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Templates


There are three major changes to templates in Octane compared to classic Ember:

  1. Angle Bracket Syntax
  2. Named Arguments
  3. Required this

Angle Bracket Syntax

When you are using a component in a template, you can invoke it using Angle Brackets (<...>) instead of curly braces ({{...}}). The component itself will work the same as it did before.

{{!-- Before --}}
{{#todo-list as |item|}}
  {{to-do item=item}}
{{/todo-list}}

{{!-- After --}}
<TodoList as |item|>
  <Todo @item={{item}} />
</TodoList>

Benefits of Angle Brackets

Angle Brackets have a number of benefits:

  • Single word component names are completely OK in angle bracket form.

  • It is clear to your collaborators you are using a component and not a helper in a template.

  • Standard attribute values applied to the component are treated like plain-old HTML attributes. This means you can assign any valid HTML attribute, and it will be reflected onto the component directly:

  <Todo
    role="list-item"
    data-test-todo-item
    data-test-id={{this.todo.id}}
    class="todo {{this.todoClass}}"
  />

As you can see, both literal and bound values can be set on attributes, and attributes can be used without setting a value at all, just like HTML attributes. The component you are invoking decides where to put these attributes by using the special ...attributes syntax. This will be discussed later in the section on components.

For classic components, only attributes that were explicitly listed by the component you are invoking would be placed on the component's wrapper element.

  • Arguments and attributes are distinguished from each other when using a component. With curly brace style components, every value you pass to the component is an argument - a JS value that is passed to the component class so it can be used in the component's template:

    {{!-- In this example, `value` is an argument --}}
    {{custom-input value=this.text}}
    

    With angle brackets, since you can pass standard HTML attributes to the component directly, we need a way to distinguish between those and the component's arguments. To do this, we use the @ symbol:

    <CustomInput @value={{this.text}} />
    

    This allows you to see at a glance whether a value is an argument, which will likely affect the JS of a component, or an attribute, which will likely affect the HTML of a component.

Getting used to Angle Brackets

Here are the main differences between angle bracket and curly syntax:

  • The component name is in CapitalCase instead of kebab-case. {{my-component}} becomes <MyComponent />.

  • Components open and close in the same way as HTML elements. Components that do not accept a block can use the self closing syntax (a trailing slash) just like <img /> or other tags.

  • Arguments are passed by adding @ to the front of the argument name:

    {{!-- Before --}}
    {{todo-item item=item}}
    
    {{!-- After --}}
    <Todo @item={{this.item}} />
    
  • When you pass a bound value to a component, remember that it needs to be wrapped in curly braces:

    <Todo @done={{this.isDone}} />
    

    Like HTML, all values for attributes that are not wrapped in strings are coerced to strings. If you want to pass a boolean or number to a component and not have it coerced to a string, wrap it in curly braces:

    <Todo @done={{false}} maxItems={{10}} />
    
  • Yielded values work the same as in curly invocation:

    <TodoList as |item|>
      <Todo @item={{item}} />
    </TodoList>
    
  • Yielded components can also be invoked with angle bracket syntax:

    <TodoList as |Item|>
    <Item />
    </TodoList>
    
  • Positional arguments (e.g. {{my-component this.someValue}}) are not available in angle bracket invocation, since there is some ambiguity between their behavior and the behavior of standard HTML attributes (HTML attributes without = default to truthy). If you still need positional arguments, you must use the component with curly bracket syntax.

    If you are updating a classic component to use angle bracket syntax, you can also overwrite the parameter array with a named argument instead. For instance, if my-greeting had the following implementation:

    import Component from '@ember/component';
    
    export default Component.extend({
    }).reopenClass({
    positionalParams: 'params'
    });
    

    To invoke it using angle bracket syntax, you would do the following:

    <MyGreeting @params={{array "Hello" "World"}}>
    
  • You can use either angle bracket or curly brackets invocation for a given component within the same app, and even within the same template. This allows for gradual migration.

  • Angle bracket syntax works for invoking components of any type, whether they are classic components, Glimmer components, or any other type of component.

  • Curly syntax is still appropriate for some types of components! Check out ember-control-flow-component for an alternative to Glimmer components and angle bracket syntax for such components.

Named Arguments

With angle brackets, there is a new syntax for passing arguments to a component:

{{!-- Passing the argument to the BlogPost component --}}
<BlogPost @title="Hello, world!"/>

Within the component, you can now access these arguments directly with the same syntax:

{{!-- inside the BlogPost component --}}
<h1>{{@title}}</h1>

Collectively, this is referred to as named arguments.

Benefits of Named Arguments

Named Arguments have a number of benefits:

  • When you see a named argument used in a component's template, you can tell immediately that it is a value that was passed to the component, without looking at the component's class.

  • Named arguments always refer to the original value that was passed to the component, so you can also be sure that the value was never mutated by the component's class.

  • Teams can gradually refactor an app to use named arguments, separately from upgrading to angle bracket invocation. You don't need to worry about whether the parent used angle brackets or curly brackets. For example, this works just fine:

    {{blog-post title="Hello, world!"}}
    
    {{!-- This still works --}}
    <h1>{{@title}}</h1>
    

Getting used to Named Arguments

The most important thing to know about named argument syntax is that an argument with an @ always refers to the original value that was passed when the component was invoked. If you change that value in a classic component, it will not update:

import Component from '@ember/component';

export default Component.extend({
  init() {
    this.set('title', this.title.toUpperCase());
  },
});
{{!-- This is still the original title, "Hello, world!" --}}
<h1>{{@title}}</h1>

{{!-- This is the uppercased title, "HELLO, WORLD!" --}}
<h1>{{this.title}}</h1>

If you need to provide a default value, you'll have to do it via a getter:

import Component from '@glimmer/component';

export default class BlogPostComponent extends Component {
  get title() {
    return this.args.title || 'Untitled';
  }
}
<h1>{{this.title}}</h1>

Note: The above sample uses Glimmer components - we'll be covering these in detail later on.

Or by using a helper in the template:

{{!-- using {{or}} from ember-truth-helpers --}}
<h1>{{or @title "Untitled"}}</h1>

If you find yourself forgetting to add the @ symbol before named arguments, it may be helpful to think of how the child template mirrors argument being passed into a component via angle bracket invocation.

Required this in templates

Finally, one thing you may have noticed in the above examples is a lot more references to this in the template. Values that are rendered from the local component or controller instance that backs the template must now have this prepended at the beginning of the path:

{{!-- Before --}}
{{title}}

{{!-- After --}}
{{this.title}}

Benefits of this in templates

The reason for this change is to provide extra clarity to both users reading templates, and the compiler. Without explicitly referring to this, a lot of handlebars statements are pretty ambiguous - for instance, {{title}} could be a helper, a local variable, or a component property.

Getting used to this in templates

You can think of this as meaning, an argument came from this component or controller, not a parent context.

Local variables, introduced via a yield, can still be referred to directly (without this) since they're unambiguous:

{{#let "Title" as |title|}}
  {{!-- This works, because it's a local variable and unambiguous --}}
  {{title}}
{{/let}}

If you forget to use this when you are supposed to, it will fall back to the context of the component or controller context that backs the template. However, the fallback behavior is deprecated and will be removed in future major versions of Ember (4+).