Knockout: `this` context with computed observables and subscribe

Published on May 29, 2017 by Jamie Munro

In JavaScript, the `this` variable inside a function (like a computed observable or a Knockout subscribe) function can be an extremely useful variable to access related properties to your observable.  Learning the following behavior has opened many doors for me when leveraging the Knockout.js framework.

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Tags: KnockoutJS | knockoutjs | this

Convert the (un)check all to a #KnockoutJS Component

Published on Apr 29, 2015 by Jamie Munro

Problem


You've created a nice feature leveraging Knockout.js and now you want to re-use this feature on another page, another site, or in a slightly different fashion. This example will extend the previous (Un)Check All using #KnockoutJS and make it easily re-usable.

Solution


A Knockout component can be created using a mixture of HTML (with data bindings) and a Knockout ViewModel. By altering the previous check all example and making it slightly more re-usable, it can be easily added to other pages with minimal effort. If you are not already familiar with the example being extended, please take a minute and give the (Un)Check All using #KnockoutJS a quick read.

Once the component is created, it can be included on any page with this simple HTML:
<checkall params="items: items, selectedItems: selectedItems"></checkall>

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Tags: KnockoutJS | knockoutjs | component

Replacing Radio Buttons with a Button Group using Bootstrap and #KnockoutJS

Published on Apr 27, 2015 by Jamie Munro

Problem


Radio buttons are hard-to-see, not easy to select, and let's face it, quite mundane. You would like to replace these radio buttons with a group of buttons that represent the same functionality, e.g. only one of the options may be selected at any given time.

Solution


Leveraging Bootstrap which provides many incredibly styled components for buttons, alert boxes, tables, forms, etc... regular radio buttons will be replaced by a button group (see screenshot below). Knockout.js will be used to create a custom data binding that will make the group of buttons act like regular radio buttons (with a nicer look of course).

radiobuttongroup

This example assumes you have a basic understanding of both Bootstrap and Knockout.js. If you don't, feel free to explore my latest book ASP.NET MVC 5 with Bootstrap and Knockout.js for a great introduction, plus many more examples.

The versions used for this example are 3.3.4 for Bootstrap and 3.3 for Knockout.js. This example should be compatible with older versions of these frameworks.

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Tags: KnockoutJS | knockoutjs | Bootstrap | bootstrap | radio buttons | buttons

(Un)Check All using #KnockoutJS

Published on Apr 23, 2015 by Jamie Munro

When I wrote Knockout.js: Building Dynamic Client-Side Web Applications I was trying to focus on demonstrating specific things, such as custom bindings, extending observables, etc. Unfortunately this didn't leave room for what I would call "random" examples of things that I do on a semi-regular basis. This blog post will demonstrate how to create a (un)check all list of checkboxes.

If I haven't said it before, examples like this are why I love working with Knockout.js on a daily basis. This example is accomplished in under 50 lines of code, with most of it being whitespace for readability!

The following example assumes you understand how to install Knockout.js and have a brief understanding of Knockout ViewModels. If you are looking for a good introduction *cough* *cough*, my book does an excellent job of it ;)

Problem


You have a list of elements - such as emails or any other list of data - and you want to provide the user a one-click button to select (or unselect) all items in the list. A nice added bonus, you want to automatically update the "global" checkbox indicating when all items are checked (and unchecking it when all items are not checked).

Solution


This problem can be solved by leveraging a computed observable. With Knockout.js, computed observables are re-evaluated each time an observable variable changes in the computed function. To avoid a circular reference, a writable computed observable will be used to force the (un)checking of all items.

This example is using Knockout.js version 3.3; however, it should be compatible with older versions as well.

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Tags: KnockoutJS | knockoutjs

Knockout - Uncaught ReferenceError: Unable to process binding

Published on Oct 27, 2014 by Jamie Munro

If you've used Knockout.js in your project, at some point or another you've probably encountered the following error "Uncaught ReferenceError: Unable to process binding". The most likely cause is a typo somewhere or forgetting to change the context. E.g. you are within a foreach binding and forget to use $parent.

I recently encountered this issue and could not find the typo anywhere. Like most developers in my situation I was getting really frustrated, banging my head against the world looking for answers.

Unfortunately, there is no easy answer to this bug; however, I can suggest an excellent technique that will hopefully help point you in the right direction.

Here is a complete example that creates a table and displays a list of books within the table. There is an unfortunate typo within one of the td data bindings.

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Tags: Javascript | KnockoutJS | knockoutjs | data binding | debugging