JS on the backend was arguably an even bigger mistake when the JS ecosystem was less sophisticated. The levels of duct tape are dizzying. Although we might go back even further and ask if JS was also a mistake when it was added to the browser.

I often wonder about a what-if, alternate history scenario where Java had been rolled out to the browser in a more thoughtful way. Poor sandboxing, the Netscape plugin paradigm and perhaps Sun's licensing needs vs. Microsoft's practices ruined it.