KnockoutJS - Linking value from a input to a datalist value

Question

I'm generating a datalist options based on a ko observable.

function Company(company) {
    this.id = company.Id;
    this.name = company.Name;
    this.state = company.State.name;
}


var self = this;
self.companies = ko.observable();
$.get('...', {}, function(result) {

    ko.utils.arrayForEach(result, function(company) {
        self.companies.push(new Company(ko.toJS(company)));
    });
});

HTML:

<input type="text" data-bind="value: selectedCompany" list="companiesList" />
<datalist id="companiesList" data-bind="foreach: companies">
    <option data-bind="value: name, text: state"></option>
</datalist>

Ok, here's the deal: in the Company function, I store the Id from that company. What I want to do? Something that able me to link the Id value to selectedCompany variable instead the name? I want to display the name property, but I need to store the Id.

Is there a way to do this? Thank you all!

Answer 1

I want to display the name property, but I need to store the Id.

Since there is currently no Knockout binding for HTML5 <datalist>, I made one.

I tried to borrow as much as I could from the select binding, so there is support for options, optionsText and optionsValue. You can specify a target observable via value.

ko.bindingHandlers.datalist = (function () {
    function getVal(rawItem, prop) {
        var item = ko.unwrap(rawItem);
        return item && prop ? ko.unwrap(item[prop]) : item;
    }

    function findItem(options, prop, ref) {
        return ko.utils.arrayFirst(options, function (item) {
            return ref === getVal(item, prop);
        });
    }
    return {
        init: function (element, valueAccessor, allBindingsAccessor) {
            var setup = valueAccessor(),
                textProperty = ko.unwrap(setup.optionsText),
                valueProperty = ko.unwrap(setup.optionsValue),
                dataItems = ko.unwrap(setup.options),
                myValue = setup.value,
                koValue = allBindingsAccessor().value,
                datalist = document.createElement("DATALIST");

            // create an associated <datalist> element
            datalist.id = element.getAttribute("list");
            document.body.appendChild(datalist);

            // when the value is changed, write to the associated myValue observable
            function onNewValue(newVal) {
                var dataItems = ko.unwrap(setup.options),
                    selectedItem = findItem(dataItems, textProperty, newVal),
                    newValue = selectedItem ? getVal(selectedItem, valueProperty) : void 0;

                if (ko.isWriteableObservable(myValue)) {
                    myValue(newValue);
                }
            }

            // listen for value changes
            // - either via KO's value binding (preferred) or the change event
            if (ko.isSubscribable(koValue)) {
                koValue.subscribe(onNewValue);
            } else {
                ko.utils.registerEventHandler(element, "change", function () {
                    onNewValue(this.value);
                });
            }

            // init the element's value
            // - either via the myValue observable (preferred) or KO's value binding
            if (ko.isObservable(myValue) && myValue()) {
                element.value = getVal(findItem(dataItems, valueProperty, myValue()), textProperty);
            } else if (ko.isObservable(koValue) && koValue()) {
                onNewValue(koValue());
            }
        },
        update: function (element, valueAccessor) {
            var setup = valueAccessor(),
                datalist = element.list,
                dataItems = ko.unwrap(setup.options),
                textProperty = ko.unwrap(setup.optionsText);

            // rebuild list of options when an underlying observable changes
            datalist.innerHTML = "";
            ko.utils.arrayForEach(dataItems, function (item) {
                var option = document.createElement("OPTION");
                option.value = getVal(item, textProperty);
                datalist.appendChild(option);
            });
            ko.utils.triggerEvent(element, "change");
        }
    };
})();

I hope I got most of this right. Corrections and improvement suggestions are welcome.

You would use it like this:

<input list="company" data-bind="datalist: {
    options: companies, 
    optionsValue: 'id', 
    optionsText: 'name', 
    value: selectedCompany
}" />

Notes.

• There is no need to create a separate <datalist> in the HTML. The custom binding does that for you.
• You must, however, supply the list="" attribute of the <input> element. I found no way of dynamically setting that in JavaScript so far.
• value is optional, but if you supply it, it must be an observable.
• You can still use the original value binding outside of datalist. It will contain whatever text the <input> displays (no surprises there). However, writing to the built-in value also updates the datalist value, and the other way around.
• The datalist value has precedence and will overwrite the built-in value upon view model init. After that they stay in sync.
• options should be an array (plain old or observable) of objects — companies in this case).
• optionsText and optionsValue are strings that should map to properties on your options.
• You do not have to use optionsValue — in that case the entire object (a single company) would be stored in value. I would prefer that over storing only the ID.
• Currently the set-up reacts to the change event. That means your view model won't update until you leave the input field.
• Tested on Chrome 32, YMMV. As I said, comments and corrections are welcome.

