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

user8269

// We start with
(! + [] + [] + ![]).length  // Remember that we are going from right to left starting 
                            // with the operation before doing -length
![] === false               // ! converts the content of [] to true, since it's not part of 
                            // the false group above, and then reverts it.
+ === +                     // the + is lonely so far with no operand to the right but sticks 
                            // around since it has something on the left.
[]+[] === ""                // The contents of [] can't be added or converted to a number so 
                            // the right operand becomes a string. There's still something to 
                            // the left so the + stays.
!+[] === true               // This the same as !+0 since +[] is 0


// Now we got: 
("false" + "" + false).length
""+false === "false"        // + adds "" to false. It then sticks around.
"true" + "" === "true"      // The + stays after adding "true" to ""


// ---
("true"++"false") 
+"false" ==== "false"       // + has nothing but an operand to the left so it goes away.
true + "" === "true"        // + adds them together and stays

// ---
("true"+"false")
"true" + "false" === "truefalse" // + still stays

// ---
(+"truefalse")
+"truefalse" === "truefalse"     // + has nothing to do after this so it goes away

// ---
("truefalse")               // The operation is done so now we are left with what's outside.

"truefalse".length === 9