how code change reflex on browser without page load in react [duplicate]

Steps I followed:

  1. setup react sample app.
  2. I modified tag in App.js
  3. went to browser.
  4. browser displayed updated change/content without refresh/reload.

Usually If I modified js file, to see the changes in browser I have to reload browser.
Question: How react update file change without browser reload? Is it related to react or node js?