Question

While going through the examples of KnockoutJS, I saw the below code.

function WebmailViewModel() {
    // Data
    var self = this;
    self.folders = ['Inbox', 'Archive', 'Sent', 'Spam'];
    self.chosenFolderId = ko.observable();

    // Behaviours    
    self.goToFolder = function(folder) { self.chosenFolderId(folder); };    
};

ko.applyBindings(new WebmailViewModel());

I am not an expert in Javascript, but bit confused by the usage self.chosenFolderId(folder);

chosenFolderId is a property, and assigned ko.observable(); From the experience with other languages,

1. How can one invoke it by passing an argument like self.chosenFolderId(folder);
2. Where is folder defines?

If you can just point to an article which explains this that will do.

Thanks.

Answer 1

chosenFolderId is a property, but properties may be functions (and must be, in this case).

So ko.observable returns a function that takes a single argument (the folder).

It’s no different than the next line:

self.goToFolder = function(folder) { ... };

where the goToFolder property is being set to a function.

folder itself is “defined” as a parameter as goToFolder‘s parameter. Whatever calls goToFolder provides a value for folder.