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Home Interviews Introduction to observables and observable arrays – MindStick
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ICSM Computer
ICSM Computer 23 Apr 2025
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Absolutely! Let's dive into the core concepts of Knockout.js: observables and observableArrays. These are what give Knockout its reactive binding magic 🪄.

What is an observable in Knockout.js?

An observable is a special JavaScript object that tracks changes to a value. When the value changes, any UI elements or computed functions bound to it automatically update.

Basic Syntax:

let myObservable = ko.observable("Hello");
  1. To get the value: myObservable()
  2. To set a new value: myObservable("World")

Example:

let name = ko.observable("Alice");

console.log(name());         // Outputs: Alice
name("Bob");
console.log(name());         // Outputs: Bob

What is an observableArray?

An observableArray is just like a regular array, but Knockout tracks additions/removals and updates the UI accordingly.

✅ Syntax:

let items = ko.observableArray(["Apple", "Banana"]);

Useful Methods:

  1. push(item)
  2. remove(item)
  3. pop(), shift(), unshift()
  4. replace(oldItem, newItem)

Example:

let fruits = ko.observableArray(["Apple", "Banana"]);

fruits.push("Cherry");         // ["Apple", "Banana", "Cherry"]
fruits.remove("Banana");       // ["Apple", "Cherry"]

Example in HTML (Binding with UI)

<p>First Name: <input data-bind="value: firstName" /></p>
<p>Hello, <span data-bind="text: firstName"></span>!</p>

<ul data-bind="foreach: fruits">
  <li data-bind="text: $data"></li>
</ul>

function ViewModel() {
  this.firstName = ko.observable("John");
  this.fruits = ko.observableArray(["Apple", "Banana"]);
}

ko.applyBindings(new ViewModel());

Recap:

Concept Type Use Case
ko.observable() Single value Text fields, numbers, booleans
ko.observableArray() Arrays Lists, tables, multi-selections
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