deep.equal({
+        add: {
+          name: 'add',
+          type: ScalarType,
+        },
+      });
+    });
+
+    it('thunk fields', () => {
+      const objType = new GraphQLInputObjectType({
+        name: 'SomeObject',
+        fields: () => ({}),
+      });
+      objType.addField('add', { name: 'add', type: ScalarType });
+
+      expect(objType.getFields()).to.deep.equal({
+        add: {
+          name: 'add',
+          type: ScalarType,
+        },
+      });
+    });
+  });
 });
 
 describe('Type System: List', () => {

src/type/definition.js

@@ -696,6 +696,13 @@ export class GraphQLObjectType {
     return this._fields;
   }
 
+  addField(name: string, field: GraphQLField<*, *>) {
+    if (typeof this._fields === 'function') {
+      this._fields = this._fields();
+    }
+    this._fields[name] = field;
+  }
+
   getInterfaces(): Array<GraphQLInterfaceType> {
     if (typeof this._interfaces === 'function') {
       this._interfaces = this._interfaces();
@@ -1338,6 +1345,13 @@ export class GraphQLInputObjectType {
     return this._fields;
   }
 
+  addField(name: string, field: GraphQLInputField) {
+    if (typeof this._fields === 'function') {
+      this._fields = this._fields();
+    }
+    this._fields[name] = field;
+  }
+
   toConfig(): {|
     ...GraphQLInputObjectTypeConfig,
     fields: GraphQLInputFieldConfigMap,