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diff --git a/toolkit/components/uniffi-js/js/UniFFI.sys.mjs b/toolkit/components/uniffi-js/js/UniFFI.sys.mjs
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+++ b/toolkit/components/uniffi-js/js/UniFFI.sys.mjs
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+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+// This JS module contains shared functionality for the generated UniFFI JS
+// code.
+
+// TypeError for UniFFI calls
+//
+// This extends TypeError to add support for recording a nice description of
+// the item that fails the type check. This is especially useful for invalid
+// values nested in objects/arrays/maps, etc.
+//
+// To accomplish this, the FfiConverter.checkType methods of records, arrays,
+// maps, etc. catch UniFFITypeError, call `addItemDescriptionPart()` with a
+// string representing the child item, then re-raise the exception.  We then
+// join all the parts together, in reverse order, to create item description
+// strings like `foo.bar[123]["key"]`
+export class UniFFITypeError extends TypeError {
+  constructor(reason) {
+    super();
+    this.reason = reason;
+    this.itemDescriptionParts = [];
+  }
+
+  addItemDescriptionPart(part) {
+    this.itemDescriptionParts.push(part);
+  }
+
+  itemDescription() {
+    const itemDescriptionParts = [...this.itemDescriptionParts];
+    itemDescriptionParts.reverse();
+    return itemDescriptionParts.join("");
+  }
+
+  get message() {
+    return `${this.itemDescription()}: ${this.reason}`;
+  }
+}