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diff --git a/comm/mailnews/base/public/nsIMsgEnumerator.idl b/comm/mailnews/base/public/nsIMsgEnumerator.idl
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+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* 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/. */
+
+#include "nsISupports.idl"
+
+interface nsIMsgDBHdr;
+interface nsIMsgThread;
+
+/**
+ * nsIJSIterator implements the JavaScript iterator protocol.
+ * For details on the JS side, see:
+ * https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Iteration_protocols#the_iterator_protocol
+ */
+[scriptable, uuid(4406175e-1aa4-414c-ad7c-86bd76e102f8)]
+interface nsIJSIterator : nsISupports {
+  /**
+   * @returns {{value: Object, done: Boolean}} the iteration state.
+   * 'value' holds the next item, and 'done' flags the end of the iteration.
+   * (notionally, both 'value' and 'done' are always present, but a missing
+   * 'done' is considered to be false, and 'value' should be ignored if
+   * 'done' is true, according to the protocol).
+   */
+  [implicit_jscontext]
+  jsval next();
+};
+
+
+/**
+ * nsIMsgEnumerator is an object for iterating forward over an ordered set of
+ * messages (nsIMsgHdrs). There is no provision to reset the iteration.
+ */
+[scriptable, uuid(5760cb6d-1a71-485f-ad85-5a98a9da2104)]
+interface nsIMsgEnumerator : nsISupports {
+  /**
+   * Implements the JavaScript iterable protocol, which allows
+   * for...of constructs and the like.
+   * https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Iteration_protocols#the_iterable_protocol
+   * NOTE: the nsIMsgEnumerator is "used up" by iterator(), just as it is by
+   * getNext()/hasMoreElements(). So you can only loop over the enumerator once.
+   */
+  [symbol]
+  nsIJSIterator iterator();
+
+  /**
+   * Called to determine whether or not there are any messages remaining
+   * to be retrieved from the enumerator.
+   *
+   * @returns true if getNext() may be called to fetch a message, or
+   *          false if there are no more messages.
+   */
+  boolean hasMoreElements();
+
+  /**
+   * Called to retrieve the next message in the set.
+   *
+   * @returns the next element in the enumeration.
+   */
+  nsIMsgDBHdr getNext();
+};
+
+/**
+ * nsIMsgThreadEnumerator is an object for iterating forward over an ordered set of
+ * message threads (nsIMsgThread). There is no provision to reset the iteration.
+ */
+[scriptable, uuid(e39ae1a8-2b94-4f2b-abb4-250171047501)]
+interface nsIMsgThreadEnumerator : nsISupports {
+  /**
+   * Implements the JavaScript iterable protocol, which allows
+   * for...of constructs and the like.
+   * https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Iteration_protocols#the_iterable_protocol
+   * NOTE: the nsIMsgThreadEnumerator is "used up" by iterator(), just as it is by
+   * getNext()/hasMoreElements(). So you can only loop over the enumerator once.
+   */
+  [symbol]
+  nsIJSIterator iterator();
+
+  /**
+   * Called to determine whether or not there are any messages remaining
+   * to be retrieved from the enumerator.
+   *
+   * @returns true if getNext() may be called to fetch a message, or
+   *          false if there are no more messages.
+   */
+  boolean hasMoreElements();
+
+  /**
+   * Called to retrieve the next thread in the set.
+   *
+   * @returns the next element in the enumeration.
+   */
+  nsIMsgThread getNext();
+};