Filtering our to-do items

Now let's move on to adding functionality to allow users to filter their to-do items, so they can view active, completed, or all items.

Prerequisites: Familiarity with the core HTML, CSS, and JavaScript languages, knowledge of the terminal/command line.
Objective: To add filtering functionality to our app.

Our filtering code

Filtering items builds on the filter property, which you previously added to app.component.ts:

filter: 'all' | 'active' | 'done' = 'all';

The default value for filter is all, but it can also be active or done.

Adding filter controls

In app.component.html, add the following HTML below the Add button but above the section that lists the items. In the following snippet, the existing sections in your HTML are in comments so you can see exactly where to put the buttons.

<!-- <button class="btn-primary" (click)="addItem(newItem.value)">Add</button>

<!-- Buttons that show all, still to do, or done items on click -->
<div class="btn-wrapper">
    class="btn btn-menu"
    []="filter == 'all'"
    (click)="filter = 'all'">

    class="btn btn-menu"
    []="filter == 'active'"
    (click)="filter = 'active'">
    To Do

    class="btn btn-menu"
    []="filter == 'done'"
    (click)="filter = 'done'">

<!-- <h2>{{items.length}} item(s)</h2>
         <ul>... -->

Clicking the buttons changes the filter values, which determines the items that show as well as the styles that Angular applies to the active button.

  • If the user clicks the All button, all of the items show.
  • If the user clicks the To do button, only the items with a done value of false show.
  • If the user clicks the Done button, only the items with a done value of true show.

A class attribute binding, using square brackets, [], controls the text color of the buttons. The class binding, [], applies the active class when the value of filter matches the expression. For example, when the user clicks the Done button, which sets the filter value to done, the class binding expression of filter == 'done' evaluates to true. When the filter value is done, Angular applies the active class to the Done button to make the text color green. As soon as the user clicks on one of the other buttons, the value a filter is no longer done, so the green text color no longer applies.


That was quick! Since you already had the filter code in app.component.ts, all you had to do was edit the template in order to provide controls for filtering the items. Our next — and last — article looks at how to build your Angular app ready for production, and provides further resources to carry on your learning journey.