【KnockoutJS】View で一時変数を使う ref: http://qiita.com/sukobuto/items/4b5af1a53bf18f81cf7d

variable-binding

ko.bindingHandlers['variable'] = {
    init: function(element, valueAccessor, allBindings, viewModel, bindingContext) {
        var variables = valueAccessor();
        for ( var v in variables ) {
            if (variables.hasOwnProperty(v)) {
                if (ko.isObservable(variables[v])) {
                    bindingContext[v] = variables[v];
                } else if (typeof variables[v] === 'function') {
                    if (ko.hasOwnProperty('defineProperty'))
                        ko.defineProperty(bindingContext, v, variables[v]);
                    else bindingContext[v] = ko.computed(variables[v]);
                } else {
                    bindingContext[v] = variables[v];
                }
            }
        }
    }
};
ko.virtualElements.allowedBindings['variable'] = true;

View

<div data-bind="variable: { foo: function(){ return doSomething(someProperty) } }">
    <!--ko if: foo == 'hoge'-->
    <span data-bind="text: foo"></span>
    <!--/ko-->
</div>