The line breaks haven't worked, here's it formatted correctly:

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

The idea behind it is not that complicated to explain... They set up a construct that spits out the word "false" and one that spits out "[object Object]". Taking the character with the Index of 2 from "false" gives us the "l" (the "2" to use as the index is of course also created using only brackets and parentheses). The character with the index of 1 from "[object Object]" is "o". So we have everything we need to spell out "lol".

There's a nice presentation that explains this behaviour quite understandable: https://youtu.be/sRWE5tnaxlI