Recently I came over quite interesting expressions ++[[]][+[]]+[+[]]
and if you open browser JavaScript console and paste it there it will return string “10”, but how it really works?
This is something that took my mind yesterday and I decided to make it clear for me:
 Firstly,
+[]
in JavaScript is the same as+""
which casts a value to number 0, which is used twice, so if we replace it, we can now see following:++[[]][0]+[0]

Now we can split it to two three separate expressions:
++ // Increment operator [[]][0] // Which returns [], that casts to 0 and then incremented to 1 (++ always returns a number) +[0] // Which returns 0

[[]][0]
means we get the first element from array[[]]
which has one element that’s empty array and then increment it by 1, let’s write it in JS:[[]][0] // [], or [[]].pop() // []

Getting 1 from
++[[]][0]
, as we figured out[[]][0]
basically means just empty array or,[]
, then we have[] + 1
which returns string “1”, and due to ++ always returns number, we have just number 1:++[[]][0] // 1 ++[[]][0] === +([] + 1) // true +([] + 1) === 1 // true

And the last part
+ [0]
, when we add array to anything in JavaScript, it will concatenate it to string of values with commas:0 + [0, 1, 2, 3] // "00,1,2,3" [1,2] + [3,4] // "1,23,4" // Or in our case 1 + [0] // "10"  Yay!
To sum it up, lets describe it in single place:
++ // Increment result of next expression, or make number 1 [ [] // Array with single empty array item ] [ // Use bracket expression to access 0 element of previous array, which is empty array "[]" +[] // Cast array to 0 ] + // Unary + Operator, or 1 + [0] [ // Array with one element of number 0 +[] // Cast array to 0 ]