Monitoring Changes in an Array of Objects with Angular's $watch Feature

Question

Monitoring Changes in an Array of Objects with Angular's $watch Feature

Imagine having an array of animal objects linked to the scope. Each object contains a 'name' field and a 'sound' field.

After that, I set up a $watch on the array with the objectEquality flag set to true (the third argument).

Then, in the view, I generate a button for each animal using an ng-repeat over the animals array. When clicked, the corresponding animal object is passed to the handling function.

If I were to update the name of one of the animals in the click handler, the change would be detected by the $watch, allowing me to manage it in the listener function.

However, the problem arises when I replace one of the animal objects with a new object instead of simply changing its properties. In this scenario, the $watch fails to recognize the change despite the clear modification in the array of objects. The listener does not get triggered.

You can see an example of this outlined situation in the following plunkr:

http://plnkr.co/edit/PHXq32?p=preview

javascript arrays angularjs object

Answer 1

Answer №1

AngularJS has no connection to this topic Remember, JavaScript operates on pass by value What does it really mean? Let's discuss how it works with primitives, arrays, and objects

Primitives - When you pass a primitive to a function, you are simply passing the value itself. Any changes made to the value within the function will not affect the original value outside of it.
```
function testPrimitive(prm){
  prm = prm+1;
}
```
```
var prm = 1;
  testPrimitive(prm);
  console.log(prm) //prm remains as 
```
still 1

Objects and Arrays - When passing objects or arrays, you are actually passing a reference to the object. This means that modifying a property of the object in a function will reflect those changes outside of it. However, creating a new object inside the function will not affect the original object. Let's see an example.

var foo = {id:1, name:'iamfoo'}, bar = {id:2, name:'iambar'};
//two objects are created with references ref1 and ref2 assigned to foo and bar respectively

function updateObj(o1, o2){
  o1.name='changed'; 
  //o1 is still referencing the same object and updates its property.
  o2 = {id:3, name:'iamnew'}; 
  //A new object is created at ref3 and assigned to o2. Note that ref2 remains unchanged.
}

updateObj(foo, bar); //ref1 and ref2 values passed here

console.log(foo);//outputs {id:1, name:'changed'}
 console.log(bar);
 //bar remains {id:2, name:'iambar'} as it still points to ref2

In your scenario, you're creating a new object inside an array which loses its reference once outside the loop. Use

animal.name ='Billy'; animal.sound='baaaa';

instead within the function

A helpful article on this topic:

Although these concepts may seem basic, many people are unaware or confused about them.

Answer 2

AngularJS has no connection to this topic Remember, JavaScript operates on pass by value What does it really mean? Let's discuss how it works with primitives, arrays, and objects

Primitives - When you pass a primitive to a function, you are simply passing the value itself. Any changes made to the value within the function will not affect the original value outside of it.
```
function testPrimitive(prm){
  prm = prm+1;
}
```
```
var prm = 1;
  testPrimitive(prm);
  console.log(prm) //prm remains as 
```
still 1

Objects and Arrays - When passing objects or arrays, you are actually passing a reference to the object. This means that modifying a property of the object in a function will reflect those changes outside of it. However, creating a new object inside the function will not affect the original object. Let's see an example.

var foo = {id:1, name:'iamfoo'}, bar = {id:2, name:'iambar'};
//two objects are created with references ref1 and ref2 assigned to foo and bar respectively

function updateObj(o1, o2){
  o1.name='changed'; 
  //o1 is still referencing the same object and updates its property.
  o2 = {id:3, name:'iamnew'}; 
  //A new object is created at ref3 and assigned to o2. Note that ref2 remains unchanged.
}

updateObj(foo, bar); //ref1 and ref2 values passed here

console.log(foo);//outputs {id:1, name:'changed'}
 console.log(bar);
 //bar remains {id:2, name:'iambar'} as it still points to ref2

In your scenario, you're creating a new object inside an array which loses its reference once outside the loop. Use

animal.name ='Billy'; animal.sound='baaaa';

instead within the function

A helpful article on this topic:

Although these concepts may seem basic, many people are unaware or confused about them.

Answer 3

Answer №2

Consider passing $index as an argument to your function and then implement the following:

$scope.animals[index] = {name:'Billy', sound:'baaaa'};

This should resolve the issue.

Answer 4

Consider passing $index as an argument to your function and then implement the following:

$scope.animals[index] = {name:'Billy', sound:'baaaa'};

This should resolve the issue.

Monitoring Changes in an Array of Objects with Angular's $watch Feature

Answer №1

Answer №2

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