Power Punch - Interactive Click Functionality

Question

Power Punch - Interactive Click Functionality

My question pertains to the click binding in Knockout and a specific behavior that I have observed. While there are numerous questions about click bindings on this platform, none seem to address the behavior I am encountering.

I have grasped that in Knockout, click bindings with parameters are executed during page load since Knockout needs a reference to a function. Therefore, when it encounters a function call within a binding, it executes that function expecting a returned function reference to bind to. Consequently, if I were to return another reference to a function, that particular function would be executed upon clicking the element.

So far, everything seems logical and clear.

To test this theory myself, I hastily created the following:

HTML:

<input data-bind="click: selectImportType(1)" type="button" />

JS

var functiontest = function () {
    alert('test');
};

viewModel.selectImportType = function (type) {
    viewModel.selectedImport(type);

    return functiontest;
}

Upon executing this code snippet, I noticed that the 'functiontest' reference is indeed returned to the binding, causing the functiontest function to be called as expected when the element is clicked.

However, here comes my confusion. I discovered that the selectImportType function is also invoked when the element is clicked. First, selectImportType is triggered followed by a call to the functiontest function.

How is this dual invocation possible? The reference resolved during the binding was specifically to the functiontest function!

javascript knockout.js data-binding

Answer 1

Answer №1

When you use the following code:

data-bind="click: selectImportType(1)"

The expression selectImportType(1) is evaluated, returning your functiontest function, which is then assigned to the click binding.

Why does this happen?

This behavior is by design. When Knockout parses a binding, it creates a bindings string evaluator that acts as a dynamic function:

function createBindingsStringEvaluator(bindingsString, options) {
  var rewrittenBindings = ko.expressionRewriting.preProcessBindings(bindingsString, options),
      functionBody = "with($context){with($data||{}){return{" + rewrittenBindings + "}}}";
  return new Function("$context", "$element", functionBody);
}

In this scenario, the function body becomes:

"with($context){with($data||{}){return{'click':function(){return selectImportType(1) }}}}"

After evaluation, the parsed bindings look like this:

{
  "click": function(){return selectImportType(1)}
}

which will then be passed to the bindginHandler in the following way:

var valueAccessor = getValueAccessor(bindingKey);
                                        ||
                                       click

Therefore, the valueAccessor becomes the function:

function(){return selectImportType(1)}

Subsequently, when looking at the event binding source code ( source link here), the valueAccessor is wrapped in newValueAccessor, and upon evaluation, you get the result of executing selectImportType(1).

Finally, eventsToHandle will be your functiontest:

function () {
    alert('test');
}

This explains why the functiontest function is invoked when clicking the button.

If you're aiming for a different outcome, consider using:

data-bind="click: selectImportType.bind($data, 1)"

With this approach, the selectImportType function will only be called after clicking on the button element.

For more information on passing parameters to functions within Knockout bindings, check out these resources:

Passing parameters to a function in knockoutjs viewmodel
Passing the model as argument in the link button click using KO

Answer 2

When you use the following code:

data-bind="click: selectImportType(1)"

The expression selectImportType(1) is evaluated, returning your functiontest function, which is then assigned to the click binding.

Why does this happen?

This behavior is by design. When Knockout parses a binding, it creates a bindings string evaluator that acts as a dynamic function:

function createBindingsStringEvaluator(bindingsString, options) {
  var rewrittenBindings = ko.expressionRewriting.preProcessBindings(bindingsString, options),
      functionBody = "with($context){with($data||{}){return{" + rewrittenBindings + "}}}";
  return new Function("$context", "$element", functionBody);
}

In this scenario, the function body becomes:

"with($context){with($data||{}){return{'click':function(){return selectImportType(1) }}}}"

After evaluation, the parsed bindings look like this:

{
  "click": function(){return selectImportType(1)}
}

which will then be passed to the bindginHandler in the following way:

var valueAccessor = getValueAccessor(bindingKey);
                                        ||
                                       click

Therefore, the valueAccessor becomes the function:

function(){return selectImportType(1)}

Subsequently, when looking at the event binding source code ( source link here), the valueAccessor is wrapped in newValueAccessor, and upon evaluation, you get the result of executing selectImportType(1).

Finally, eventsToHandle will be your functiontest:

function () {
    alert('test');
}

This explains why the functiontest function is invoked when clicking the button.

If you're aiming for a different outcome, consider using:

data-bind="click: selectImportType.bind($data, 1)"

With this approach, the selectImportType function will only be called after clicking on the button element.

For more information on passing parameters to functions within Knockout bindings, check out these resources:

Passing parameters to a function in knockoutjs viewmodel
Passing the model as argument in the link button click using KO

Power Punch - Interactive Click Functionality

Answer №1

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