How does the ~[] construction work in JavaScript?

Question

I've come across a working JavaScript code that I can't explain. For example:

• +[]===0
• -[]===0
• ~[]===-1
• ~-~[]===-2
• ~-~-~-~-~[]===-5
• ~-~-~-~-~[]+~[]===-6
• ~+~[]===0
• ~+~+~[]===-1
• ~+~+~+~[]===0

Can you explain the logic of these expressions?

Answer 1

[] is an empty array object, so:

+[]: force empty array to be positive integer, aka 0, which is === to 0
-[]: force empty array to be negative integer, aka 0, which is === to 0
~[]: bitwise NOT empty array, which evaluates to -1, which is === to -1
~-~[]: bitwise NOT of negated NOTted empty array: ~-(-1) -> ~1 -> -2

etc...

Answer 2

When you use the + or - operator on anything, it calls Number on it. Number([]) returns 0, so you get your first two answers.

The ~ operator is bitwise NOT. Basically it inverses all the bits in a number, which changes 0 to -1. More on bitwise operators here.

The rest are just combinations of those cases. You can pretty much combine those things to make any number you want.