snappymail/vendors/knockout/src/binding/defaultBindings/foreach.js

// "foreach: someExpression" is equivalent to "template: { foreach: someExpression }"
// "foreach: { data: someExpression, afterAdd: myfn }" is equivalent to "template: { foreach: someExpression, afterAdd: myfn }"
ko.bindingHandlers['foreach'] = {
    makeTemplateValueAccessor: valueAccessor => {
        return () => {
            var modelValue = valueAccessor(),
                // Unwrap without setting a dependency here
                unwrappedValue = ko.isObservable(modelValue) ? modelValue.peek() : modelValue;

            // If unwrappedValue is the array, pass in the wrapped value on its own
            // The value will be unwrapped and tracked within the template binding
            // (See https://github.com/SteveSanderson/knockout/issues/523)
            if ((!unwrappedValue) || typeof unwrappedValue.length == "number")
                return { 'foreach': modelValue };

            // If unwrappedValue.data is the array, preserve all relevant options and unwrap again value so we get updates
            ko.utils.unwrapObservable(modelValue);
            return {
                'foreach': unwrappedValue['data'],
                'as': unwrappedValue['as'],
                'beforeRemove': unwrappedValue['beforeRemove']
            };
        };
    },
    'init': (element, valueAccessor) =>
        ko.bindingHandlers['template']['init'](element, ko.bindingHandlers['foreach'].makeTemplateValueAccessor(valueAccessor))
    ,
    'update': (element, valueAccessor, allBindings, viewModel, bindingContext) =>
        ko.bindingHandlers['template']['update'](element, ko.bindingHandlers['foreach'].makeTemplateValueAccessor(valueAccessor), allBindings, viewModel, bindingContext)
};
ko.expressionRewriting.bindingRewriteValidators['foreach'] = false; // Can't rewrite control flow bindings
ko.virtualElements.allowedBindings['foreach'] = true;