Knockout firing click binding on applyBindings

The reason is, that you're indeed invoking the preview function (because writing functionName means referring to the function, writing functionName() means calling it).

So data-bind="click: Report.preview" would be working as expected, but without handing over the parameter.

As the manual states (on a different topic, but this still applies):

If you need to pass more parameters, one way to do it is by wrapping your handler in a function literal that takes in a parameter, as in this example:

<button data-bind="click: function(data, event) { myFunction(data, event, 'param1', 'param2') }">
    Click me
</button>

or in your case:

data-bind="click: function() { Report.preview('url/to/report') }"

Another solution would be to make preview() return a function (pretty much the same thing actually):

preview: function(path) {
    return function() {
        // ...
    }
}

Another solution is to use 'bind' construct:

data-bind="click: Report.preview.bind($data, 'url/to/report')" 

where the first parameter to bind() will become the 'this' in the called function.