AngularJS watch array of objects for data change

Question

I'm implementing a shopping cart and want to store the data in localStorage. I want to watch the variable $scope.cart for change so that I can update the localStorage

The cart variable looks like this:

[{'name':'foo', 'id':'bar', 'amount': 1 },...]

This is the code for the watch.

$scope.updateCart = function(){
    localStorageService.add('cart',JSON.stringify($scope.cart));
    alert('cart updated');
};

$scope.$watch($scope.cart, $scope.updateCart(), true);

This is the HTML where the user changes the model.

<li ng-repeat="item in cart">
  {{item.name}} <input type="text" ng-model="item.amount">
</li>

With the following code, the updateCart() method is never triggered. I'm not sure what I'm doing wrong. I check that the $scope.cart variable did change, but the update was not triggered.

score 41 · Accepted Answer · edited Nov 12 '15 at 03:58

41

$watch only evaluate string or function parameter in its first argument. Change your $watch like this :

$scope.$watch('cart.name + cart.id + cart.amount', $scope.updateCart());

OR

$scope.$watch('cart', $scope.updateCart, true);

See reference API

edited Nov 12 '15 at 03:58

Rubens Mariuzzo

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answered Jul 02 '13 at 07:18

EpokK

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Isn't `true` unnecessary in the first example? – Tomasz Kowalczyk Oct 20 '13 at 20:21
6

$scope.updateCart will never trigger if you pass it in with parentheses as you would pass the result of the function to the $watch which would be undefined in this case. The correct way is: $scope.$watch('cart', $scope.updateCart, true); – SKuijers Apr 20 '15 at 13:11
1

`cart` is an array of objects, it does not have properties like `amount` – Hovo Jul 04 '16 at 06:10
@SKuijers I guess that was fixed in this edit: http://stackoverflow.com/revisions/17419684/3 – The Red Pea Apr 16 '17 at 02:00
@TheRedPea yep it seems fixed. Thanks to rubensMaruizzo for the edit – SKuijers Apr 24 '17 at 10:48

score 28 · Answer 2 · edited Mar 10 '21 at 09:30

28

It is a common mistake to use a variable as the first parameter to the watch expression.

For $scope.cart, you should use the string cart:

$scope.$watch('cart', function () {...}, true)

AngularJs has a new watch method, $watchCollection. It is basically the same as $watch with the object equality option set to true, to watch for a change in a property or an array item.

$scope.$watchCollection('cart', function () {...});

Documentation: $watchCollection.

edited Mar 10 '21 at 09:30

Lee Goddard

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answered Mar 23 '14 at 05:32

Jin

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$watchColletion saved me, when pushing / removing items from an array. Thanks! – Ziarno May 12 '15 at 14:58
1

It's unclear what kind of change the OP wanted, but [as seen here](http://stackoverflow.com/a/29189252/1175496) `$watch(.., .., true)` detects even more changes, namely this type of change: `$scope.cart[0].name= "Foo Bar";` – The Red Pea Nov 27 '16 at 17:26

score 1 · Answer 3 · edited Jan 31 '14 at 21:40

1

try to change

$scope.$watch($scope.cart, $scope.updateCart(), true);

to

$scope.$watch('cart', $scope.updateCart(), true);

$watch only evaluate string or function parameter in its first argument

edited Jan 31 '14 at 21:40

Blazemonger

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answered Jul 02 '13 at 05:20

Roger Jin

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AngularJS watch array of objects for data change

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