diff -u b/core/lib/Drupal/Core/Ajax/AddJsCommand.php b/core/lib/Drupal/Core/Ajax/AddJsCommand.php
--- b/core/lib/Drupal/Core/Ajax/AddJsCommand.php
+++ b/core/lib/Drupal/Core/Ajax/AddJsCommand.php
@@ -3,10 +3,11 @@
 namespace Drupal\Core\Ajax;
 
 /**
- * An AJAX command for adding JS to the page via ajax.
+ * An AJAX command for adding JS to the page via AJAX.
  *
- * This command is implemented by Drupal.AjaxCommands.prototype.add_js()
- * defined in misc/ajax.js.
+ * This command will make sure all the files are loaded before continuing
+ * executing the next AJAX command. This command is implemented by
+ * Drupal.AjaxCommands.prototype.add_js() defined in misc/ajax.js.
  *
  * @see misc/ajax.js
  *
@@ -15,8 +16,7 @@
 class AddJsCommand implements CommandInterface {
 
   /**
-   * An array containing the attributes of the 'script' tags to be added to the
-   * page.
+   * An array containing attributes of the scripts to be added to the page.
    *
    * @var string[]
    */
@@ -28,7 +28,7 @@
    * If the command is a response to a request from an #ajax form element then
    * this value can be NULL.
    *
-   * @var string|null
+   * @var string
    */
   protected $selector;
 
@@ -38,7 +38,7 @@
    * @param array $scripts
    *   An array containing the attributes of the 'script' tags to be added to
    *   the page.
-   * @param string|null $selector
+   * @param string $selector
    *   A CSS selector.
    */
   public function __construct(array $scripts, string $selector = 'body') {
diff -u b/core/misc/ajax.es6.js b/core/misc/ajax.es6.js
--- b/core/misc/ajax.es6.js
+++ b/core/misc/ajax.es6.js
@@ -550,7 +550,7 @@
           // when the return is not a Promise we make sure it can be used as
           // one. This is useful for code that overrides the success method.
           Promise.resolve(ajax.success(response, status))
-            // Ajaxing status is back to false when all the ajax commands have
+            // Ajaxing status is back to false when all the AJAX commands have
             // finished executing.
             .then(() => {
               ajax.ajaxing = false;
@@ -1042,9 +1042,9 @@
 
     return (
       this.commandExecutionQueue(response, status)
-        // If the focus hasn't been changed by the ajax commands, try to refocus the
-        // triggering element or one of its parents if that element does not exist
-        // anymore.
+        // If the focus hasn't been changed by the AJAX commands, try to refocus
+        // the triggering element or one of its parents if that element does not
+        // exist anymore.
         .then(() => {
           if (
             !focusChanged &&
@@ -1065,9 +1065,9 @@
             }
           }
           // Reattach behaviors, if they were detached in beforeSerialize(). The
-          // attachBehaviors() called on the new content from processing the response
-          // commands is not sufficient, because behaviors from the entire form need
-          // to be reattached.
+          // attachBehaviors() called on the new content from processing the
+          // response commands is not sufficient, because behaviors from the
+          // entire form need to be reattached.
           if (this.$form && document.body.contains(this.$form.get(0))) {
             const settings = this.settings || drupalSettings;
             Drupal.attachBehaviors(this.$form.get(0), settings);
@@ -1686,14 +1686,15 @@
     add_js(ajax, response, status) {
       const parentEl = document.querySelector(response.selector || 'body');
       const settings = ajax.settings || drupalSettings;
-      const allUniqueBundleIDs = response.data.map((script) => {
+      const allUniqueBundleIds = response.data.map((script) => {
         // loadjs requires a unique ID, AJAX instances' `instanceIndex` are
         // guaranteed to be unique.
         // @see Drupal.behaviors.AJAX.detach
-        const uniqueBundleID = script.src + ajax.instanceIndex;
-        loadjs(script.src, uniqueBundleID, {
-          // By default, dynamically added scripts are marked as async. Only
-          // explicitly marked async scripts should be loaded async.
+        const uniqueBundleId = script.src + ajax.instanceIndex;
+        loadjs(script.src, uniqueBundleId, {
+          // The default loadjs behavior is to load script with async, in Drupal
+          // we need to explicitly tell scripts to load async, this is set in
+          // the before callback below if necessary.
           async: false,
           before(path, scriptEl) {
             // This allows all attributes to be added, like defer, async and
@@ -1707,18 +1708,19 @@
             // consistency, because it doesn't actually matter for the script
             // where it is added. Developers however expect library assets to
             // show up where they declared them, so this makes things consistent
-            // with the assets that are not loaded with ajax.
+            // with the assets that are not loaded with AJAX.
             parentEl.appendChild(scriptEl);
-            // Return `false` to bypass loadjs' default DOM insertion mechanism.
+            // Return false to bypass loadjs' default DOM insertion mechanism.
             return false;
           },
         });
-        return uniqueBundleID;
+        return uniqueBundleId;
       });
-      // Returns the promise so that the next AJAX command waits on the completion
-      // of this one to execute, ensuring the JS is loaded before executing.
+      // Returns the promise so that the next AJAX command waits on the
+      // completion of this one to execute, ensuring the JS is loaded before
+      // executing.
       return new Promise((resolve, reject) => {
-        loadjs.ready(allUniqueBundleIDs, {
+        loadjs.ready(allUniqueBundleIds, {
           success() {
             Drupal.attachBehaviors(parentEl, settings);
             // All JS files were loaded and new and old behaviors have
@@ -1728,8 +1730,8 @@
           },
           error(depsNotFound) {
             const message = Drupal.t(
-              `The following files could not be loaded: @deps`,
-              { '@deps': depsNotFound.join(', ') },
+              `The following files could not be loaded: @dependencies`,
+              { '@dependencies': depsNotFound.join(', ') },
             );
             reject(message);
           },
diff -u b/core/misc/ajax.js b/core/misc/ajax.js
--- b/core/misc/ajax.js
+++ b/core/misc/ajax.js
@@ -686,9 +686,9 @@
     add_js(ajax, response, status) {
       const parentEl = document.querySelector(response.selector || 'body');
       const settings = ajax.settings || drupalSettings;
-      const allUniqueBundleIDs = response.data.map(script => {
-        const uniqueBundleID = script.src + ajax.instanceIndex;
-        loadjs(script.src, uniqueBundleID, {
+      const allUniqueBundleIds = response.data.map(script => {
+        const uniqueBundleId = script.src + ajax.instanceIndex;
+        loadjs(script.src, uniqueBundleId, {
           async: false,
 
           before(path, scriptEl) {
@@ -700,18 +700,18 @@
           }
 
         });
-        return uniqueBundleID;
+        return uniqueBundleId;
       });
       return new Promise((resolve, reject) => {
-        loadjs.ready(allUniqueBundleIDs, {
+        loadjs.ready(allUniqueBundleIds, {
           success() {
             Drupal.attachBehaviors(parentEl, settings);
             resolve();
           },
 
           error(depsNotFound) {
-            const message = Drupal.t(`The following files could not be loaded: @deps`, {
-              '@deps': depsNotFound.join(', ')
+            const message = Drupal.t(`The following files could not be loaded: @dependencies`, {
+              '@dependencies': depsNotFound.join(', ')
             });
             reject(message);
           }