Question

I have this dropdown which have options if vehicle is new or used.

<select name="VehicleType" id="vehicleTypeDropdown" data-bind="value: VehicleType">    
    <option value="New" selected="selected">Nuevo</option>
    <option value="Used">Usado</option>
</select> `

And this input:

<input type="text" name="Mileage" data-bind="disable: VehicleType() === 'New',value:  Mileage" class="input-large"> `

If the value in the dropdown selected is New the input must be disabled, and if used the input should be enabled, but if I enter a value the observable will grab this value and if I change the dropdown value to new the observable must become zero.

Answer 1

A manual subscription in your view model is a good way to handle something like this. The subscription might look like:

this.VehicleType.subscribe(function(newValue) {
    if (newValue === "New") {
        this.Mileage(0);
    }
}, this);

Here is a sample using it: http://jsfiddle.net/rniemeyer/h4cKC/

The HTML:

<select name="VehicleType" id="vehicleTypeDropdown" data-bind="value: VehicleType">
    <option value="New" selected="selected">Nuevo</option> 
    <option value="Used">Usado</option> 
</select>

<input type="text" name="Mileage" data-bind="disable: VehicleType() === 'New', value: Mileage" class="input-large">
<hr/>

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

The JS:

var ViewModel = function() {
    this.VehicleType = ko.observable();
    this.Mileage = ko.observable();

    this.VehicleType.subscribe(function(newValue) {
        if (newValue === "New") {
            this.Mileage(0);
        }
    }, this);
};

ko.applyBindings(new ViewModel());