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Passing Properties to a Component

Components are isolated from their surroundings, so any data that the component needs has to be passed in.

For example, imagine you have a blog-post component that is used to display a blog post:

app/templates/components/blog-post.hbs

<article class="blog-post">
  <h1>{{title}}</h1>
  <p>{{body}}</p>
</article>

Now imagine we have the following template and route:

app/routes/index.js

import Route from '@ember/routing/route';

export default Route.extend({
  model() {
    return this.store.findAll('post');
  }
});

If we tried to use the component like this:

app/templates/index.hbs

{{#each model as |post|}}
  {{blog-post}}
{{/each}}

The following HTML would be rendered:

<article class="blog-post">
  <h1></h1>
  <p></p>
</article>

In order to make a property available to a component, you must pass it in like this:

app/templates/index.hbs

{{#each model as |post|}}
  {{blog-post title=post.title body=post.body}}
{{/each}}

It is important to note that these properties stay in sync (technically known as being "bound"). That is, if the value of componentProperty changes in the component, outerProperty will be updated to reflect that change. The reverse is true as well.

Positional Params

In addition to passing parameters in by name, you can pass them in by position. In other words, you can invoke the above component example like this:

app/templates/index.hbs

{{#each model as |post|}}
  {{blog-post post.title post.body}}
{{/each}}

To set the component up to receive parameters this way, you need to set the positionalParams attribute in your component class.

app/components/blog-post.js

import Component from '@ember/component';

export default Component.extend({}).reopenClass({
  positionalParams: ['title', 'body']
});

Then you can use the attributes in the component exactly as if they had been passed in like {{blog-post title=post.title body=post.body}}.

Notice that the positionalParams property is added to the class as a static variable via reopenClass. Positional params are always declared on the component class and cannot be changed while an application runs.

Alternatively, you can accept an arbitrary number of parameters by setting positionalParams to a string, e.g. positionalParams: 'params'. This will allow you to access those params as an array like so:

app/components/blog-post.js

import Component from '@ember/component';
import { computed } from '@ember/object';

export default Component.extend({
  title: computed('params.[]', function(){
    return this.params[0];
  }),
  body: computed('params.[]', function(){
    return this.params[1];
  })
}).reopenClass({
  positionalParams: 'params'
});

Positional Params