[moron4hire]

No, certainly not the first in aggregate to use JS heavily. Actually, the browser-based Outlook client was the first to have all the parts that we would now consider to be essential for JS Single Page Apps (because Microsoft has to invent AJAX first). But Google Maps was definitely the first to have a major impact and start changing the public perception of what could be done with browser-based apps.

I consider Google Maps to be the first well-adopted, no-traditional-alternative app to be what we recognize today as a JS SPA. Gmail had a pure HTTP mode, and otherwise was not interesting to people who were happy with their current email. Outlook wasn't really used by that many people, not at the scale that Google Maps was Google Maps has broad appeal and was significantly better because of it's SPA-ness to change how people thought about browser-based apps.