diff --git a/packages/commands/src/index.ts b/packages/commands/src/index.ts
index b1d8d76a9..3957e2fa5 100644
--- a/packages/commands/src/index.ts
+++ b/packages/commands/src/index.ts
@@ -12,7 +12,7 @@ import {
 } from '@lumino/algorithm';
 
 import {
-  JSONExt, ReadonlyJSONObject
+  JSONExt, ReadonlyPartialJSONObject
 } from '@lumino/coreutils';
 
 import {
@@ -168,7 +168,7 @@ class CommandRegistry {
    * @returns The display label for the command, or an empty string
    *   if the command is not registered.
    */
-  label(id: string, args: ReadonlyJSONObject = JSONExt.emptyObject): string {
+  label(id: string, args: ReadonlyPartialJSONObject = JSONExt.emptyObject): string {
     let cmd = this._commands[id];
     return cmd ? cmd.label.call(undefined, args) : '';
   }
@@ -183,7 +183,7 @@ class CommandRegistry {
    * @returns The mnemonic index for the command, or `-1` if the
    *   command is not registered.
    */
-  mnemonic(id: string, args: ReadonlyJSONObject = JSONExt.emptyObject): number {
+  mnemonic(id: string, args: ReadonlyPartialJSONObject = JSONExt.emptyObject): number {
     let cmd = this._commands[id];
     return cmd ? cmd.mnemonic.call(undefined, args) : -1;
   }
@@ -191,7 +191,7 @@ class CommandRegistry {
   /**
    * @deprecated Use `iconClass()` instead.
    */
-  icon(id: string, args: ReadonlyJSONObject = JSONExt.emptyObject): string {
+  icon(id: string, args: ReadonlyPartialJSONObject = JSONExt.emptyObject): string {
     return this.iconClass(id, args);
   }
 
