Angular2 Custom Pipes on Two-Way Bound Input Controls

In my previous blog, I showed you how to create a custom pipe for phone numbers.  If you haven’t read it yet, I recommend that you do so before continuing with this article.  Click here for that blog. 

The custom pipe works great for formatting data in labels (i.e. one-way bound controls).

But what if you wanted to apply the custom pipe to an input control, and have it bind bi-directionally?  That is, the data from the data model is formatted in the view, and the data entered by the user is automatically validated and formatted as well?  What if you wanted to do something like the code below?

When you run this code, Angular2 will complain and throw a nasty exception like the one below:

The code is failing exactly for the reason that you see in the error message: You cannot use a pipe in an action expression, which is what the two-way binding of the input control is.  Double binding with pipes will not work.  Angular2 does not support it.

So, what do we do then?  Well, I’m glad you asked.

While we cannot do two-way data binding, we can still do one-way bindings.  We can break down what we are trying to do into two one-way data bindings, so that we format on the way out, and validate and format on the way in.  For this trick, we will need to listen to the Angular2 “ngModelChange” event for the input control. 

We filter the “ngModel” through our custom pipe, and we wire up our data model to the “ngModelChange” event.  Our code would now look like this:

Now your input control would exhibit behavior similar to a two-way-bound control, with the benefit of a pipe filter.  That's it.

I’d like to add a bonus tip.  If all you are looking for is to have the phone number formatted a certain way, and you did not want to take the trouble of building a custom pipe, then you can take advantage of HTML5’s new Phone Number Input Type Form Validation. 

This is simply an HTML5 input control with type=”tel” and a pattern attribute with the phone number regular expression that you would like to validate against.  You also have the option of adding a title, which would serve as a “hint” to the user.  It would require your user to comply with the pattern, but it would ensure that the input is valid.  Your markup code would now look like below:

When your user attempts to submit an invalid entry, the user would get a friendly error message and a hint.

And that is how you take advantage of HTML5’s built-in phone number input type form validation.

In this blog, I showed you how to make use of pipes in an input control that is intended to bind both ways.  I also showed an alternative way to validate two-way-bound input controls using HTML5 form validation.

Angular2 Custom Pipe for Phone

Angular2 pipes are great for formatting data labels.  And Angular2 already comes with a bunch of built-in pipes out-of-the-box, including the Date Pipe, Decimal Pipe, Currency Pipe, LowerCase and UpperCase Pipes, JSON Pipe, Percent Pipe, Slice Pipe, and Async Pipe.

“Why isn’t there a Pipe for telephone numbers?” you might ask.  Standardizing on phone number formats is notoriously tricky because, with all the different international phone number conventions, there are no standards.  So, if you need a Pipe for phone numbers, you need to build a custom Pipe yourself.  Unfortunately I could not find one on the web, so I wrote one myself.

You will need to import “Pipe” from Angular2 Core, give your new pipe a name, and implement or override the “Transform” function.

As you can see in the code, this Pipe will transform the input by a) stripping out any non-numeric characters, b) take only the first 10 valid characters, and c) apply the area code parenthesis and the hyphen separator.

To consume our Phone Pipe in our component, we would need to a) import the Pipe and b) declare it in the pipes array inside our Component decorator.

In our view, we apply the Phone Pipe as simply as below:

Regardless of the user’s input (entering invalid characters or entering more than 10 characters), the Phone Pipe will format it as you desired.

This Phone Pipe works great when you are just formatting a label.  But what if you wish to format the data in an input control?  Then we are talking about applying pipes to a control with two-way data binding, and that would be much trickier.

In this article, I showed you how to implement a custom Phone Pipe.  In my next blog, I will discuss how you can apply formatting and validation using pipes on an input control.