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

Answer 1

Since I couldn't find any info on this elsewhere, I'll add a reply for how to use this with TypeScript.

The key here was to use the KnockoutArrayChange interface as TEvent for subscribe. If you don't do that, it'll try to use the other (non-generic) subscribe and will complain about status, index, and value not existing.

class ZoneDefinition {
    Name: KnockoutObservable;
}

class DefinitionContainer
{
    ZoneDefinitions: KnockoutObservableArray;
    constructor(zoneDefinitions?: ZoneDefinition[]){
        this.ZoneDefinitions = ko.observableArray(zoneDefinitions);
        // you'll get an error if you don't use the generic version of subscribe
        // and you need to use the KnockoutArrayChange interface as T
        this.ZoneDefinitions.subscribe[]>(function (changes) {
            changes.forEach(function (change) {
                if (change.status === 'added') {
                    // do something with the added value
                    // can use change.value to get the added item
                    // or change.index to get the index of where it was added
                } else if (change.status === 'deleted') {
                    // do something with the deleted value
                    // can use change.value to get the deleted item
                    // or change.index to get the index of where it was before deletion
                }
            });
        }, null, "arrayChange");
}