Because JMeter is not a browser, it won't interpret the JavaScript code on the page you GETting:
JMeter does not process Javascript or applets embedded in HTML pages.
[JMeter Wiki]
So, what can you do? You can add WebDriver to JMeter test and by this, evaluate the web pages.
Web Driver Sampler automates the execution and collection of
Performance metrics on the Browser (client-side). A large part of
performance testing, up to this point, has been on the server side of
things. However, with the advancement of technology, HTML5, JS and CSS
improvements, more and more logic and behaviour have been pushed down
to the client. This adds to the overall perceived performance of
website/webapp, but this metric is not available in JMeter. Things
that add to the overall browser execution time may include:
- Client-side Javascript execution - eg. AJAX, JS templates
- CSS transforms - eg. 3D matrix transforms, animations
- 3rd party plugins - eg. Facebook like, Double click ads, site analytics, etc
All these things add to the overall browser execution time, and this
project aims to measure the time it takes to complete rendering all
this content.
Official guide: https://jmeter-plugins.org/wiki/WebDriverTutorial/