diff --git a/src/user.js b/src/user.js
index 50b7eadf..23f6ff30 100644
--- a/src/user.js
+++ b/src/user.js
@@ -19,7 +19,7 @@ function MockFirebaseUser(ref, data) {
   this.emailVerified = !!data.emailVerified;
   this.isAnonymous = !!data.isAnonymous;
   this.metadata = data.metadata;
-  this.providerData = data.providerData;
+  this.providerData = data.providerData || [];
   this.providerId = data.providerId;
   this.refreshToken = data.refreshToken;
 }
@@ -123,6 +123,26 @@ MockFirebaseUser.prototype.getIdToken = function (forceRefresh) {
   });
 };
 
+MockFirebaseUser.prototype.toJSON = function() {
+  const json = {
+    uid: this.uid,
+    email: this.email,
+    emailVerified: this.emailVerified,
+    displayName: this.displayName,
+    photoURL: this.photoURL,
+    phoneNumber: this.phoneNumber,
+  };
+  if (this.metadata) {
+    json.createdAt = this.metadata.createdAt;
+    json.lastLoginAt = this.metadata.lastLoginAt;
+  }
+  json.providerData = [];
+  for (const entry of this.providerData) {
+    json.providerData.push(entry.toJSON());
+  }
+  return json;
+};
+
 MockFirebaseUser.prototype.getIdTokenResult = function (forceRefresh) {
   if (forceRefresh) {
     this._refreshIdToken();
diff --git a/test/unit/user.js b/test/unit/user.js
index 5ea3082f..d9e70514 100644
--- a/test/unit/user.js
+++ b/test/unit/user.js
@@ -463,6 +463,48 @@ describe('User', function() {
       return expect(user._refreshIdToken()).not.to.be.rejected;
     });
   });
+
+  describe('#toJSON', () => {
+    describe('most fields', () => {
+      it('should be the same', () => {
+        const user = new User(auth, {});
+        const json = user.toJSON();
+        [
+          'uid',
+          'email',
+          'emailVerified',
+          'displayName',
+          'photoURL',
+          'phoneNumber',
+          'providerData',
+        ].forEach(k => expect(json[k]).to.deep.equal(user[k]));
+      });
+    });
+
+    describe('.metadata', () => {
+      it('keys should be missing if omitted', () => {
+        const user = new User(auth, {});
+        expect(user.toJSON()).not.to.haveOwnProperty('lastLoginAt');
+        expect(user.toJSON()).not.to.haveOwnProperty('createdAt');
+      });
+
+      it('should populate to lastLogin if present', () => {
+        const metadata = {
+          lastLoginAt: new Date(11).getTime().toString(10),
+        };
+        const user = new User(auth, { metadata: metadata, });
+        expect(user.toJSON().lastLoginAt).to.equal(metadata.lastLoginAt);
+      });
+
+      it('should populate to createdAt if present', () => {
+        const metadata = {
+          createdAt: new Date(12).getTime().toString(10),
+        };
+        const user = new User(auth, { metadata: metadata, });
+        expect(user.toJSON().createdAt).to.equal(metadata.createdAt);
+      });
+    });
+  });
 });
 
 function randomTimestamp() {