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Revati S Misra

Revati S Misra

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Aryan Kumar

Aryan Kumar

24-Nov-2023

In Knockout.js, the virtual element syntax is a way to conditionally include or exclude DOM elements in the UI based on the evaluation of an expression. This syntax uses the <!-- ko --> and <!-- /ko --> comment-based markers to define a block of HTML elements that are conditionally rendered. These markers indicate to Knockout.js that it should consider the enclosed HTML as part of its data-binding logic. Here's how the virtual element syntax is used:

Basic Syntax:

<!-- ko if: condition -->
   <!-- HTML elements to render if condition is true -->
<!-- /ko -->

<!-- ko ifnot: condition -->
   <!-- HTML elements to render if condition is false -->
<!-- /ko -->

Example:

<div>
    <p>This is always visible.</p>

    <!-- ko if: showAdditionalContent -->
        <p>This content is conditionally visible.</p>
    <!-- /ko -->

    <!-- ko ifnot: showAdditionalContent -->
        <p>This content is conditionally visible when showAdditionalContent is false.</p>
    <!-- /ko -->
</div>

Explanation:

<!-- ko if: condition --> and <!-- /ko -->:

  • These markers define a block of HTML elements that are conditionally rendered based on the specified condition.

<!-- ko ifnot: condition -->:

  • This marker is used when you want to conditionally render HTML elements when the specified condition is false.

condition:

  • Replace this with the actual condition that you want to evaluate. It can be an observable, a computed observable, or any expression that resolves to a truthy or falsy value.

Usage:

  • The virtual element syntax is often used in conjunction with observables or computed observables. When the condition changes and the observable updates, Knockout.js automatically reevaluates the virtual elements and updates the DOM accordingly.

Example with Observables:

<div>
    <p>This is always visible.</p>

    <!-- ko if: shouldShowContent -->
        <p>This content is conditionally visible.</p>
    <!-- /ko -->
</div>

function ViewModel() {
    this.shouldShowContent = ko.observable(true);
}

ko.applyBindings(new ViewModel());

In this example, the content inside the <!-- ko if: shouldShowContent --> block will be visible initially because the shouldShowContent observable is set to true. If you later set shouldShowContent(false), the content will be hidden, demonstrating the dynamic nature of the virtual element syntax.

The virtual element syntax in Knockout.js is a powerful tool for creating dynamic and responsive user interfaces based on changing data conditions.

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