You should create a js file in your own module and extend initial function. Depending on an Odoo version, it should be done differently. E.g. for Odoo 10:

odoo.define('YOURMODULENAME.ChatThread',
function (require) {
"use strict";

var core = require('web.core');
var MailThread = require('mail.Thread');

var Thread = MailThread.extend({
   
className: 'o_mail_thread',
   
init: function (parent, options) {
       
this._super.apply(this, arguments);
       
this.options = _.defaults(options || {}, {
       
    display_order: ORDER.ASC,
       
    display_needactions: true,
       
    display_stars: true,
       
    display_document_link: true,
       
    display_avatar: true,
       
    squash_close_messages: true,
       
    display_email_icon: true,
       
    display_reply_icon: true,
       
});
       
this.expanded_msg_ids = [];
       
this.selected_id = null;
    },
)};

Do not forget to init your js in the xml template:

       
<template id="assets_backend" name="Genial
Selection Assets Backend" inherit_id="web.assets_backend">
       
    <xpath expr="." position="inside">
       
        <script type="text/javascript"
src="/YOURMODULENAME/static/src/js/ChatThread.js"></script>
       
    </xpath>
</template>