Unique ids in knockout.js templates

Question

Suppose I have knockout.js template like this:

<script type="text/html" id="mytemplate">
    <label for="inputId">Label for input</label>
    <input type="text" id="inputId" data-bind="value: inputValue"/>
</script>

If I render this template in several places on the page I end up with several inputs with the same id (and several labels with the same for value), which has bad consequences. In particular, all code that depends on ids may not work properly (in my case I use jquery.infieldlabel plugin that gets confused by multiple inputs with the same id). The way I solve this issue now is I add unique id attribute to the model that I bind to the template:

<script type="text/html" id="mytemplate">
    <label data-bind="attr: {for: id}>Label for input</label>
    <input type="text" data-bind="attr: {id: id}, value: inputValue"/>
</script>

This works, but it's not very elegant since I have to have this artificial id attribute in my models that is not used for anything else. I wonder if there is a better solution here.

Answer 1

An alternative that does not rely on the order that the fields are bound is to have the binding set an id property on the data itself, which would need to be an observable.

ko.bindingHandlers.uniqueId = {
  init: function(element, valueAccessor) {
    var value = valueAccessor();
    value.id = value.id || ko.bindingHandlers.uniqueId.prefix + (++ko.bindingHandlers.uniqueId.counter);

    element.id = value.id;
  },
  counter: 0,
  prefix: "unique"
};

ko.bindingHandlers.uniqueFor = {
  init: function(element, valueAccessor) {
    var value = valueAccessor();
    value.id = value.id || ko.bindingHandlers.uniqueId.prefix + (++ko.bindingHandlers.uniqueId.counter);

    element.setAttribute("for", value.id);
  }
};

var viewModel = {
  items: [{
      name: ko.observable("one")
    },
    {
      name: ko.observable("two")
    },
    {
      name: ko.observable("three")
    }
  ]
};

ko.applyBindings(viewModel);

<script src="https://cdnjs.cloudflare.com/ajax/libs/knockout/3.4.2/knockout-min.js"></script>

<ul data-bind="foreach: items">
  <li>
    <label data-bind="uniqueFor: name">Before</label>
    <input data-bind="uniqueId: name, value: name" />
    <label data-bind="uniqueFor: name">After</label>
  </li>
</ul>

(a JSFiddle sample is also available)

The nice thing about adding a property to the observable function is that when you turn it into JSON to send back to the server, then it will just naturally disappear as the observable will just turn into its unwrapped value.