Passing $index and $data as arguments to function

Question

I am passing $index and $data to the change_model function. The function is expecting 2 parameters in the following order: (index, data).

From the viewModel I am passing click: $root.change_model.bind($data, $index()). Within the function index prints $data, and data prints index: values are reversed.

self.change_model = function(index, data) {
  self.patternSelectedIndex(index);
  selected_door = data.file;
  create_door();
};

<div data-bind="foreach: x.patterns">
    <div class="thumbnail" data-bind="css: { selected: $index() === $root.patternSelectedIndex() }">
      <img class='img model' style='width:164px;height:90px;padding:5px' data-bind="attr:{src:'images/models/' + $data.file + '.png'}, click: $root.change_model.bind($data, $index())" />
      <div class="caption">
        <span data-bind="text: $data.name"></span>
      </div>
    </div>
</div>

Answer 1

The first argument of bind will become this inside your function, because Knockout is merely using the regular bind function.

You can either pass $data or $root as the first (thisArg) argument, or pass null or undefined, as you don't really need it since you seem to use the self = this idiom.

For example:

var ViewModel = function () {
    var self = this;

    self.change_model = function (index, data) {
        console.log(this);
        console.log(index);
        console.log(data);
        // Actual code here
    };

    self.x = { patterns: [{ file: 'some-file', name: 'some-name' }] };
};

ko.applyBindings(new ViewModel());

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

<div data-bind="foreach: x.patterns">
    <button data-bind="click: $root.change_model.bind($data, $index(), $data)">Click me!</button>
    <span data-bind="text: $data.name"></span>
</div>