Finding missing numbers in an array
I think, there is an error:

If I enter the values:

[-3,-2,1,5]


I get the result:

[-2,0,2,3,4]


but it should be:

[-1,0,2,3,4]


The error occurs when there are two adjacent numbers in the array (here -3 and -2).



Naming:

Index is not an index, but the actual value in the valid sequence (from min to max). I would call it value or curValue or something like that.

I don't like the name target either, because the target is actually the sequence of numbers from min to max. input would be better IMO.



Analysis

Basically you want to compare two sequences of numbers index by index. If they differ then save the target value, if not then increment to the next value of the input array. The index of the target sequence (here from min to max) is always incremented. Instead of the target sequence you can just increment a value starting from input[0] and ending on input.[input.length - 1]. 

All in all it could be done something like this:

function findMissing(input) {

  var result = [];

  for (var inputIndex = 0, targetValue = input[0]; targetValue &lt;= input[input.length - 1]; targetValue++) {
    if (input[inputIndex] != targetValue) {
      result.push(targetValue);
    }
    else {
      inputIndex++;
    }
  }

  return result;
}