Getting started

  1. How KO works and what benefits it brings
  2. Downloading and installing


  1. Creating view models with observables
  2. Working with observable arrays

Computed observables

  1. Using computed observables
  2. Writable computed observables
  3. How dependency tracking works
  4. Pure computed observables
  5. Reference


Controlling text and appearance

  1. The visible binding
  2. The text binding
  3. The html binding
  4. The css binding
  5. The style binding
  6. The attr binding

Control flow

  1. The foreach binding
  2. The if binding
  3. The ifnot binding
  4. The with binding
  5. The component binding

Working with form fields

  1. The click binding
  2. The event binding
  3. The submit binding
  4. The enable binding
  5. The disable binding
  6. The value binding
  7. The textInput binding
  8. The hasFocus binding
  9. The checked binding
  10. The options binding
  11. The selectedOptions binding
  12. The uniqueName binding

Rendering templates

  1. The template binding

Binding syntax

  1. The data-bind syntax
  2. The binding context

Creating custom bindings

  1. Creating custom bindings
  2. Controlling descendant bindings
  3. Supporting virtual elements
  4. Custom disposal logic
  5. Preprocessing: Extending the binding syntax


  1. Overview: What components and custom elements offer
  2. Defining and registering components
  3. The component binding
  4. Using custom elements
  5. Advanced: Custom component loaders

Further techniques

  1. Loading and saving JSON data
  2. Extending observables
  3. Deferred updates
  4. Rate-limiting observables
  5. Unobtrusive event handling
  6. Using fn to add custom functions
  7. Microtasks
  8. Asynchronous error handling


  1. The mapping plugin

More information

  1. Browser support
  2. Getting help
  3. Links to tutorials & examples
  4. Usage with AMD using RequireJs (Asynchronous Module Definition)

The "css" binding


The css binding adds or removes one or more named CSS classes to the associated DOM element. This is useful, for example, to highlight some value in red if it becomes negative.

(Note: If you don’t want to apply a CSS class but instead want to assign a style attribute value directly, see the style binding.)

Example with static classes

<div data-bind="css: { profitWarning: currentProfit() < 0 }">
   Profit Information

<script type="text/javascript">
    var viewModel = {
        currentProfit: ko.observable(150000) // Positive value, so initially we don't apply the "profitWarning" class
    viewModel.currentProfit(-50); // Causes the "profitWarning" class to be applied

This will apply the CSS class profitWarning whenever the currentProfit value dips below zero, and remove that class whenever it goes above zero.

Example with dynamic classes

<div data-bind="css: profitStatus">
   Profit Information

<script type="text/javascript">
    var viewModel = {
        currentProfit: ko.observable(150000)

    // Evalutes to a positive value, so initially we apply the "profitPositive" class
    viewModel.profitStatus = ko.pureComputed(function() {
        return this.currentProfit() < 0 ? "profitWarning" : "profitPositive";
    }, viewModel);

    // Causes the "profitPositive" class to be removed and "profitWarning" class to be added

This will apply the CSS class profitPositive when the currentProfit value is positive, otherwise it will apply the profitWarning CSS class.


  • Main parameter

    If you are using static CSS class names, then you can pass a JavaScript object in which the property names are your CSS classes, and their values evaluate to true or false according to whether the class should currently be applied.

    You can set multiple CSS classes at once. For example, if your view model has a property called isSevere,

    <div data-bind="css: { profitWarning: currentProfit() < 0, majorHighlight: isSevere }">

    You can even set multiple CSS classes based on the same condition by wrapping the names in quotes like:

    <div data-bind="css: { profitWarning: currentProfit() < 0, 'major highlight': isSevere }">

    Non-boolean values are interpreted loosely as boolean. For example, 0 and null are treated as false, whereas 21 and non-null objects are treated as true.

    If your parameter references an observable value, the binding will add or remove the CSS class whenever the observable value changes. If the parameter doesn’t reference an observable value, it will only add or remove the class once and will not do so again later.

    If you want to use dynamic CSS class names, then you can pass a string that corresponds to the CSS class or classes that you want to add to the element. If the parameter references an observable value, then the binding will remove any previously added classes and add the class or classes corresponding to the observable’s new value.

    As usual, you can use arbitrary JavaScript expressions or functions as parameter values. KO will evaluate them and use the resulting values to determine the appropriate CSS classes to add or remove.

  • Additional parameters

    • None

If you want to apply the CSS class my-class, you can’t write this:

<div data-bind="css: { my-class: someValue }">...</div>

… because my-class isn’t a legal identifier name at that point. The solution is simple: just wrap the identifier name in quotes so that it becomes a string literal, which is legal in a JavaScript object literal. For example,

<div data-bind="css: { 'my-class': someValue }">...</div>


None, other than the core Knockout library.