Merge two arrays and sort the final one

Question

In an interview I was asked the following question. I am given two arrays, both of them are sorted.

BUT

Array 1 will have few -1\'s and Array 2 will have to

Answer 1

Since they are both sorted, the order of arrayTwo's items should match the order of -1s in arrayOne. Then the job becomes simple and can be implemented as follows;

function replaceMissing(a,b){
  var i = 0;
  return a.map(n => n < 0 ? b[i++] : n);
}
var arrayOne = [3,6,-1,11,15,-1,23,34,-1,42],
    arrayTwo = [7,19,38];
      result = replaceMissing(arrayOne,arrayTwo);
console.log(result);

Edit: I believe the upper solution does make more sense in the general logic of the question. If the position of -1s does not mean anything then what use do they have? Let's just delete the -1's and do a simple insertion of arrayTwo items at proper indices in arrayOne. This can very simply be done as follows.

function replaceMissing(a,b){
  var j = b.length-1;
  return b.concat(a.reduceRight((r,m,i) => (m < 0 ? r.splice(i,1)
                                                  : m < b[j] && r.splice(i+1,0,b.splice(j--,1)[0]),
                                            r), a.slice()));
}
var arrayOne = [3,6,-1,11,15,-1,23,34,-1,42],
    arrayTwo = [1,25,38];
      result = replaceMissing(arrayOne,arrayTwo);
console.log(result);