Explanation: it's mostly due to how js does type conversion. for the Ls, it's
[] is an empty array ![] is treated as false combining a boolean with the empty array returns "false" as a string (so true + [] = "true", false + [] = "false") ! + [] is treated as true ! + [] + ! + [] is treated as 2 since true + true = 1 + 1 = 2 so you have "false"[2], which is l for the o it's [] is an empty array [] + {} returns "[object Object]" as a string ({} + [] returns 0) ![] is false !![] is true +!![] casts it to an integer so that part is "[object Object]"[1], which returns "o"
Iirc this is called "JSFuck", and it has a number of compilers