Use spoiler flag.

Pooh! That was really difficult but exciting!
But it had to make a hack on the case with one element in the array.
Now will examine other solutions.
Thank you!

I somehow doubt the test case is wrong. Have you considered that leading zeroes disappear?

It's also testing backwards.

Your input is [3]. Allowed return values would be 12, 21 or 30.

Because there can be multiple possibilities, the test checks the resulting sums of digits, not the return value itself. Those digit sums are unique.

test case which make me so confused!

testing for 12, Expected: [3].

??

// 判定数组是否是1-9

const containNumber  = arr => [...new Set(arr)].length === 9

const containNumber = arr => arr.sort().join('') === '123456789'