diff --git a/js/src/builtin/Sorting.js b/js/src/builtin/Sorting.js
index 4fead92dc178c..c1efcca58ec96 100644
--- a/js/src/builtin/Sorting.js
+++ b/js/src/builtin/Sorting.js
@@ -165,18 +165,31 @@ function MergeSortTypedArray(array, len, comparefn) {
     assert(IsPossiblyWrappedTypedArray(array),
            "MergeSortTypedArray works only with typed arrays.");
 
+    // Use the same TypedArray kind for the buffer.
+    var C = ConstructorForTypedArray(array);
+
+    var lBuffer = new C(len);
+
+    // Copy all elements into a temporary buffer, so that any modifications
+    // when calling |comparefn| are ignored.
+    for (var i = 0; i < len; i++) {
+        lBuffer[i] = array[i];
+    }
+
     // Insertion sort for small arrays, where "small" is defined by performance
     // testing.
     if (len < 8) {
-        InsertionSort(array, 0, len - 1, comparefn);
+        InsertionSort(lBuffer, 0, len - 1, comparefn);
+
+        // Move the sorted elements into the array.
+        for (var i = 0; i < len; i++) {
+            array[i] = lBuffer[i];
+        }
+
         return array;
     }
 
-    // Use the same TypedArray kind for the buffer.
-    var C = ConstructorForTypedArray(array);
-
     // We do all of our allocating up front.
-    var lBuffer = array;
     var rBuffer = new C(len);
 
     // Use insertion sort for the initial ranges.
@@ -204,10 +217,8 @@ function MergeSortTypedArray(array, len, comparefn) {
     }
 
     // Move the sorted elements into the array.
-    if (lBuffer !== array) {
-        for (var i = 0; i < len; i++) {
-            array[i] = lBuffer[i];
-        }
+    for (var i = 0; i < len; i++) {
+        array[i] = lBuffer[i];
     }
 
     return array;
diff --git a/js/src/tests/non262/TypedArray/sort_modifications.js b/js/src/tests/non262/TypedArray/sort_modifications.js
new file mode 100644
index 0000000000000..2a6ed49eb79fa
--- /dev/null
+++ b/js/src/tests/non262/TypedArray/sort_modifications.js
@@ -0,0 +1,73 @@
+const TAConstructors = [
+  Int8Array,
+  Uint8Array,
+  Int16Array,
+  Uint16Array,
+  Int32Array,
+  Uint32Array,
+  Uint8ClampedArray,
+  Float32Array,
+  Float64Array,
+  BigInt64Array,
+  BigUint64Array,
+];
+
+// Use different size classes to catch any implementation-specific
+// optimisations.
+const sizes = [
+  4, 8, 64, 128, 1024
+];
+
+function ToNumeric(TA) {
+  if (TA === BigInt64Array || TA === BigUint64Array) {
+    return BigInt;
+  }
+  return Number;
+}
+
+function ascending(a, b) {
+  return a < b ? -1 : a > b ? 1 : 0;
+}
+
+function descending(a, b) {
+  return -ascending(a, b);
+}
+
+for (let TA of TAConstructors) {
+  let toNumeric = ToNumeric(TA);
+  for (let size of sizes) {
+    let sorted = new TA(size);
+
+    // Fill with |1..size| and then sort to account for wrap-arounds.
+    for (let i = 0; i < size; ++i) {
+      sorted[i] = toNumeric(i + 1);
+    }
+    sorted.sort();
+
+    // Create a copy in descending order.
+    let ta = new TA(sorted);
+    ta.sort(descending);
+
+    // Sort the copy in ascending order and on the first call reset all
+    // elements to zero.
+    let called = false;
+    ta.sort(function(a, b) {
+      if (!called) {
+        called = true;
+        ta.fill(toNumeric(0));
+      }
+      return ascending(a, b);
+    });
+
+    // Ensure the comparator function was called.
+    assertEq(called, true);
+
+    // All elements should be sorted correctly. No elements should be zero.
+    for (let i = 0; i < size; ++i) {
+      assertEq(ta[i], sorted[i], `${TA.name} at index ${i} for size ${size}`);
+    }
+  }
+}
+
+if (typeof reportCompare === "function")
+  reportCompare(true, true);