JSFiddle - React, Tailwind, and code Playground

HTML

<script src="https://cdnjs.cloudflare.com/ajax/libs/sinon.js/7.2.2/sinon.min.js"></script>

JavaScript

const toBeMocked = { MockedMethod: () => alert('original') };

const r = document.getElementById('result');

const stub = sinon.stub(toBeMocked , 'MockedMethod');
const fakeMethod = () => alert('mocked');
stub.callsFake(fakeMethod);

toBeMocked.MockedMethod();