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diff --git a/node_modules/jsdom/lib/jsdom/living/events/EventModifierMixin-impl.js b/node_modules/jsdom/lib/jsdom/living/events/EventModifierMixin-impl.js
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+++ b/node_modules/jsdom/lib/jsdom/living/events/EventModifierMixin-impl.js
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+"use strict";
+
+// This mixin doesn't have an IDL equivalent, but since MouseEvent and KeyboardEvent implement getModifierState() the
+// same way, its implementation is shared here.
+
+class EventModifierMixinImpl {
+  // Event's constructor assumes all options correspond to IDL attributes with the same names, and sets them on `this`.
+  // That is not the case for these modifier boolean options, but since the options are set on `this` anyway we'll
+  // access them that way. The spec doesn't say much about the case where keyArg is not one of the valid ones
+  // (https://w3c.github.io/uievents-key/#keys-modifier), but at least Chrome returns false for invalid modifiers. Since
+  // these invalid modifiers will be undefined on `this` (thus `false` after casting it to boolean), we don't need to do
+  // extra checking for validity.
+  getModifierState(keyArg) {
+    return Boolean(this[`modifier${keyArg}`]);
+  }
+}
+
+exports.implementation = EventModifierMixinImpl;