Compare 2 arrays of objects with Underscore to find the differnce

Building on @Jonathan's answer, I have tweaked a version that returns a usable list of differences including which side had diff and the index of the record.

var a = [
{"ListingId":1762276,"Rating":3,"ListPrice":7411828,"PropertyType":"Residential"},
{"ListingId":1826692,"Rating":3,"ListPrice":650000,"PropertyType":"Residential"},
{"ListingId":1833283,"Rating":4,"ListPrice":950000,"PropertyType":"Residential"},
{"ListingId":1832134,"Rating":3,"ListPrice":850000,"PropertyType":"Residential"},
{"ListingId":1829932,"Rating":4,"ListPrice":750000,"PropertyType":"Residential"},
{"ListingId":1827548,"Rating":5,"ListPrice":650000,"PropertyType":"Residential"}
];

var b = [
{"ListingId":1762276,"Rating":2,"ListPrice":7411828,"PropertyType":"Residential"},
{"ListingId":1826692,"Rating":3,"ListPrice":650000,"PropertyType":"Residential"},
{"ListingId":1833283,"Rating":4,"ListPrice":950000,"PropertyType":"Residential"},
{"ListingId":1832131,"Rating":3,"ListPrice":850000,"PropertyType":"Residential"},
{"ListingId":1829932,"Rating":4,"ListPrice":750000,"PropertyType":"Residential"},
{"ListingId":1827548,"Rating":5,"ListPrice":650000,"PropertyType":"Residential"}
]

function diff(arr1, arr2){
    var aDiffs=[]
    function __comp(arrL, arrR, side){
        var rL, rR, bFound
        for(var i=0;i<arrL.length;i++){rL=arrL[i]
            bFound=false
            for(var j=0;j<arrR.length;j++){rR=arrR[j]
                if (_.isEqual(rL, rR)){ 
                    bFound=true
                    break
                }
            }
            if (!bFound)aDiffs.push({side: side, ind: i, obj: rL})
        }
    }
    __comp(arr1, arr2, 'l')
    __comp(arr2, arr1, 'r')
    return aDiffs
};


console.log(JSON.stringify(diff(b, a)).replace(/},{/g, "}\n,{"));

Output:

[{"side":"l","ind":0,"obj":{"ListingId":1762276,"Rating":2,"ListPrice":7411828,"PropertyType":"Residential"}}
,{"side":"l","ind":3,"obj":{"ListingId":1832131,"Rating":3,"ListPrice":850000,"PropertyType":"Residential"}}
,{"side":"r","ind":0,"obj":{"ListingId":1762276,"Rating":3,"ListPrice":7411828,"PropertyType":"Residential"}}
,{"side":"r","ind":3,"obj":{"ListingId":1832134,"Rating":3,"ListPrice":850000,"PropertyType":"Residential"}}]

Take a look to this:

var a = [
{"ListingId":1762276,"Rating":3,"ListPrice":7411828,"PropertyType":"Residential"},
{"ListingId":1826692,"Rating":3,"ListPrice":650000,"PropertyType":"Residential"},
{"ListingId":1833283,"Rating":4,"ListPrice":950000,"PropertyType":"Residential"},
{"ListingId":1832134,"Rating":3,"ListPrice":850000,"PropertyType":"Residential"},
{"ListingId":1829932,"Rating":4,"ListPrice":750000,"PropertyType":"Residential"},
{"ListingId":1827548,"Rating":5,"ListPrice":650000,"PropertyType":"Residential"}
];

var b = [
{"ListingId":1762276,"Rating":2,"ListPrice":7411828,"PropertyType":"Residential"},
{"ListingId":1826692,"Rating":3,"ListPrice":650000,"PropertyType":"Residential"},
{"ListingId":1833283,"Rating":4,"ListPrice":950000,"PropertyType":"Residential"},
{"ListingId":1832134,"Rating":3,"ListPrice":850000,"PropertyType":"Residential"},
{"ListingId":1829932,"Rating":4,"ListPrice":750000,"PropertyType":"Residential"},
{"ListingId":1827548,"Rating":5,"ListPrice":650000,"PropertyType":"Residential"}
]

var difference = function(array){
   var rest = Array.prototype.concat.apply(Array.prototype, Array.prototype.slice.call(arguments, 1));

   var containsEquals = function(obj, target) {
    if (obj == null) return false;
    return _.any(obj, function(value) {
      return _.isEqual(value, target);
    });
  };

  return _.filter(array, function(value){ return ! containsEquals(rest, value); });
};

console.log(JSON.stringify(difference(b, a)));
> [{"ListingId":1762276,"Rating":2,"ListPrice":7411828,"PropertyType":"Residential"}]

The code is based on the original function difference from underscore, but it performs a deep comparison between objects using isEqual.

If the order does not change, a simple iteration will do it. Underscore provides the find method for this task:

var changedObj = _.find(newVal, function(obj, index) {
    return obj.Rating != oldVal[index].Rating;
});