diff --git a/packages/application/src/mimerenderers.ts b/packages/application/src/mimerenderers.ts
index e2f370c1c134..a30f15dd1f87 100644
--- a/packages/application/src/mimerenderers.ts
+++ b/packages/application/src/mimerenderers.ts
@@ -137,7 +137,7 @@ export function createRendermimePlugin(
           rendermime,
           modelName: option.modelName,
           name: option.name,
-          primaryFileType: registry.getFileType(option.primaryFileType)!, // TODO: Can we assume this is not undefined?
+          primaryFileType: registry.getFileType(option.primaryFileType),
           fileTypes: option.fileTypes,
           defaultFor: option.defaultFor,
           defaultRendered: option.defaultRendered,
diff --git a/packages/application/src/router.ts b/packages/application/src/router.ts
index aa5e4867388b..72aada0998d3 100644
--- a/packages/application/src/router.ts
+++ b/packages/application/src/router.ts
@@ -7,7 +7,12 @@ import { URLExt } from '@jupyterlab/coreutils';
 
 import { CommandRegistry } from '@lumino/commands';
 
-import { PromiseDelegate, Token, ReadonlyJSONObject } from '@lumino/coreutils';
+import {
+  PromiseDelegate,
+  Token,
+  ReadonlyPartialJSONObject,
+  PartialJSONObject
+} from '@lumino/coreutils';
 
 import { DisposableDelegate, IDisposable } from '@lumino/disposable';
 
@@ -159,10 +164,7 @@ export class Router implements IRouter {
 
       try {
         const request = this.current.request;
-        const result = await commands.execute(
-          command,
-          (current as unknown) as ReadonlyJSONObject // TODO: Should typings in Lumino be updated to be partial?
-        );
+        const result = await commands.execute(command, current);
         if (result === stop) {
           queue.length = 0;
           console.log(`Routing ${request} was short-circuited by ${command}`);
diff --git a/packages/application/src/tokens.ts b/packages/application/src/tokens.ts
index 2cf9088a0bc5..c1756adcdfc3 100644
--- a/packages/application/src/tokens.ts
+++ b/packages/application/src/tokens.ts
@@ -5,7 +5,7 @@ import { CommandRegistry } from '@lumino/commands';
 
 import { ServerConnection, ServiceManager } from '@jupyterlab/services';
 
-import { Token } from '@lumino/coreutils';
+import { Token, ReadonlyPartialJSONObject } from '@lumino/coreutils';
 
 import { IDisposable } from '@lumino/disposable';
 
@@ -109,7 +109,7 @@ export namespace IRouter {
   /**
    * The parsed location currently being routed.
    */
-  export interface ILocation {
+  export interface ILocation extends ReadonlyPartialJSONObject {
     /**
      * The location hash.
      */
diff --git a/packages/docregistry/src/mimedocument.ts b/packages/docregistry/src/mimedocument.ts
index 5849ef9dc458..9341ee0cad0e 100644
--- a/packages/docregistry/src/mimedocument.ts
+++ b/packages/docregistry/src/mimedocument.ts
@@ -272,7 +272,7 @@ export class MimeDocumentFactory extends ABCWidgetFactory<MimeDocument> {
    */
   protected createNewWidget(context: DocumentRegistry.Context): MimeDocument {
     const ft = this._fileType;
-    const mimeType = ft.mimeTypes.length ? ft.mimeTypes[0] : 'text/plain';
+    const mimeType = ft?.mimeTypes.length ? ft.mimeTypes[0] : 'text/plain';
 
     const rendermime = this._rendermime.clone({
       resolver: context.urlResolver
@@ -287,8 +287,8 @@ export class MimeDocumentFactory extends ABCWidgetFactory<MimeDocument> {
       dataType: this._dataType
     });
 
-    content.title.iconClass = ft.iconClass ?? '';
-    content.title.iconLabel = ft.iconLabel ?? '';
+    content.title.iconClass = ft?.iconClass ?? '';
+    content.title.iconLabel = ft?.iconLabel ?? '';
 
     const widget = new MimeDocument({ content, context });
 
@@ -298,7 +298,7 @@ export class MimeDocumentFactory extends ABCWidgetFactory<MimeDocument> {
   private _rendermime: IRenderMimeRegistry;
   private _renderTimeout: number;
   private _dataType: 'string' | 'json';
-  private _fileType: DocumentRegistry.IFileType;
+  private _fileType: DocumentRegistry.IFileType | undefined;
 }
 
 /**
@@ -313,7 +313,7 @@ export namespace MimeDocumentFactory {
     /**
      * The primary file type associated with the document.
      */
-    primaryFileType: DocumentRegistry.IFileType;
+    primaryFileType: DocumentRegistry.IFileType | undefined;
 
     /**
      * The rendermime instance.
diff --git a/packages/observables/src/observablejson.ts b/packages/observables/src/observablejson.ts
index 2bb7ebaa963e..80f0eac6e605 100644
--- a/packages/observables/src/observablejson.ts
+++ b/packages/observables/src/observablejson.ts
@@ -5,7 +5,7 @@ import {
   JSONExt,
   JSONObject,
   PartialJSONObject,
-  PartialJSONValue
+  ReadonlyPartialJSONValue
 } from '@lumino/coreutils';
 
 import { Message } from '@lumino/messaging';
@@ -16,7 +16,7 @@ import { IObservableMap, ObservableMap } from './observablemap';
  * An observable JSON value.
  */
 export interface IObservableJSON
-  extends IObservableMap<PartialJSONValue | undefined> {
+  extends IObservableMap<ReadonlyPartialJSONValue | undefined> {
   /**
    * Serialize the model to JSON.
    */
@@ -30,13 +30,15 @@ export namespace IObservableJSON {
   /**
    * A type alias for observable JSON changed args.
    */
-  export type IChangedArgs = IObservableMap.IChangedArgs<PartialJSONValue>;
+  export type IChangedArgs = IObservableMap.IChangedArgs<
+    ReadonlyPartialJSONValue
+  >;
 }
 
 /**
  * A concrete Observable map for JSON data.
  */
-export class ObservableJSON extends ObservableMap<PartialJSONValue> {
+export class ObservableJSON extends ObservableMap<ReadonlyPartialJSONValue> {
   /**
    * Construct a new observable JSON object.
    */
@@ -58,7 +60,7 @@ export class ObservableJSON extends ObservableMap<PartialJSONValue> {
       const value = this.get(key);
 
       if (value !== undefined) {
-        out[key] = JSONExt.deepCopy(value);
+        out[key] = JSONExt.deepCopy(value) as PartialJSONObject;
       }
     }
     return out;
diff --git a/packages/rendermime/src/attachmentmodel.ts b/packages/rendermime/src/attachmentmodel.ts
index 8eceacf48767..0255172ab579 100644
--- a/packages/rendermime/src/attachmentmodel.ts
+++ b/packages/rendermime/src/attachmentmodel.ts
@@ -160,8 +160,7 @@ export class AttachmentModel implements IAttachmentModel {
       let oldValue = observable.get(key);
       let newValue = data[key];
       if (oldValue !== newValue) {
-        // TODO: Should we do a deepcopy? Or make observalbe type as readonly?
-        observable.set(key, newValue as PartialJSONValue | undefined);
+        observable.set(key, newValue);
       }
     }
   }