Class Ember.Templates.helpers

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Module: ember

The {{action}} helper provides a way to pass triggers for behavior (usually just a function) between components, and into components from controllers.

Passing functions with the action helper

There are three contexts an action helper can be used in. The first two contexts to discuss are attribute context, and Handlebars value context.

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{{! An example of attribute context }}
<div onclick={{action "save"}}></div>
{{! Examples of Handlebars value context }}
{{input on-input=(action "save")}}
{{yield (action "refreshData") andAnotherParam}}

In these contexts, the helper is called a "closure action" helper. Its behavior is simple: If passed a function name, read that function off the actions property of the current context. Once that function is read, or immediately if a function was passed, create a closure over that function and any arguments. The resulting value of an action helper used this way is simply a function.

For example, in the attribute context:

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{{! An example of attribute context }}
<div onclick={{action "save"}}></div>

The resulting template render logic would be:

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var div = document.createElement('div');
var actionFunction = (function(context){
  return function() {
    return context.actions.save.apply(context, arguments);
  };
})(context);
div.onclick = actionFunction;

Thus when the div is clicked, the action on that context is called. Because the actionFunction is just a function, closure actions can be passed between components and still execute in the correct context.

Here is an example action handler on a component:

component.js
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import Component from '@glimmer/component';
import { action } from '@ember/object';

export default class extends Component {
  @action setName(model, name) {
    model.set('name', name);
  }
}
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<MyInput @submit={{action 'setName' @model}} />

or

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{{my-input submit=(action 'setName' @model)}}
component.js
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import Component from '@ember/component';

export default Component.extend({
  click() {
    // Note that model is not passed, it was curried in the template
    this.sendAction('submit', 'bob');
  }
});

Attaching actions to DOM elements

The third context of the {{action}} helper can be called "element space". For example:

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{{! An example of element space }}
<div {{action "save"}}></div>

Used this way, the {{action}} helper provides a useful shortcut for registering an HTML element in a template for a single DOM event and forwarding that interaction to the template's context (controller or component). If the context of a template is a controller, actions used this way will bubble to routes when the controller does not implement the specified action. Once an action hits a route, it will bubble through the route hierarchy.

Event Propagation

{{action}} helpers called in element space can control event bubbling. Note that the closure style actions cannot.

Events triggered through the action helper will automatically have .preventDefault() called on them. You do not need to do so in your event handlers. If you need to allow event propagation (to handle file inputs for example) you can supply the preventDefault=false option to the {{action}} helper:

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<div {{action "sayHello" preventDefault=false}}>
  <input type="file" />
  <input type="checkbox" />
</div>

To disable bubbling, pass bubbles=false to the helper:

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<button {{action 'edit' post bubbles=false}}>Edit</button>

To disable bubbling with closure style actions you must create your own wrapper helper that makes use of event.stopPropagation():

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<div onclick={{disable-bubbling (action "sayHello")}}>Hello</div>
bubbling.js
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import { helper } from '@ember/component/helper';

export function disableBubbling([action]) {
  return function(event) {
    event.stopPropagation();
    return action(event);
  };
}
export default helper(disableBubbling);

If you need the default handler to trigger you should either register your own event handler, or use event methods on your view class. See "Responding to Browser Events" in the documentation for Component for more information.

Specifying DOM event type

{{action}} helpers called in element space can specify an event type. By default the {{action}} helper registers for DOM click events. You can supply an on option to the helper to specify a different DOM event name:

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<div {{action "anActionName" on="doubleClick"}}>
  click me
</div>

See "Event Names" for a list of acceptable DOM event names.

Specifying whitelisted modifier keys

{{action}} helpers called in element space can specify modifier keys. By default the {{action}} helper will ignore click events with pressed modifier keys. You can supply an allowedKeys option to specify which keys should not be ignored.

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<div {{action "anActionName" allowedKeys="alt"}}>
  click me
</div>

This way the action will fire when clicking with the alt key pressed down. Alternatively, supply "any" to the allowedKeys option to accept any combination of modifier keys.

