Combining Value Converters with i18n in Aurelia

Posted by AureliaEffect on March 17, 2017

Today, I'd like to introduce you to Peter Vermeulen, an Aurelia community member with some great thoughts on Aurelia Value Converters and i18n. Peter, take it away....

My name is Peter Vermeulen, a freelance Front-End engineer and UX/UI designer with several years of experience in enterprise software. I spend my time helping forward-thinking companies building software with a strong focus on experiences.

Most if not all projects at one point or another will have to provide multilingual support.

Adding i18n support in aurelia is as easy as adding the aurelia-i18n plugin and configuring it .

But as a project becomes larger, centralizing your logic into reusable components is the way to go.

This article describes a way of combining aurelia's value-converters with i18n, to allow reusing them in your locales.

A live preview can be found here .

0. Getting Started

Create a new aurelia project by running au new (assuming you are using the aurelia-cli ).

After this has completed, follow the steps at the i18n-with-aurelia hub section to configure your aurelia project for internationalization.

1. Adjust the i18n Configuration

Aurelia-i18n is a wrapper that does two things, and does them well:

  • It acts as a proxy around i18next to allow you to configure an aurelia application for i18next support.
  • It provides some syntactic sugar to allow for easier translations

The fact that it acts as a proxy, allows us to still hook into the i18next instance without too much hassle.

i18next provides interpolation support. This is a way of defining how the specific key-value pairs in your locale file should be processed.

Aurelia provides thin wrappers such as the df (dateFormat) value converter for hooking into this. But what if we wanted to add our own custom value converters into the mix?

A very interesting configuration option for i18next's interpolation feature is the format property. This provides us with a hook into the processing pipeline of i18next. Go ahead and extend your i18n configuration like so:

    return instance.setup({
      // ...
      interpolation : {
        format : function (value, format, lng) {
          const parts = format.split(':');
          //  Check if the value converter is registered as a resource
          const vc = aurelia.resources.valueConverters[parts.shift()];
          return vc ? vc.toView(value, : value;
      // ...

There's a couple of things going on here, let's use an example to explain this step by step.

2. Using Value Converters In Your Locales, An Example

Let's consider a value converter called formatDate. All this value converter does is receive a date and a format, and return a string representation of that date in the provided format. To make that easier, we're using the date manipulation library Moment.js

    import moment from 'moment';
    export class FormatDateValueConverter {
      toView (value, format) {
        return moment(value, 'YYYY-MM-DD').format(format);

To use this in your view you could have a setup like so:

    <span t="a" t-params.bind="{date: '2017-02-20}"></span>

And your locale file could look like this:

      "a" : "Today's date is : {{date, formatDate:MMMM D YYYY}}"

When executed, i18next will recognize that the key you're trying to translate contains a section that needs to be interpolated (because of the default prefix and suffix {{ ... }} in your locale file).

Make sure to visit the i18next docs section targeting the interpolation options for further info.

As such it will execute the interpolation.format function that we configured in the previous step. This function will receive a value, format and an optional lng parameter.

  • Value : The date that we passed; in this case it will be 2017-02-20
  • Format : Our locale file will define this as being formatDate:MMMM D YYYY

Our interpolation function will split the format on the : character to become ['formatDate', 'MMMM D YYYY'] where the first part (formatDate) would be the name of the value converter we want to use, and the second part (MMMM D YYYY) would be the format parameter our value converter requires. Our format function will then look for the value converter in aurelia's registered resources, and will execute the toView function with the provided parameters.

And behold, the eventual result would be "Today's date is : February 20 2017".

Take note

  • If no value converter was registered under the name that you're trying to format, it will simply render the value of the variable you passed to it.
  • This approach will only work if the value converter that you are trying to use is registered as a global resource.
  • Using this approach, you could easily pass in additional parameters to your value converter by doing the following : valueConverterName:param1:param2:param3


Extending aurelia-i18n is extremely easy, and i'd definitely encourage anyone to try this for him/herself.

For example try writing any of the following and see for yourselves:

  • A value converter to render out currency values using something like numeral !
  • A value converter to automatically render out a person's fullname, when provided with an object containing personal details
  • Automatically prefix the correct title (Miss/Mr./...) based on the locale
  • ... Anything you can imagine :)

But most of all, have fun and be creative!

If you'd like to get in touch with Peter, you can visit his company web site Valkyrie Studios or contact him on Github or through his personal Twitter handle dragovianDesign or his company Twitter handle valkyrie_be.