Binding context

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 properties that you can reference in any binding:

• $parent

  This is the view model object in the parent context, the one immeditely outside the current context. In the root context, this is undefined. Example:

    <h1 data-bind="text: name"></h1>
  
    <div data-bind="with: manager">
        <!-- Now we're inside a nested binding context -->
        <span data-bind="text: name"></span> is the
        manager of <span data-bind="text: $parent.name"></span>
    </div>
  

• $parents

  This is an array representing all of the parent view models:

  $parents[0] is the view model from the parent context (i.e., it’s the same as $parent)

  $parents[1] is the view model from the grandparent context

  $parents[2] is the view model from the great-grandparent context

  … and so on.

• $root

  This is the main view model object in the root context, i.e., the topmost parent context. It is equivalent to $parents[$parents.length - 1].

• $data

  This is the view model object in the current context. In the root context, $data and $root are equivalent. $data is useful when you don’t want to reference a property on the viewmodel, but instead want to reference the viewmodel itself. Example:

    <ul data-bind="foreach: ['cats', 'dogs', 'fish']">
        <li>The value is <span data-bind="text: $data"></span></li>
    </ul>
  

• $index (only available within foreach bindings)

  This is the zero-based index of the current array entry being rendered by a foreach binding. Unlike the other binding context properties, $index is an observable and is updated whenever the index of the item changes (e.g., if items are added to or removed from the array).

• $parentContext

  This refers to the binding context object at the parent level. This is different from $parent, which refers to the data (not binding context) at the parent level. This is useful, for example, if you need to access the index value of an outer foreach item from an inner context (usage: $parentContext.$index). This is undefined in the root context.

The following special variables are also available in bindings, but are not part of the binding context object:

• $context

  This refers to the current binding context object. This may be useful if you want to access properties of the context when they might also exist in the view model, or if you want to pass the context object to a helper function in your view model.

• $element

  This is the element DOM object (for virtual elements, it will be the comment DOM object) of the current binding. This can be useful if a binding needs to access an attribute of the current element. Example:

    <div id="item1" data-bind="text: $element.id"></div>
  

Controlling or modifying the binding context in custom bindings

Just like the built-in bindings with and foreach, custom bindings can change the binding context for their descendant elements, or provide special properties by extending the binding context object. This is described in detail under creating custom bindings that control descendant bindings.