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<div {{action "anActionName" allowedKeys="any"}}>
  click me with any key pressed
</div>

Specifying a Target

A target option can be provided to the helper to change which object will receive the method call. This option must be a path to an object, accessible in the current context:

app/templates/application.hbs
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<div {{action "anActionName" target=someService}}>
  click me
</div>
app/controllers/application.js
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import Controller from '@ember/controller';
import { inject as service } from '@ember/service';

export default class extends Controller {
  @service someService;
}
Module: ember

Available since v3.8.0

options
Array
returns
Array
Array

Use the {{array}} helper to create an array to pass as an option to your components.

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<MyComponent @people={{array
  'Tom Dade'
  'Yehuda Katz'
  this.myOtherPerson}}
/>

or

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{{my-component people=(array
  'Tom Dade'
  'Yehuda Katz'
  this.myOtherPerson)
}}

Would result in an object such as:

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['Tom Date', 'Yehuda Katz', this.get('myOtherPerson')]

Where the 3rd item in the array is bound to updates of the myOtherPerson property.

Module: ember

Available since v1.11.0

The {{component}} helper lets you add instances of Component to a template. See Component for additional information on how a Component functions. {{component}}'s primary use is for cases where you want to dynamically change which type of component is rendered as the state of your application changes. This helper has three modes: inline, block, and nested.

Inline Form

Given the following template:

app/application.hbs
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{{component this.infographicComponentName}}

And the following application code:

app/controllers/application.js
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import Controller from '@ember/controller';
import { tracked } from '@glimmer/tracking';

export default class ApplicationController extends Controller {
  @tracked isMarketOpen = 'live-updating-chart'

  get lastUpdateTimestamp() {
    return new Date();
  }

  get infographicComponentName() {
    return this.isMarketOpen ? 'live-updating-chart' : 'market-close-summary';
  }
}

And the following component template:

chart.hbs
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{{! chart }}
{{yield}}

The Last Update: {{this.lastUpdateTimestamp}} will be rendered in place of the {{yield}}.

Nested Usage

The component helper can be used to package a component path with initial attrs. The included attrs can then be merged during the final invocation. For example, given a person-form component with the following template:

form.hbs
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{{yield (hash
  nameInput=(component "my-input-component" value=@model.name placeholder="First Name")
)}}

When yielding the component via the hash helper, the component is invoked directly. See the following snippet:

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<PersonForm as |form|>
  <form.nameInput @placeholder="Username" />
</PersonForm>

or

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{{#person-form as |form|}}
  {{form.nameInput placeholder="Username"}}
{{/person-form}}

Which outputs an input whose value is already bound to model.name and placeholder is "Username".

When yielding the component without the hash helper use the component helper. For example, below is a full-name component template:

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{{yield (component "my-input-component" value=@model.name placeholder="Name")}}
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<FullName as |field|>
  {{component field placeholder="Full name"}}
</FullName>

or

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{{#full-name as |field|}}
  {{component field placeholder="Full name"}}
{{/full-name}}
Module: ember

Available since v1.13.0

Concatenates the given arguments into a string.

Example:

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{{some-component name=(concat firstName " " lastName)}}

{{! would pass name="<first name value> <last name value>" to the component}}

or for angle bracket invocation, you actually don't need concat at all.

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<SomeComponent @name="{{firstName}} {{lastName}}" />
Module: ember

Execute the debugger statement in the current template's context.

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{{debugger}}

When using the debugger helper you will have access to a get function. This function retrieves values available in the context of the template. For example, if you're wondering why a value {{foo}} isn't rendering as expected within a template, you could place a {{debugger}} statement and, when the debugger; breakpoint is hit, you can attempt to retrieve this value:

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> get('foo')

get is also aware of keywords. So in this situation

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{{#each items as |item|}}
  {{debugger}}
{{/each}}

You'll be able to get values from the current item:

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> get('item.name')

You can also access the context of the view to make sure it is the object that you expect:

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> context
Module: ember

The {{#each}} helper loops over elements in a collection. It is an extension of the base Handlebars {{#each}} helper.

The default behavior of {{#each}} is to yield its inner block once for every item in an array passing the item as the first block parameter.

