What is the difference between $root and $parent in Knockout?

They are similar but different:

• $root refers to the view model applied to the DOM with ko.applyBindings;
• $parent refers to the immediate outer scope;

Or, visually, from $data‘s perspective:

Or, in words of the relevant documentation:

• $parent: This is the view model object in the parent context, the one immeditely outside the current context.
• $root: This is the main view model object in the root context, i.e., the topmost parent context. It’s usually the object that was passed to ko.applyBindings. 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.

You’ll only see a practical difference if you have view models nested more than one level, otherwise they will amount to the same thing.

It benefit is rather simple to demonstrate:

var Person = function(name) {
  var self = this;
  self.name = ko.observable(name);
  self.children = ko.observableArray([]);
}
  
var ViewModel = function() {
  var self = this;
  self.name = 'root view model';
  self.mainPerson = ko.observable();
}

var vm = new ViewModel(),
    grandpa = new Person('grandpa'),
    daddy = new Person('daddy'),
    son1 = new Person('marc'),
    son2 = new Person('john');

vm.mainPerson(grandpa);
grandpa.children.push(daddy);
daddy.children.push(son1);
daddy.children.push(son2);

ko.applyBindings(vm);

th, td { padding: 10px; border: 1px solid gray; }

<script src="https://cdnjs.cloudflare.com/ajax/libs/knockout/3.2.0/knockout-min.js"></script>

<script type="text/html" id="person">
  <tr>
    <td data-bind="text: $root.name"></td>
    <td data-bind="text: $parent.name"></td>
    <td data-bind="text: $data.name"></td>
  </tr>
  <!-- ko template: { name: 'person', foreach: children } --><!-- /ko -->
</script>

<table>
  <tr>
    <th>$root</th>
    <th>$parent</th>
    <th>$data</th>
  </tr>
  <!-- ko template: { name: 'person', data: mainPerson } --><!-- /ko -->
</table>

The $root is always the same. The $parent is different, depending on how deeply nested you are.

Here is another good example on StackOverflow.

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