Subscribe to observable array for new or removed entry only

Question

So yes I can subscribe to an observable array:

vm.myArray = ko.observableArray();
vm.myArray.subscribe(function(newVal){...});

The problem is the newVal passed to the function is the entire array. Is there anyway I can get only the delta part? Say the added or removed element?

Answer 1

As of KnockoutJS 3.0, there's an arrayChange subscription option on ko.observableArray.

var myArray = ko.observableArray(["Alpha", "Beta", "Gamma"]);

myArray.subscribe(function(changes) {

    // For this example, we'll just print out the change info
    console.log(changes);

}, null, "arrayChange");

myArray.push("newitem!");

In the above callback, the changes argument will be an array of change objects like this:

[ 
   { 
      index: 3, 
      status: 'added', 
      value: 'newitem!' 
   }
]

For your specific problem, you want to be notified of new or removed items. To implement that using Knockout 3, it'd look like this:

myArray.subscribe(function(changes) {

    changes.forEach(function(change) {
        if (change.status === 'added' || change.status === 'deleted') {
            console.log("Added or removed! The added/removed element is:", change.value);
        }
    });

}, null, "arrayChange");