It's called WallyFOV. It's basically an implementation of recursive shadowcasting in TypeScript, but with support for walls.
Go ahead and check out the demo. (and thanks to EasyStar.js for the pathfinding algorithm used in that demo, and from which I kindof copied the demo page style)
Recursive shadowcasting works by scanning outwards from the player looking for obstructions. When something is found, a shadow (represented by two angles) is added to a shadow list. Overlapping shadows are merged to keep the algorithm running quickly.
In this particular variant, a tile is considered visible if there exists any line from the center of the player's tile to any point within the target tile. There are some complications around dealing with diagonally-adjacent bodies, which you can read about in the algorithm overview, but overall it's a pretty simple algorithm to understand. It's a bit tricky to implement and optimize, however.
V8 (at least the version of V8 in node.js 7.9). Along the way I learned how to use node's --trace-deopt, and discovered some very unexpected optimization tips, mainly having to do with newer ECMAScript features.