I had an assortment of those types of things back in the day, both the genuine article and myriad knockoffs of varying levels of quality. Photon was the big name brand for those at the time. These days it's astonishing to think that LEDs used to be an "exotic" technology at least for the purposes of illumination and not just being indicators on things, and they went through a period of being deemed neat but inferior in performance to their analog counterpart in almost exactly the same way that LCD screens did for a while. Which is an interesting but irrelevant parallel.

Anyway, the best one of those I ever got my hands on is one that I still have, which is an "Inova" branded Photon knockoff which among its admittedly rather short list of features included a sliding switch that would keep the thing powered on for you without having to pinch it the entire time.

Those little jobbers were and still are powered by a pair of CR2016 coin cells which the LED is extremely unkind to. There are no active electronics and also no current limiting resistor. The sum total of the light's components consist of the casing, the batteries, and the LED. They're switched by just mashing one leg of the LED into contact with the batteries, and the current draw of the LED is so immense compared to the couple of milliamps that the coin cells expect to be faced with that they're literally incapable of supplying enough current to damage the LED. As a result of this their battery life is famously awful, and their full brightness runtime is probably less than two minutes. They'll continue to illuminate in at least some capacity off of extremely beat batteries, though. I unearthed my little Inova light the other day and it still turns on, albeit not at a terribly useful brightness, on its original batteries. Which are probably from 2004.