First, you define a template in your module to add your js assets located in static/src/js:

<template id="assets_backend" name="newmodule_hr_timesheet_sheet assets" inherit_id="web.assets_backend"> 
           <xpath expr="." position="inside"> 
               <script type="text/javascript" src="/newmodule_hr_timesheet_sheet/static/src/js/timesheet.js"></script> 
           </xpath>
</template>

Then, in your module's new js file, you extend the widget from hr_timesheet_sheet like this:

odoo.define('newmodule_hr_timesheet_sheet.sheet', function (require){
"use strict";
// require original module JS
var timesheet = require('hr_timesheet_sheet.sheet');

// Extend widget
timesheet.WeeklyTimesheet.include({
    ....
    your code to override here    
    ....
});

}