Extending Knockout's binding syntax using preprocessing

Note: This is an advanced technique, typically used only when creating libraries of reusable bindings or extended syntaxes. It’s not something you’ll normally need to do when building applications with Knockout.

Starting with Knockout 3.0, developers can define custom syntaxes by providing callbacks that rewrite DOM nodes and binding strings during the binding process.

Preprocessing binding strings

You can hook into Knockout’s logic for interpreting data-bind attributes by providing a binding preprocessor for a specific binding handler (such as click, visible, or any custom binding handler).

To do this, attach a preprocess function to the binding handler:

ko.bindingHandlers.yourBindingHandler.preprocess = function(stringFromMarkup) {
    // Return stringFromMarkup if you don't want to change anything, or return
    // some other string if you want Knockout to behave as if that was the
    // syntax provided in the original HTML

See later on this page for an API reference.

Example 1: Setting a default value for a binding

If you leave off the value of a binding, it’s bound to undefined by default. If you want to have a different default value for a binding, you can do so with a preprocessor. For example, you can allow uniqueName to be bound without a value by making its default value true:

ko.bindingHandlers.uniqueName.preprocess = function(val) {
    return val || 'true';

Now you can bind it like this:

<input data-bind="value: someModelProperty, uniqueName" />

Example 2: Binding expressions to events

If you’d like to be able to bind expressions to click events (rather than a function reference as Knockout expects), you can set up a preprocessor for the click handler to support this syntax:

ko.bindingHandlers.click.preprocess = function(val) {
    return 'function($data,$event){ ' + val + ' }';

Now you can bind click like this:

<button type="button" data-bind="click: myCount(myCount()+1)">Increment</button>

Binding preprocessor reference

  • ko.bindingHandlers.<name>.preprocess(value, name, addBindingCallback)

    If defined, this function will be called for each <name> binding before the binding is evaluated.


    • value: the syntax associated with the binding value before Knockout attempts to parse it (e.g., for yourBinding: 1 + 1, the associated value is "1 + 1" as a string).

    • name: the name of the binding (e.g., for yourBinding: 1 + 1, the name is "yourBinding" as a string).

    • addBinding: a callback function you can optionally use to insert another binding on the current element. This requires two parameters, name and value. For example, inside your preprocess function, call addBinding('visible', 'acceptsTerms()'); to make Knockout behave as if the element had a visible: acceptsTerms() binding on it.

    Return value:

    Your preprocess function must return the new string value to be parsed and passed to the binding, or return undefined to remove the binding.

    For example, if you return 'value + ".toUpperCase()"' as a string, then yourBinding: "Bert" would be interpreted as if the markup contained yourBinding: "Bert".toUpperCase(). Knockout will parse the returned value in the normal way, so it has to be a legal JavaScript expression.

    Don’t return non-string values. That wouldn’t make sense, because markup is always a string.

Preprocessing DOM nodes

You can hook into Knockout’s logic for traversing the DOM by providing a node preprocessor. This is a function that Knockout will call once for each DOM node that it walks over, both when the UI is first bound, and later when any new DOM subtrees are injected (e.g., via a foreach binding).

To do this, define a preprocessNode function on your binding provider:

ko.bindingProvider.instance.preprocessNode = function(node) {
    // Use DOM APIs such as setAttribute to modify 'node' if you wish.
    // If you want to leave 'node' in the DOM, return null or have no 'return' statement.
    // If you want to replace 'node' with some other set of nodes,
    //    - Use DOM APIs such as insertChild to inject the new nodes
    //      immediately before 'node'
    //    - Use DOM APIs such as removeChild to remove 'node' if required
    //    - Return an array of any new nodes that you've just inserted
    //      so that Knockout can apply any bindings to them

See later on this page for an API reference.

Example 3: Virtual template elements

If you commonly include template content using virtual elements, the normal syntax can feel a bit verbose. Using preprocessing, you can add a new template format that uses a single comment:

ko.bindingProvider.instance.preprocessNode = function(node) {
    // Only react if this is a comment node of the form <!-- template: ... -->
    if (node.nodeType == 8) {
        var match = node.nodeValue.match(/^\s*(template\s*:[\s\S]+)/);
        if (match) {
            // Create a pair of comments to replace the single comment
            var c1 = document.createComment("ko " + match[1]),
                c2 = document.createComment("/ko");
            node.parentNode.insertBefore(c1, node);
            node.parentNode.replaceChild(c2, node);

            // Tell Knockout about the new nodes so that it can apply bindings to them
            return [c1, c2];

Now you can include a template in your view like this:

<!-- template: 'some-template' -->

Preprocessing Reference

  • ko.bindingProvider.instance.preprocessNode(node)

    If defined, this function will be called for each DOM node before bindings are processed. The function can modify, remove, or replace node. Any new nodes must be inserted immediately before node, and if any nodes were added or node was removed, the function must return an array of the new nodes that are now in the document in place of node.

© Steven Sanderson, the Knockout.js team, and other contributors
Licensed under the MIT License.