diff --git a/packages/coreutils/src/json.ts b/packages/coreutils/src/json.ts
index 018f3e08b..9116c2909 100644
--- a/packages/coreutils/src/json.ts
+++ b/packages/coreutils/src/json.ts
@@ -25,7 +25,7 @@ type JSONValue = JSONPrimitive | JSONObject | JSONArray;
  * A type definition for a JSON object.
  */
 export
-interface JSONObject { [key: string]: JSONValue; }
+interface JSONObject { [key: string]: JSONValue | undefined; }
 
 
 /**
@@ -39,7 +39,7 @@ interface JSONArray extends Array<JSONValue> { }
  * A type definition for a readonly JSON object.
  */
 export
-interface ReadonlyJSONObject { readonly [key: string]: ReadonlyJSONValue; }
+interface ReadonlyJSONObject { readonly [key: string]: ReadonlyJSONValue | undefined; }
 
 
 /**
@@ -220,21 +220,36 @@ namespace JSONExt {
 
     // Check for the first object's keys in the second object.
     for (let key in first) {
-      if (!(key in second)) {
+      if (first[key] !== undefined && !(key in second)) {
         return false;
       }
     }
 
     // Check for the second object's keys in the first object.
     for (let key in second) {
-      if (!(key in first)) {
+      if (second[key] !== undefined && !(key in first)) {
         return false;
       }
     }
 
     // Compare the values for equality.
     for (let key in first) {
-      if (!deepEqual(first[key], second[key])) {
+      // Get the values.
+      let firstValue = first[key];
+      let secondValue = second[key];
+
+      // If both are undefined, ignore the key.
+      if (firstValue === undefined && secondValue === undefined) {
+        continue;
+      }
+
+      // If only one value is undefined, the objects are not equal.
+      if (firstValue === undefined || secondValue === undefined) {
+        return false;
+      }
+
+      // Compare the values.
+      if (!deepEqual(firstValue, secondValue)) {
         return false;
       }
     }
@@ -260,7 +275,12 @@ namespace JSONExt {
   function deepObjectCopy(value: any): any {
     let result: any = {};
     for (let key in value) {
-      result[key] = deepCopy(value[key]);
+      // Ignore undefined values.
+      let subvalue = value[key];
+      if (subvalue === undefined) {
+        continue;
+      }
+      result[key] = deepCopy(subvalue);
     }
     return result;
   }
diff --git a/tests/test-coreutils/src/json.spec.ts b/tests/test-coreutils/src/json.spec.ts
index 62dc6c29b..71ae9e9b0 100644
--- a/tests/test-coreutils/src/json.spec.ts
+++ b/tests/test-coreutils/src/json.spec.ts
@@ -14,6 +14,11 @@ import {
 } from '@phosphor/coreutils';
 
 
+interface IFoo extends JSONObject {
+  bar?: string;
+}
+
+
 describe('@phosphor/coreutils', () => {
 
   describe('JSONExt', () => {
@@ -74,6 +79,15 @@ describe('@phosphor/coreutils', () => {
         expect(JSONExt.deepEqual({ b: 1 }, { a: 1 })).to.equal(false);
       });
 
+      it('should handle optional keys', () => {
+        let a: IFoo = { };
+        let b: IFoo = { bar: 'a' };
+        let c: IFoo = { bar: undefined };
+        expect(JSONExt.deepEqual(a, b)).to.equal(false);
+        expect(JSONExt.deepEqual(a, c)).to.equal(true);
+        expect(JSONExt.deepEqual(c, a)).to.equal(true);
+      });
+
     });
 
     describe('deepCopy()', () => {
@@ -110,6 +124,18 @@ describe('@phosphor/coreutils', () => {
         expect(v7['c']).to.not.equal(r7['c']);
       });
 
+      it('should handle optional keys', () => {
+        let v1: IFoo = { };
+        let v2: IFoo = { bar: 'a' };
+        let v3: JSONObject = { a: false, b: { bar: undefined }};
+        let r1 = JSONExt.deepCopy(v1);
+        let r2 = JSONExt.deepCopy(v2);
+        let r3 = JSONExt.deepCopy(v3);
+        expect(Object.keys(r1).length).to.equal(0);
+        expect(v2.bar).to.equal(r2.bar);
+        expect(Object.keys(r3.b).length).to.equal(0);
+      });
+
     });
 
   });