The "value" binding
value binding links the associated DOM element’s value with a property on your view model. This is typically useful with form elements such as
When the user edits the value in the associated form control, it updates the value on your view model. Likewise, when you update the value in your view model, this updates the value of the form control on screen.
Note: If you’re working with checkboxes or radio buttons, use the
checked binding to read and write your element’s checked state, not the
KO sets the element’s
valueproperty to your parameter value. Any previous value will be overwritten.
If this parameter is an observable value, the binding will update the element’s value whenever the value changes. If the parameter isn’t observable, it will only set the element’s value once and will not update it again later.
If you supply something other than a number or a string (e.g., you pass an object or an array), the displayed text will be equivalent to
yourParameter.toString()(that’s usually not very useful, so it’s best to supply string or numeric values).
Whenever the user edits the value in the associated form control, KO will update the property on your view model. KO will always attempt to update your view model when the value has been modified and a user transfers focus to another DOM node (i.e., on the
changeevent), but you can also trigger updates based on other events by using the
valueUpdateparameter described below.
If your binding also includes a parameter called
valueUpdate, this defines additional browser events KO should use to detect changes besides the
changeevent. The following string values are the most commonly useful choices:
"input"- updates your view model when the value of an
<textarea>element changes. Note that this event is only raised by reasonably modern browsers (e.g., IE 9+).
"keyup"- updates your view model when the user releases a key
"keypress"- updates your view model when the user has typed a key. Unlike
keyup, this updates repeatedly while the user holds a key down
"afterkeydown"- updates your view model as soon as the user begins typing a character. This works by catching the browser’s
keydownevent and handling the event asynchronously.
Of these options,
"input"is the best choice if you want to keep your view model updated in real-time, and you only need to support reasonably modern browsers such as IE 9+ (whereas
"afterkeydown"is a good choice for older browsers). For example:
See Note 1 below. Note that
valueAllowUnsetis only applicable when using
valueto control selection on a
<select>element. On other elements it has no effect.
Note 1: Working with drop-down lists (i.e.,
Knockout has special support for drop-down lists (i.e.,
<select> elements). The
value binding works in conjunction with the
options binding or for handling multi-select lists, see the documentation for the
You can also use the
value binding with a
<select> element that does not use the
options binding. In this case, you can choose to specify your
<option> elements in markup or build them using the
template bindings. You can even nest options within
<optgroup> elements and Knockout will set the selected value appropriately.
Normally, when you use the
value binding on a
<select> element, it means that you want the associated model value to describe which item in the
<select> is selected. But what happens if you set the model value to something that has no corresponding entry in the list? The default behavior is for Knockout to overwrite your model value to reset it to whatever is already selected in the dropdown, thereby preventing the model and UI from getting out of sync.
However, sometimes you might not want that behavior. If instead you want Knockout to allow your model observable to take values that have no corresponding entry in the
<select>, then specify
valueAllowUnset: true. In this case, whenever your model value cannot be represented in the
<select>, then the
<select> simply has no selected value at that time, which is visually represented by it being blank. When the user later selects an entry from the dropdown, this will be written to your model as usual. For example:
In the above example,
selectedCountry will retain the value
'Latvia', and the dropdown will be blank, because there is no corresponding option.
valueAllowUnset had not been enabled, then Knockout would have overwritten
undefined, so that it would match the value of the
'Choose one...' caption entry.
Note 2: Updating observable and non-observable property values
If you use
value to link a form element to an observable property, KO is able to set up a 2-way binding so that changes to either affect the other.
However, if you use
If you reference a simple property, i.e., it is just a regular property on your view model, KO will set the form element’s initial state to the property value, and when the form element is edited, KO will write the changes back to your property. It cannot detect when the property changes (because it isn’t observable), so this is only a 1-way binding.
If you reference something that is not a simple property, e.g., the result of a function call or comparison operation, KO will set the form element’s initial state to that value, but it will not be able to write any changes back when the user edits the form element. In this case it’s a one-time-only value setter, not an ongoing binding that reacts to changes.
None, other than the core Knockout library.