Javascript Quirks

[] + [] // equals ''

Arrays are converted to strings and then concatenated. As there are no elements in either of these arrays the concatenation is "" + "" which is "".

true + true // equals 2

The + operator is interpreted and javascript converts the boolean values to numbers. The numeric value of true is 1 so therefore it is just a simple 1+1.

false + false // equals 0

Similar to above, the numberic value of false is 0.

!![] // equals true

!! in front of a value converts it to its Boolean representation and the Boolean representation of [] is true.

[,,,].length // equals 3

N number commas within an array creates N number of elements within that array.

[1,2,3] + [4,5,6] // equals '1,2,34,5,6'

Array are converted to strings and then concatenated. Adding in a trailing comma to the first array would also not affect the outcome. [1,2,3,]+ [4,5,6] still equals '1,2,34,5,6'.