They basically started designing a jet 50 years ago
This became the Viggen (development actually started over 60 years ago), with some help from the US, especially for the engine, which is basically a P&W JT8D with afterburner. Still a very impressive STOL-capable Mach-2 fighter jet, one of the very few with thrust reversers.
The Gripen is the 4th-gen successor, with much of the original design parameters retained, but with modern “relaxed stability”-aerodynamics for higher agility, and digital avionics.