What is a computed observable in KnockoutJS, and how is it used?
What is a computed observable in KnockoutJS, and how is it used?
11918-Apr-2023
Updated on 24-Nov-2023
Home / DeveloperSection / Forums / What is a computed observable in KnockoutJS, and how is it used?
What is a computed observable in KnockoutJS, and how is it used?
Aryan Kumar
24-Nov-2023In Knockout.js, a computed observable is a special type of observable that is automatically updated whenever its dependencies change. It allows you to create a computed value based on one or more other observables or plain JavaScript values. Computed observables are useful when you want to perform calculations or transformations based on the values of other observables in your view model. Here's how computed observables work and how they are used:
Creating a Computed Observable:
You can create a computed observable using the ko.computed function. Here's a basic example:
In this example, the fullName computed observable depends on firstName and lastName. When either firstName or lastName changes, fullName is automatically updated.
Automatic Dependency Tracking:
One of the key features of computed observables is automatic dependency tracking. Knockout.js keeps track of which observables are accessed within the computed observable's function. When any of these dependencies change, the computed observable is marked as "dirty," and its value is re-evaluated.
In this example, changing the value of a triggers an automatic update of the sum computed observable.
Read-Only vs. Read-Write Computed Observables:
Computed observables can be either read-only or read-write:
Read-Only: These computed observables are used for calculations and transformations, and you should not attempt to write to them directly.
Read-Write: If you need to manually control the value of a computed observable, you can provide both a read and a write function.
Using Computed Observables in Data Bindings:
You can use computed observables directly in your data bindings. When you bind to a computed observable, Knockout.js automatically updates the UI whenever the computed value changes.
Subscribing to Changes:
You can subscribe to changes in a computed observable using the subscribe function. This is useful when you need to perform additional actions when the computed value changes.
In summary, computed observables in Knockout.js provide a powerful mechanism for automatically updating values based on dependencies. They are particularly useful for performing calculations and transformations in a declarative and reactive manner within your view model.