For the test at https://codility.com/demo/take-sample-test

the following provides a solution in Javascript:

function solution(A) {
    // write your code in JavaScript (Node.js 8.9.4)

    let tmp = []
    
    A.forEach(e => {if (0 <= e) tmp[e] = true})
    
    for (let i=1; i<=tmp.length; i++) {
        if (tmp[i]===undefined) return i
    }

    return 1
}