The first version of Vue came bundled with some text filters that helped format text and solve some common problems.

In this new version, there are no built-in filters (except the equivalent of the JSON filter covered in the next recipe). I think this is because it's very easy to write your own filter and also very easy to find online libraries that do a much better job in specialized situations. Finally, filters have somewhat changed purpose: they are more for post-processing now and less for actual filtering and sorting arrays.

To demonstrate how easy it is to create a filter, we will recreate a filter of the old version of Vue: capitalize.

You don't need any particular knowledge to complete this recipe.

Sometimes we have some strings floating around in our variables like labels. When we put them in the middle of a sentence they work fine, but on the other hand they don't look very good at the beginning of a sentence or bullet point.

We want to write a filter that will capitalize whatever string we put into it. If, for example, we want the string hello world to start with a capital H, we'd like to be able to write:

{{'hello world' | capitalize }}

If we try to run this as HTML in a Vue app, it will complain [Vue warn]: Failed to resolve filter: capitalize.

Let's create the filter and add it to Vue's internal list of filters:

1. Write the following JavaScript to register a filter and instantiate Vue:

        Vue.filter('capitalize', function (string) { 
          var capitalFirst = string.charAt(0).toUpperCase() 
          var noCaseTail = string.slice(1, string.length) 
            return capitalFirst + noCaseTail 
        }) 
        new Vue({el:'#app'})

2. In the HTML section, write:

        {{'hello world' | capitalize }}

3. Run your code and notice how the text now reads Hello world.

The pipe sign indicates that the following is the name of a filter; in our case capitalize is not in Vue's list of filters, hence the warning. Vue will print the string as is.

What Vue will do before even starting is register our filter (with Vue.filter) in its asset library. Vue has an the internal filters object and will create a new entry: capitalize. Every time it sees the pipe symbol it will look for a corresponding filter. Remember to write it before the actual instantiation of a Vue instance because otherwise Vue will not find it.

The working of the filter is very basic JavaScript, in fact, a better way to write this filter with ES6 would be:

Vue.filter('capitalize', function (string) { 
  var [first, ...tail] = string 
  return first.toUpperCase() + tail.join('') 
})

If you are not familiar with ES6, here is a brief explanation. The second line is called a destructuring assignment of string; in particular we are interpreting string as an array of characters, separating the first character into first and putting all the other characters in tail. This is a faster way to assign different parts of an array to multiple variables. The other thing that may seems mysterious is that join(''). Since tail is now an array of characters, we need some means to re-join the single letters into a compact string. The argument of join represents a separator between the single characters. We don't want any, so we pass an empty string.

In the next chapter, you will find more recipe for filters and cover other real use cases.