Purpose The text binding causes the associated DOM element to display the text value of your parameter. Typically this is useful with elements like <span> or <em> that traditionally display text, but technically you can use it with any element. Example Today's message is: <span data-bind="text: myMessage"></span> <script type="text/javascript"> var viewModel = { myMessage: ko.observable() // Initially blank }; viewModel.myMessage("Hello, world!

Note: This documentation applies to Knockout 3.4.0 and later. ko.onError Knockout wraps internal asynchronous calls and looks for an optional ko.onError callback to execute, if an exception is encountered, before throwing the original error. This gives you the opportunity to run custom logic, such as passing the error to a logging module. Additionally, since the original call is wrapped in a try/catch, the error passed to ko.onError contains a stack property, which is not true in many browsers

Purpose The hasFocus binding links a DOM element’s focus state with a viewmodel property. It is a two-way binding, so: If you set the viewmodel property to true or false, the associated element will become focused or unfocused. If the user manually focuses or unfocuses the associated element, the viewmodel property will be set to true or false accordingly. This is useful if you’re building sophisticated forms in which editable elements appear dynamically, and you would like to control where t

In a typical Knockout application, DOM elements are dynamically added and removed, for example using the template binding or via control-flow bindings (if, ifnot, with, and foreach). When creating a custom binding, it is often desirable to add clean-up logic that runs when an element associated with your custom binding is removed by Knockout. Registering a callback on the disposal of an element To register a function to run when a node is removed, you can call ko.utils.domNodeDisposal.addDispos

Purpose The submit binding adds an event handler so that your chosen JavaScript function will be invoked when the associated DOM element is submitted. Typically you will only want to use this on form elements. When you use the submit binding on a form, Knockout will prevent the browser’s default submit action for that form. In other words, the browser will call your handler function but will not submit the form to the server. This is a useful default because when you use the submit binding, it’

A binding context is an object that holds data that you can reference from your bindings. While applying bindings, Knockout automatically creates and manages a hierarchy of binding contexts. The root level of the hierarchy refers to the viewModel parameter you supplied to ko.applyBindings(viewModel). Then, each time you use a control flow binding such as with or foreach, that creates a child binding context that refers to the nested view model data. Bindings contexts offer the following special

Note: This rate-limit API was added in Knockout 3.1.0. For previous versions, the throttle extender provides similar functionality. Normally, an observable that is changed notifies its subscribers immediately, so that any computed observables or bindings that depend on the observable are updated synchronously. The rateLimit extender, however, causes an observable to suppress and delay change notifications for a specified period of time. A rate-limited observable therefore updates dependencies a

Pure computed observables, introduced in Knockout 3.2.0, provide performance and memory benefits over regular computed observables for most applications. This is because a pure computed observable doesn’t maintain subscriptions to its dependencies when it has no subscribers itself. This feature: Prevents memory leaks from computed observables that are no longer referenced in an application but whose dependencies still exist. Reduces computation overhead by not re-calculating computed observab

Purpose The event binding allows you to add an event handler for a specified event so that your chosen JavaScript function will be invoked when that event is triggered for the associated DOM element. This can be used to bind to any event, such as keypress, mouseover or mouseout. Example <div> <div data-bind="event: { mouseover: enableDetails, mouseout: disableDetails }"> Mouse over me </div> <div data-bind="visible: detailsEnabled"> Details

What if you’ve got an observable for firstName, and another for lastName, and you want to display the full name? That’s where computed observables come in - these are functions that are dependent on one or more other observables, and will automatically update whenever any of these dependencies change. For example, given the following view model class, function AppViewModel() { this.firstName = ko.observable('Bob'); this.lastName = ko.observable('Smith'); } … you could add a computed ob