AngularJS: How to get changed key and value from existing object in form?

I would achieve this by using the default ngForm directives like $dirty and a "save state" object of your original input values. This is a easy solution to achieve what you want. ngForm itself does not provide the original values so you have to create a "save object" and compare them manually.

View

<div ng-controller="MyCtrl">
  <form name="myForm" ng-submit="submit()">
    <input type="text" name="key1" ng-model="obj.key1"><br />
    <input type="text" name="key2" ng-model="obj.key2"><br />
    <input type="text" name="key3" ng-model="obj.key3">
    <input type="submit" id="submit" value="Submit" />
  </form>
</div>

AngularJS application

var myApp = angular.module('myApp',[]);

myApp.controller('MyCtrl', function ($scope) {

    $scope.obj = {
      key1: 'test 1',
      key2: 'test 2',
      key3: 'test 3',
    }

    var originalValues = angular.copy($scope.obj);

    $scope.submit = function () {

      var paramsChanged = {};

      angular.forEach($scope.obj, function (value, key) {
          if ($scope.myForm[key].$dirty && originalValues[key] !== value) {
            paramsChanged[key] = value;
          }
      });

      console.log(paramsChanged);
    }
});

> demo fiddle

Have a copy of the variable and just before sending it to the server, compare the model object with the copied variable using a compare function like the following:

// At the time of initialization
$scope.obj = [ ... ];
var copiedObj = angular.copy($scope.obj);

// At the time of submit
var changed = {};

Object.keys(copiedObj).forEach(key => {
  if(copiedObj[key] !== $scope.obj[key]) {
    changed[key] = $scope.obj[key];
  }
});

// Submit `changed` to the server

Assuming copiedObj is the copy of original $scope.obj, changed will contain the keys which are actually changed. So you can send changed to the server.

You can use $watch to watch for specific changes in $scope.

$scope.$watch('obj.key1', function(newValue, oldValue) {
  console.log('Key1 was updated from' + oldValue + ' to ' + newValue');
});

You can do this way have a ng-change bind to your HTML and whenever the ng-change happens for a particular Element write a code to push into an array and send that array to the server. A sample Code

    <input value="obj.key100" ng-model="obj.key100" ng-change="selectedValue(obj.keyx)" />

Angular Code

 $scope.tempValues=[];
 $scope.selectedValue = function(x) {
    $scope.tempValues.push(x);
 }

You can use ng-change to detect any changes, as well as a (key, value) syntax in ng-repeat to list your inputs. Here is a demo:

var app = angular.module('myApp', []);
app.controller('myCtrl', function($scope) {
  $scope.obj = {
    "key1": "value1",
    "key2": "value2",
    "key3": "value3"
  }

  var toSend = [];
  $scope.select = function(key) {
    var s = {};
    s[key] = $scope.obj[key]
    toSend.push(s); // or just the value: $scope.obj[key]
  }

  $scope.submit = function() {
    console.log(toSend)
    $scope.sent = toSend;
  }

});

<script src="https://ajax.googleapis.com/ajax/libs/angularjs/1.6.4/angular.min.js"></script>

<div ng-app="myApp" ng-controller="myCtrl">

  <form ng-submit="submit()">
    <div ng-repeat="(key,value) in obj">
      <input ng-change="select(key)" ng-model="value" /><br>
    </div>
    <button type="submit">Send</button>
  </form>

  <pre>{{sent | json}}</pre>

</div>