JavaScript - Comparing 2 arrays - Would like to check if I'm reinventing the wheel or am I good to go?
You should take a look at the Underscore javascript library, which contains a number of methods for comparing and manipulating arrays and objects.

I'm using the _.intersection and _.keys methods to compare your two objects. I've also taken the liberty of restructuring your arrays into objects, which I think makes more sense for what you're trying to do.

// Adds the values of keys that are the same between any two objects    
function addObjects(a, b) {
  var sameKeys = _.intersection(_.keys(a), _.keys(b)),      // Creates array of similary keys. eg ["John", "Jenny", "Amanda"]
      sumObj = {}

  // Loop through sameKeys, and add values from both objects
  for(i=0; i &lt; sameKeys.length; i++) {
    var thisKey = sameKeys[i];
    sumObj[thisKey] = a[thisKey] + b[thisKey];              // eg. sumbObj["John"] = a["John"] + b["John"] = 7 
  }

  return sumObj;
}

var firstObj = {
  "John" : 2,
  "Jenny" : 3,
  "Beavis" : 5,
  "Toby": 6,
   "Amanda": 12
}

var secondObj = {
  "John": 5,
  "Jenny": 18,
  "Amanda": 2
}


mySumObj = addObjects(firstObj, secondObj);
/* 
mySumObj = {
    "Amanda": 14,
    "Jenny": 21,
    "John": 7
}
*/


I hope this helps you. It may help if you could explain a bit more what your end goal here is.