(!+[]+[]+![]).length

> (!+[]+[]+![]).length
9

+[] === 0
!+[] === true
!+[]+[] === "true"
(!+[]+[])+![] === (!+[]+[])+false === "truefalse"
"truefalse".length === 9

+[] ⟶ +"" ⟶ 0

!+[] ⟶ !0 ⟶ !false ⟶ true

!+[]+[] ⟶ true + [] ⟶ true + "" ⟶ "true"

![]  ⟶ !true ⟶ false

"true" + false ⟶ "truefalse"