I tried to override open_composer function and preprocess_message function of chatter.js . When i tried to override using extend or include, it throws an error Cannot read property 'extend' of undefined or Cannot read property 'include' of undefined. I tried like this 

odoo.define('demo.call', function (require) {
"use strict";
var composer = require('mail.composer');
var chatter = require('mail.Chatter');
chatter.ChatterComposer.extend({
  preprocess_message: function(){
   //code here
  },
 });
chatter.Chatter.include({
open_composer: function(options){
  //code here
},
 });
});