Assuming the @developers argument contains this array:

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[{ name: 'Yehuda' },{ name: 'Tom' }, { name: 'Paul' }];
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<ul>
  {{#each @developers as |person|}}
    <li>Hello, {{person.name}}!</li>
  {{/each}}
</ul>

The same rules apply to arrays of primitives.

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['Yehuda', 'Tom', 'Paul']
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<ul>
  {{#each @developerNames as |name|}}
    <li>Hello, {{name}}!</li>
  {{/each}}
</ul>

During iteration, the index of each item in the array is provided as a second block parameter.

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<ul>
  {{#each @developers as |person index|}}
    <li>Hello, {{person.name}}! You're number {{index}} in line</li>
  {{/each}}
</ul>

Specifying Keys

In order to improve rendering speed, Ember will try to reuse the DOM elements where possible. Specifically, if the same item is present in the array both before and after the change, its DOM output will be reused.

The key option is used to tell Ember how to determine if the items in the array being iterated over with {{#each}} has changed between renders. By default the item's object identity is used.

This is usually sufficient, so in most cases, the key option is simply not needed. However, in some rare cases, the objects' identities may change even though they represent the same underlying data.

For example:

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people.map(person => {
  return { ...person, type: 'developer' };
});

In this case, each time the people array is map-ed over, it will produce an new array with completely different objects between renders. In these cases, you can help Ember determine how these objects related to each other with the key option:

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<ul>
  {{#each @developers key="name" as |person|}}
    <li>Hello, {{person.name}}!</li>
  {{/each}}
</ul>

By doing so, Ember will use the value of the property specified (person.name in the example) to find a "match" from the previous render. That is, if Ember has previously seen an object from the @developers array with a matching name, its DOM elements will be re-used.

{{else}} condition

{{#each}} can have a matching {{else}}. The contents of this block will render if the collection is empty.

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<ul>
  {{#each @developers as |person|}}
    <li>{{person.name}} is available!</li>
  {{else}}
    <li>Sorry, nobody is available for this task.</li>
  {{/each}}
</ul>
Module: ember

Available since v2.1.0

The {{each-in}} helper loops over properties on an object.

For example, if the @user argument contains this object:

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{
  "name": "Shelly Sails",
  "age": 42
}

This template would display all properties on the @user object in a list:

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<ul>
{{#each-in @user as |key value|}}
  <li>{{key}}: {{value}}</li>
{{/each-in}}
</ul>

Outputting their name and age.

Module: ember

Available since v3.11.0

The fn helper allows you to ensure a function that you are passing off to another component, helper, or modifier has access to arguments that are available in the template.

For example, if you have an each helper looping over a number of items, you may need to pass a function that expects to receive the item as an argument to a component invoked within the loop. Here's how you could use the fn helper to pass both the function and its arguments together:

listing.hbs
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{{#each @items as |item|}}
  <DisplayItem @item=item @select={{fn this.handleSelected item}} />
{{/each}}
list.js
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import Component from '@glimmer/component';
import { action } from '@ember/object';

export default class ItemsList extends Component {
  @action handleSelected(item) {
    // ...snip...
  }
}

In this case the display-item component will receive a normal function that it can invoke. When it invokes the function, the handleSelected function will receive the item and any arguments passed, thanks to the fn helper.

Let's take look at what that means in a couple circumstances:

  • When invoked as this.args.select() the handleSelected function will receive the item from the loop as its first and only argument.
  • When invoked as this.args.select('foo') the handleSelected function will receive the item from the loop as its first argument and the string 'foo' as its second argument.

In the example above, we used @action to ensure that handleSelected is properly bound to the items-list, but let's explore what happens if we left out @action:

list.js
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import Component from '@glimmer/component';

export default class ItemsList extends Component {
  handleSelected(item) {
    // ...snip...
  }
}

In this example, when handleSelected is invoked inside the display-item component, it will not have access to the component instance. In other words, it will have no this context, so please make sure your functions are bound (via @action or other means) before passing into fn!

See also partial application.

Module: ember

Available since v2.1.0

Dynamically look up a property on an object. The second argument to {{get}} should have a string value, although it can be bound.

