Superman2971 Superman2971 - 3 months ago 19
Javascript Question

Why and when to use angular.copy? (Deep Copy)

I've been saving all the data received from services direct to local variable, controller, or scope. What I suppose would be considered a shallow copy, is that correct?

Example:

DataService.callFunction()
.then(function(response) {
$scope.example = response.data;
});


Recently I was told to use angular.copy in order to create a deep copy.

$scope.example = angular.copy(response.data);


However, the deep copy information seems to be working in the same way when used by my Angular application. Are there specific benefits to using a deep copy (angular.copy) and can you please explain them to me?

Answer

Use angular.copy when assigning value of object or array to another variable and that object value should not be changed.

Without deep copy or using angular.copy, changing value of property or adding any new property update all object referencing that same object.

var app = angular.module('copyExample', []);
app.controller('ExampleController', ['$scope',
  function($scope) {
    $scope.printToConsole = function() {
      $scope.main = {
        first: 'first',
        second: 'second'
      };

      $scope.child = angular.copy($scope.main);
      console.log('Main object :' + $scope.main);
      console.log('Child object with angular.copy :' + $scope.child);
      $scope.child.first = 'last';
      console.log('New Child object :' + $scope.child);
      console.log('Main object after child change and using angular.copy :' + $scope.main);
      console.log('Assing main object without copy and updating child');
      $scope.child = $scope.main;
      $scope.child.first = 'last';
      console.log('Main object after update:' + $scope.main);
      console.log('Child object after update:' + $scope.child);
    }
  }
]);

// Basic object assigning example

var main = {
  first: 'first',
  second: 'second'
};
var one = main; // same as main
var two = main; // same as main

console.log('main :' + JSON.stringify(main)); // All object are same
console.log('one :' + JSON.stringify(one)); // All object are same
console.log('two :' + JSON.stringify(two)); // All object are same

two = {
  three: 'three'
}; // two changed but one and main remains same
console.log('main :' + JSON.stringify(main)); // one and main are same
console.log('one :' + JSON.stringify(one)); // one and main are same
console.log('two :' + JSON.stringify(two)); // two is changed

two = main; // same as main

two.first = 'last'; // change value of object's property so changed value of all object property 

console.log('main :' + JSON.stringify(main)); // All object are same with new value
console.log('one :' + JSON.stringify(one)); // All object are same with new value
console.log('two :' + JSON.stringify(two)); // All object are same with new value
<script src="https://ajax.googleapis.com/ajax/libs/angularjs/1.2.23/angular.min.js"></script>
<div ng-controller="ExampleController">
  <button ng-click='printToConsole()'>Explain</button>
</div>

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