ok.js

function solution(A) {
    // permutation -> if numbers in sequences, return 1
    // ex. [3,1,2] is a permutation, [3,4,1] isn't
    let result = 1
    let sortedArr = A.sort((a,b) => a - b)
    let next = sortedArr[0]
    for (let i=0; i < sortedArr.length; i++) {
        (sortedArr[i] !== next) ? result = 0 : next += 1
    }
    return result
}

A = [1,2,3]

console.log(solution(A))