For example, these two usages are equivalent:

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{{person.height}}
{{get person "height"}}

If there were several facts about a person, the {{get}} helper can dynamically pick one:

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{{get person factName}}

For a more complex example, this template would allow the user to switch between showing the user's height and weight with a click:

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{{get person factName}}
<button {{action (fn (mut factName)) "height"}}>Show height</button>
<button {{action (fn (mut factName)) "weight"}}>Show weight</button>

The {{get}} helper can also respect mutable values itself. For example:

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{{input value=(mut (get person factName)) type="text"}}
<button {{action (fn (mut factName)) "height"}}>Show height</button>
<button {{action (fn (mut factName)) "weight"}}>Show weight</button>

Would allow the user to swap what fact is being displayed, and also edit that fact via a two-way mutable binding.

Module: ember
the
String
name of the block. The name (at the moment) is either "main" or "inverse" (though only curly components support inverse)
returns
Boolean
`true` if the component was invoked with a block

{{has-block}} indicates if the component was invoked with a block.

This component is invoked with a block:

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{{#my-component}}
  Hi Jen!
{{/my-component}}

This component is invoked without a block:

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{{my-component}}

Using angle bracket invocation, this looks like:

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<MyComponent>Hi Jen!</MyComponent> {{! with a block}}
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<MyComponent/> {{! without a block}}

This is useful when you want to create a component that can optionally take a block and then render a default template when it is not invoked with a block.

component.hbs
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{{#if (has-block)}}
  Welcome {{yield}}, we are happy you're here!
{{else}}
  Hey you! You're great!
{{/if}}
Module: ember
the
String
name of the block. The name (at the moment) is either "main" or "inverse" (though only curly components support inverse)
returns
Boolean
`true` if the component was invoked with block params

{{has-block-params}} indicates if the component was invoked with block params.

This component is invoked with block params:

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{{#my-component as |favoriteFlavor|}}
  Hi Jen!
{{/my-component}}

This component is invoked without block params:

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{{#my-component}}
  Hi Jenn!
{{/my-component}}

This is useful when you want to create a component that can render itself differently when it is not invoked with block params.

component.hbs
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{{#if (has-block-params)}}
  Welcome {{yield favoriteFlavor}}, we're happy you're here and hope you
  enjoy your favorite ice cream flavor.
{{else}}
  Welcome {{yield}}, we're happy you're here, but we're unsure what
  flavor ice cream you would enjoy.
{{/if}}
Module: ember

Available since v2.3.0

options
Object
returns
Object
Hash

Use the {{hash}} helper to create a hash to pass as an option to your components. This is specially useful for contextual components where you can just yield a hash:

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{{yield (hash
   name='Sarah'
   title=office
)}}

Would result in an object such as:

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{ name: 'Sarah', title: this.get('office') }

Where the title is bound to updates of the office property.

Note that the hash is an empty object with no prototype chain, therefore common methods like toString are not available in the resulting hash. If you need to use such a method, you can use the call or apply approach:

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function toString(obj) {
  return Object.prototype.toString.apply(obj);
}
Module: ember

The if helper allows you to conditionally render one of two branches, depending on the "truthiness" of a property. For example the following values are all falsey: false, undefined, null, "", 0, NaN or an empty array.

This helper has two forms, block and inline.

Block form

You can use the block form of if to conditionally render a section of the template.

To use it, pass the conditional value to the if helper, using the block form to wrap the section of template you want to conditionally render. Like so:

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{{! will not render if foo is falsey}}
{{#if foo}}
  Welcome to the {{foo.bar}}
{{/if}}

You can also specify a template to show if the property is falsey by using the else helper.

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{{! is it raining outside?}}
{{#if isRaining}}
  Yes, grab an umbrella!
{{else}}
  No, it's lovely outside!
{{/if}}

You are also able to combine else and if helpers to create more complex conditional logic.

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{{#if isMorning}}
  Good morning
{{else if isAfternoon}}
  Good afternoon
{{else}}
  Good night
{{/if}}

Inline form

The inline if helper conditionally renders a single property or string.