Below is a demo, copied from the original fiddle.

function main() {
  function Company(company) {
      this.id = company.Id;
      this.name = company.Name;
      this.state = company.State.name;
  }

  function ViewModel(sampleData) {
      var self = this;

      self.companies = ko.observableArray();
      ko.utils.arrayForEach(sampleData, function (companyData) {
          self.companies.push(new Company(companyData));
      });

      // KO's "value" binding _can_ supply a start value
      self.val = ko.observable("Microsoft");

      // ... but it is overridden by datalist's "value" binding - in any case you can use both
      self.selectedCompany = ko.observable(1);
  }

  // -----------------------------------------------------------------------------------

  ko.applyBindings(new ViewModel([{
      Id: 1,
      Name: "Apple",
      State: {
          name: "California"
      }
  }, {
      Id: 2,
      Name: "Microsoft",
      State: {
          name: "Washington"
      }
  }, {
      Id: 3,
      Name: "IBM",
      State: {
          name: "New York"
      }
  }]));
}

// -----------------------------------------------------------------------------------
ko.bindingHandlers.datalist = (function () {
    function getVal(rawItem, prop) {
        var item = ko.unwrap(rawItem);
        return item && prop ? ko.unwrap(item[prop]) : item;
    }

    function findItem(options, prop, ref) {
        return ko.utils.arrayFirst(options, function (item) {
            return ref === getVal(item, prop);
        });
    }
    return {
        init: function (element, valueAccessor, allBindingsAccessor) {
            var setup = valueAccessor(),
                textProperty = ko.unwrap(setup.optionsText),
                valueProperty = ko.unwrap(setup.optionsValue),
                dataItems = ko.unwrap(setup.options),
                myValue = setup.value,
                koValue = allBindingsAccessor().value,
                datalist = document.createElement("DATALIST");

            // create an associated <datalist> element
            datalist.id = element.getAttribute("list");
            document.body.appendChild(datalist);

            // when the value is changed, write to the associated myValue observable
            function onNewValue(newVal) {
                var dataItems = ko.unwrap(setup.options),
                    selectedItem = findItem(dataItems, textProperty, newVal),
                    newValue = selectedItem ? getVal(selectedItem, valueProperty) : void 0;

                if (ko.isWriteableObservable(myValue)) {
                    myValue(newValue);
                }
            }

            // listen for value changes
            // - either via KO's value binding (preferred) or the change event
            if (ko.isSubscribable(koValue)) {
                koValue.subscribe(onNewValue);
            } else {
                ko.utils.registerEventHandler(element, "change", function () {
                    onNewValue(this.value);
                });
            }

            // init the element's value
            // - either via the myValue observable (preferred) or KO's value binding
            if (ko.isObservable(myValue) && myValue()) {
                element.value = getVal(findItem(dataItems, valueProperty, myValue()), textProperty);
            } else if (ko.isObservable(koValue) && koValue()) {
                onNewValue(koValue());
            }
        },
        update: function (element, valueAccessor) {
            var setup = valueAccessor(),
                datalist = element.list,
                dataItems = ko.unwrap(setup.options),
                textProperty = ko.unwrap(setup.optionsText);

            // rebuild list of options when an underlying observable changes
            datalist.innerHTML = "";
            ko.utils.arrayForEach(dataItems, function (item) {
                var option = document.createElement("OPTION");
                option.value = getVal(item, textProperty);
                datalist.appendChild(option);
            });
            ko.utils.triggerEvent(element, "change");
        }
    };
})();
main();

.hint {
    color: silver;
    font-size: 80%;
}

<script src="https://cdnjs.cloudflare.com/ajax/libs/knockout/3.0.0/knockout-min.js"></script>
<input list="company" type="text" placeholder="Select a company..." data-bind="value: val, datalist: {
    options: companies, 
    optionsValue: 'id',  /* try removing 'optionsValue' and see what happens to the view model */
    optionsText: 'name', 
    value: selectedCompany
}" />
<span class="hint">(note the "change" event occurs after the field loses focus!)</span>

<hr />
<p>View Model:</p>
<pre data-bind="text: ko.toJSON($root, null, 2)"></pre>