var a = [1,5,1];
var x = [];
x.push(a[0]);
for (var i=0; i<a.length-1; i++){
    if (x[x.length-1] < a[i+1]) {
        for (var j = x[x.length-1]+1; j < a[i+1]; j++) {
            if(j != a[+1])
                x.push(j)
        }
    }else{
        var p = x[x.length-1]-1;
        while (p > a[i+1]) {
            x.push(p)
            p--;
        }
    }
    x.push(a[i+1])
}

console.log(x)