Strange JavaScript notation, all parenthesis and brackets [duplicate]

Question

So I participated in a Capture The Flag this week and one of the challenges really confused me.

Challenge file (.txt)

Preview:

[][(![]+[])[+[]]+([![]]+[][[]])[+!+[]+[+[]]]+(![]+[])[!+[]+!+[]]+(!![]+[])[+[]]+(!![]+[])[!+[]+!+[]+!+[]]+(!![]+[])[+!+[]]][([][(![]+[])[+[]]+([![]]+[][[]])[+!+[]+[+[]]]+(![]+[])[!+[]+!+[]]+(!![]+[])[+[]]+(!![]+[])[!+

This was the file given, it reminded me of this, after playing with it for a bit I entered it into the JavaScript console on my browser and an alert with the flag popped up.

If someone could explain why this works, and how I could create something like this. Also is it simple to translate this into normal looking JavaScript, I just imagine this could be a way for an attacker to execute sketchy code in my browser.

Answer 1

i put some link that could be helpful

• transaltejs (github code) translate string into this pattern

• explanetion 1, jsfuck

• explanation 2, JavaScript, the weird parts

the key to do this

• false => ![]

• true => !![]

• undefined => [][[]]

• NaN => +[![]]

• 0 => +[]

• 1 => +!+[]

• 2 => !+[]+!+[]

• 10 => [+!+[]]+[+[]]

• Array => []

• Number => +[]

• String => []+[]

• Boolean => ![]

• Function => []["filter"]

• eval => []["filter"]"constructor"()

• window => []["filter"]"constructor"()