Setting a combined optionsText in knockout.js

Question

I want to combine two return values in my optionsText field, I thought this would be simple, but I am apparently wrong. I'm also using a value for the options field that is outside the normal view model. Here is the original:

<select style="width: 180px" data-bind="visible: false, value: fields.person, options: $root.extra.personList, optionsText: 'FirstName', optionsValue: 'LastName' }, select2: {}">

This works fine, displays the person's FirstName and sets the value as LastName.

This next piece throws an error:

<select style="width: 180px" data-bind="visible: false, value: fields.person, options: $root.extra.personList, optionsText: function (item) { return item.FirstName() + '-' + item.LastName() }, optionsValue: 'LastName', select2: {}">

Uncaught TypeError: Property 'FirstName' of object # is not a function

Answer 1

how optionsText works is it looks for the property 'FirstName' on the options array that is is bound to. So if your extra person array has a 'FirstName' and a 'LastName' you can extend a computed property to hold full name.

function ViewModel(){
    var self = this; 
    self.peopleList = ko.observableArray(); 
}
function Person(f, l){
    var self = this; 
    self.FirstName = ko.obserable(f); 
    self.LastName = ko.obserable(l); 
    self.FullName = ko.computed(function(){
        return self.FirstName() + ' ' + self.LastName();
    });
}

If they are not observables you don't want to be calling them [ what () does ] it would in that case I would just have a function living on the view model that puts them together which you call from the binding.

Answer 2

You sure FirstName and LastName are observables? If you're not sure, try this:

optionsText: function(item) { return ko.unwrap(item.FirstName) + '-' + ko.unwrap(item.LastName); }

Or better yet, create a computed in your viewmodel:

self.FullName = function() {
    return ko.unwrap(self.FirstName) + '-' + ko.unwrap(self.LastName);
};

And:

optionsText: 'FullName'