@@ -205,7 +205,7 @@ class CommandRegistry {
    * @returns The icon class for the command, or an empty string if
    *   the command is not registered.
    */
-  iconClass(id: string, args: ReadonlyJSONObject = JSONExt.emptyObject): string {
+  iconClass(id: string, args: ReadonlyPartialJSONObject = JSONExt.emptyObject): string {
     let cmd = this._commands[id];
     return cmd ? cmd.iconClass.call(undefined, args) : '';
   }
@@ -220,7 +220,7 @@ class CommandRegistry {
    * @returns The icon label for the command, or an empty string if
    *   the command is not registered.
    */
-  iconLabel(id: string, args: ReadonlyJSONObject = JSONExt.emptyObject): string {
+  iconLabel(id: string, args: ReadonlyPartialJSONObject = JSONExt.emptyObject): string {
     let cmd = this._commands[id];
     return cmd ? cmd.iconLabel.call(undefined, args) : '';
   }
@@ -235,7 +235,7 @@ class CommandRegistry {
    * @returns The caption for the command, or an empty string if the
    *   command is not registered.
    */
-  caption(id: string, args: ReadonlyJSONObject = JSONExt.emptyObject): string {
+  caption(id: string, args: ReadonlyPartialJSONObject = JSONExt.emptyObject): string {
     let cmd = this._commands[id];
     return cmd ? cmd.caption.call(undefined, args) : '';
   }
@@ -250,7 +250,7 @@ class CommandRegistry {
    * @returns The usage text for the command, or an empty string if
    *   the command is not registered.
    */
-  usage(id: string, args: ReadonlyJSONObject = JSONExt.emptyObject): string {
+  usage(id: string, args: ReadonlyPartialJSONObject = JSONExt.emptyObject): string {
     let cmd = this._commands[id];
     return cmd ? cmd.usage.call(undefined, args) : '';
   }
@@ -265,7 +265,7 @@ class CommandRegistry {
    * @returns The class name for the command, or an empty string if
    *   the command is not registered.
    */
-  className(id: string, args: ReadonlyJSONObject = JSONExt.emptyObject): string {
+  className(id: string, args: ReadonlyPartialJSONObject = JSONExt.emptyObject): string {
     let cmd = this._commands[id];
     return cmd ? cmd.className.call(undefined, args) : '';
   }
@@ -280,7 +280,7 @@ class CommandRegistry {
    * @returns The dataset for the command, or an empty dataset if
    *   the command is not registered.
    */
-  dataset(id: string, args: ReadonlyJSONObject = JSONExt.emptyObject): CommandRegistry.Dataset {
+  dataset(id: string, args: ReadonlyPartialJSONObject = JSONExt.emptyObject): CommandRegistry.Dataset {
     let cmd = this._commands[id];
     return cmd ? cmd.dataset.call(undefined, args) : {};
   }
@@ -295,7 +295,7 @@ class CommandRegistry {
    * @returns A boolean indicating whether the command is enabled,
    *   or `false` if the command is not registered.
    */
-  isEnabled(id: string, args: ReadonlyJSONObject = JSONExt.emptyObject): boolean {
+  isEnabled(id: string, args: ReadonlyPartialJSONObject = JSONExt.emptyObject): boolean {
     let cmd = this._commands[id];
     return cmd ? cmd.isEnabled.call(undefined, args) : false;
   }
@@ -310,7 +310,7 @@ class CommandRegistry {
    * @returns A boolean indicating whether the command is toggled,
    *   or `false` if the command is not registered.
    */
-  isToggled(id: string, args: ReadonlyJSONObject = JSONExt.emptyObject): boolean {
+  isToggled(id: string, args: ReadonlyPartialJSONObject = JSONExt.emptyObject): boolean {
     let cmd = this._commands[id];
     return cmd ? cmd.isToggled.call(undefined, args) : false;
   }
@@ -325,7 +325,7 @@ class CommandRegistry {
    * @returns A boolean indicating whether the command is visible,
    *   or `false` if the command is not registered.
    */
-  isVisible(id: string, args: ReadonlyJSONObject = JSONExt.emptyObject): boolean {
+  isVisible(id: string, args: ReadonlyPartialJSONObject = JSONExt.emptyObject): boolean {
     let cmd = this._commands[id];
     return cmd ? cmd.isVisible.call(undefined, args) : false;
   }
@@ -343,7 +343,7 @@ class CommandRegistry {
    * The promise will reject if the command throws an exception,
    * or if the command is not registered.
    */
-  execute(id: string, args: ReadonlyJSONObject = JSONExt.emptyObject): Promise<any> {
+  execute(id: string, args: ReadonlyPartialJSONObject = JSONExt.emptyObject): Promise<any> {
     // Reject if the command is not registered.
     let cmd = this._commands[id];
     if (!cmd) {
@@ -583,7 +583,7 @@ namespace CommandRegistry {
    * A type alias for a user-defined command function.
    */
   export
-  type CommandFunc<T> = (args: ReadonlyJSONObject) => T;
+  type CommandFunc<T> = (args: ReadonlyPartialJSONObject) => T;
 
   /**
    * A type alias for a simple immutable string dataset.
@@ -808,7 +808,7 @@ namespace CommandRegistry {
     /**
      * The arguments object passed to the command.
      */
-    readonly args: ReadonlyJSONObject;
+    readonly args: ReadonlyPartialJSONObject;
 
     /**
      * The promise which resolves with the result of the command.
@@ -866,7 +866,7 @@ namespace CommandRegistry {
      *
      * The default value is an empty object.
      */
-    args?: ReadonlyJSONObject;
+    args?: ReadonlyPartialJSONObject;
 
     /**
      * The key sequence to use when running on Windows.
@@ -916,7 +916,7 @@ namespace CommandRegistry {
     /**
      * The arguments for the command.
      */
-    readonly args: ReadonlyJSONObject;
+    readonly args: ReadonlyPartialJSONObject;
   }
 
   /**