Bindings in KnockoutJS - Part II

Part I explained the bindings that control the appearance and style. Part II explains the control statements to control the rendering of the HTML markups.

KnockoutJS provides the following control statements:
  1. foreach
  2. if
  3. ifnot
  4. with
The following explains each one of the bindings.
The foreach bindings
The "foreach" binding renders the specific section of HTML markup repeatedly for the each item of a specified array. The following is the syntax and parameters for the foreach binding.
<b>foreach binding </b>

    <ul data-bind="foreach: { data: people, as: 'person', includeDestroyed: true, afterRender: yellowFadeIn }">
          <li data-bind="text:"></li>
    <a class="link" data-bind="click: addPerson">Add Item</a>

In the preceding markup, we have the foreach binding associated with the ul element. We will see the options passed as an object with the foreach binding. KnockoutJS renders the li element repeatedly depending on the observable array items. Each item will be bound with the respective li element.

  • data - Should be a collection,  in other wods an array or observable array. 
  • as - Refers to the current item of the array or observable array. The value assigned to 'as' should be a string literal.
  • includeDestroyed - Should be true or false. If it is true then the deleted item will be removed from the rendered markup but not from the collection bound.
  • afterRender/afterAdd/beforeRemove/beforeMove/afterMove - Should be a callback function and will be executed corresondingly.
In our foreach binding example, if you click on the "Add Item" link then an item will be added into the observableArray and the same will be rendered on the HTML page. Since we have a "afterRender" callback bound with the ul element, the callback will be executed after the render.
The if binding
The "if" binding adds/removes the associated element based on the value/expression assigned to the binding. If the value is true or true-ish, then the element will be added to the parent node else removed from the DOM.
<b>if binding</b>
    <p> <inputtype="checkbox"data-bind="checked: isChecked"/>Check/Uncheck the checkbox to add/remove Hello node </p>
    <pdata-bind="if: isChecked">Hello.... You have clicked the checkbox </p>  
In the preceding markup, if you click on the checkbox then the paragraph node will be added to the DOM. If you uncheck it then the paragraph will be removed from the DOM.
The ifnot binding
The "ifnot" binding is same as the if binding but it inverts the supplied value. 
<b>if binding</b>
<p> <input type="checkbox" data-bind="checked: isChecked" />Check/Uncheck the checkbox to add/remove Hello node </p>
<p data-bind="ifnot: isChecked">Hello.... You have clicked the checkbox </p>  

The with binding
The "with" binding will create a new binding context for the descendant elements of the associated element. If the value supplied to the with binding is null or undefined then the underlying descendant elements will not be added; it will be removed from the DOM node.
    <divdata-bind="with: withBinding">
        <p>Name:<spandata-bind="text: name"></span></p>
        <p>College:<spandata-bind="text: college"></span></p>


If the with Binding value is null or undefined then the two paragraphs will be removed from the parent div. If you supply a valid object with the name and college property then the Name and College node will be rendered with the supplied value.
So we had an overview of control statement bindings in KnockoutJS.

Reference: KnockoutJS

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