Why does ++[[]][+[]]+[+[]] return the string “10”?

Question

This is valid and returns the string \"10\" in JavaScript (more examples here):

Answer 1

Let’s make it simple:

++[[]][+[]]+[+[]] = "10"

var a = [[]][+[]];
var b = [+[]];

// so a == [] and b == [0]

++a;

// then a == 1 and b is still that array [0]
// when you sum the var a and an array, it will sum b as a string just like that:

1 + "0" = "10"