How can I add CommandBar buttons in SharePoint online using JSOM?

Question

As shown in figure I want to add button (right side by send by mail) on each CommandBar in SharePoint site?

Answer 1

You can insert some code into React script editor web part to append the custom button.

<script src="https://code.jquery.com/jquery-1.12.4.min.js" type="text/javascript"></script>
<script type="text/javascript">
        $(document).ready(function () {
        var commandset= $("button[name='Send by email'][role='menuitem']").parent().parent();
        commandset.append( "<p>Test</p>" );
});

Answer 2

You should use a SharePoint Framework Extension to do this.