Merge.js

var merge = function(A,p,q,r) {
  // 1, 4 , 9, intervalli da [1..4] incluso, [5...9] 
  var n1 = q - p +1; // = 4 -1 +1 = 4
  var n2 = r - q; // 5
  var L = [], R = [];
  for(var i=1; i<=n1;i++) { // da 1 a 4 
    L[i] = A[p + i - 1]; // setta L[i] = A[1+1-1]=  6, L[1+2-1]=L[2]=7, L[1+3-1]=L[3]=8, L[1 + 4 -1]=L[4] = 9
  }
  for (var j=1;j<=n2;j++) { // da 1 a 5 
    R[j] = A[q + j] ; // setta R[1] = A[4+1]=1, ..., R[n2]=R[5]=A[9]=5
  }
  L[n1+1] = 9999;
  R[n2+1] = 9999;
  i=1;
  j=1;
  for (var k=p;k<=r;k++) {
    // console.log('confronto: '+L[i]+' con '+R[j]);
    if(L[i]<R[j]) {
      A[k] = L[i];
      i++;
    }
    else {
      A[k] = R[j];
      j++;
    }
  }

  return A;

}

var ar=[null,6,7,8,9,1,2,3,4,5];

console.log(merge(ar,1,4,9));