REPEAT binding for Knockout

The repeat binding can replace foreach in many instances and is faster and simpler for some tasks. In contrast to foreach:

• repeat does not create a new binding context. Therefore, the variables for the binding context ($data, $parent, and $parents) are unaffected. Instead, you can access the current item using $item() and $index.
• repeat operates on the outer HTML instead of the inner HTML.
• repeat can either loop a number of times (count) or iterate over an array or observableArray (foreach).
• repeat avoids excessive re-rendering of DOM nodes by updating only the child bindings when the view model changes.

Here’s a comparison between foreach and repeat for a data table:

<table>
    <tbody data-bind="foreach: data">
        <tr data-bind="foreach: $parent.columns">
            <td data-bind="text: $parent[$data.propertyName]"></td>
        </tr>
    </tbody>
</table>

<table>
    <tbody>
        <tr data-bind="repeat: { foreach: data, item: '$row' }">
            <td data-bind="repeat: { foreach: columns, item: '$col' }"
                data-repeat-bind="text: $row()[$col().propertyName]"></td>
        </tr>
    </tbody>
</table>

Main Parameters

The repeat binding accepts a single parameter of the number of repetitions or an array to iterate. It also accepts an object literal with these parameters provided through the count or foreach property. If the parameter is an observable, the repeat binding will add or remove elements whenever you update it. Here are the main parameters:

• foreach — an array (or observableArray) over which to iterate, or the number of repetitions
• count — the number of repetitions. If both the foreach and (non-zero) count parameters are given, the count value takes precedence. This allows you to provide an array using foreach but always output a fixed number of items, even if it’s larger than the array length.
• limit — an upper limit on the number of repetitions, if non-zero (optional)

The following optional parameters do not support updates (and can’t be observable):

• reverse — if true, the elements will be repeated in reverse order, with the lowest value at the bottom and items added to the top (default is false)
• step — the increment value (default is 1)
• index — the name of the index context property (default is $index; see section below)
• item — the name of the context property used to access the item in the array (default is $item; see section below)
• bind — the binding used for the repeated elements (see section below)

Context Properties

The repeat binding makes the following context properties available to bindings in the repeated nodes.

• $index — the zero-based index of the current item. The name of this property can be changed using the index option.
• $item — the array item matching the current index. This property in available only when an array is supplied to the repeat binding. It is a pseudo-observable, which can be passed directly to bindings that accept observables (most do), including two-way bindings; or the item value can be accessed using observable syntax: $item(). The name of this property can be changed using the item option.

Repeated Element Binding

The repeat binding allows you to specify the binding for the repeated elements in a number of ways. Note that you cannot do this in the normal way you set additional bindings for an element—for example, <span data-bind="repeat: 5, text: $index"> will not set the text of the repeated elements and will probably generate an error.

1. The simplest and cleanest way is to use a data-repeat-bind attribute, which becomes the data-bind attribute of the repeated elements.

   <span data-bind="repeat: 5" data-repeat-bind="text: $index">

2. Similarly, you can specify a binding string using the bind parameter to repeat.

   <span data-bind="repeat: { count: 5, bind: 'text: $index' }">

3. If you are using a custom binding provider that doesn’t support the standard binding method of using a data-bind attribute, you can specify the binding for repeated elements using a function provided through the bind parameter to repeat. If using this option with foreach, the first parameter to the function is the item and the second is the index. If used with just count, the first parameter is the index. In both cases, the last parameter to the function is the binding context object, which is useful if you want to access additional context properties such as $parent.

   <span data-bind="repeat: { count: 5, bind: function($index) { return { text: $index } } }">

   <div data-bind="repeat: { foreach: availableCountries, item: '$country',
       bind: function($country) { return { css: { sel: $country() == selectedCountry() } } } }">
       <span data-bind="text: $index+1"></span>. <span data-bind="text: $country"></span>
   </div>

License and Contact

License: MIT (http://www.opensource.org/licenses/mit-license.php)

Michael Best
https://github.com/mbest
mbest@dasya.com