In this form, the if helper receives three arguments, the conditional value, the value to render when truthy, and the value to render when falsey.

For example, if useLongGreeting is truthy, the following:

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{{if useLongGreeting "Hello" "Hi"}} Alex

Will render:

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Hello Alex

Nested if

You can use the if helper inside another helper as a nested helper:

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<SomeComponent @height={{if isBig "100" "10"}} />

or

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{{some-component height=(if isBig "100" "10")}}

One detail to keep in mind is that both branches of the if helper will be evaluated, so if you have {{if condition "foo" (expensive-operation "bar"), expensive-operation will always calculate.

Module: ember

The let helper receives one or more positional arguments and yields them out as block params.

This allows the developer to introduce shorter names for certain computations in the template.

This is especially useful if you are passing properties to a component that receives a lot of options and you want to clean up the invocation.

For the following example, the template receives a post object with content and title properties.

We are going to call the my-post component, passing a title which is the title of the post suffixed with the name of the blog, the content of the post, and a series of options defined in-place.

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{{#let
    (concat post.title ' | The Ember.js Blog')
    post.content
    (hash
      theme="high-contrast"
      enableComments=true
    )
    as |title content options|
}}
  <MyPost @title={{title}} @content={{content}} @options={{options}} />
{{/let}}

or

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{{#let
    (concat post.title ' | The Ember.js Blog')
    post.content
    (hash
      theme="high-contrast"
      enableComments=true
    )
    as |title content options|
}}
  {{my-post title=title content=content options=options}}
{{/let}}
Module: ember
str
String
The string to format.

Calls String.loc with the provided string. This is a convenient way to localize text within a template. For example:

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Ember.STRINGS = {
  '_welcome_': 'Bonjour'
};
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<div class='message'>
  {{loc '_welcome_'}}
</div>
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<div class='message'>
  Bonjour
</div>

See String.loc for how to set up localized string references.

Module: ember
params
Array

log allows you to output the value of variables in the current rendering context. log also accepts primitive types such as strings or numbers.

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{{log "myVariable:" myVariable }}
Module: ember
name
String
Name of the engine to mount.
model
Object
Object that will be set as the model of the engine.

The {{mount}} helper lets you embed a routeless engine in a template. Mounting an engine will cause an instance to be booted and its application template to be rendered.

For example, the following template mounts the ember-chat engine:

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{{! application.hbs }}
{{mount "ember-chat"}}

Additionally, you can also pass in a model argument that will be set as the engines model. This can be an existing object:

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<div>
  {{mount 'admin' model=userSettings}}
</div>

Or an inline hash, and you can even pass components:

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<div>
  <h1>Application template!</h1>
  {{mount 'admin' model=(hash
      title='Secret Admin'
      signInButton=(component 'sign-in-button')
  )}}
</div>
Module: ember
attr
Object
the "two-way" attribute that can be modified.

The mut helper lets you clearly specify that a child Component can update the (mutable) value passed to it, which will change the value of the parent component.

To specify that a parameter is mutable, when invoking the child Component:

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<MyChild @childClickCount={{fn (mut totalClicks)}} />

or

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{{my-child childClickCount=(mut totalClicks)}}

The child Component can then modify the parent's value just by modifying its own property:

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// my-child.js
export default Component.extend({
  click() {
    this.incrementProperty('childClickCount');
  }
});

Note that for curly components ({{my-component}}) the bindings are already mutable, making the mut unnecessary.

Additionally, the mut helper can be combined with the fn helper to mutate a value. For example:

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<MyChild @childClickCount={{this.totalClicks}} @click-count-change={{fn (mut totalClicks))}} />

or

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{{my-child childClickCount=totalClicks click-count-change=(fn (mut totalClicks))}}

The child Component would invoke the function with the new click value:

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// my-child.js
export default Component.extend({
  click() {
    this.get('click-count-change')(this.get('childClickCount') + 1);
  }
});

The mut helper changes the totalClicks value to what was provided as the fn argument.

The mut helper, when used with fn, will return a function that sets the value passed to mut to its first argument. As an example, we can create a button that increments a value passing the value directly to the fn:

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{{! inc helper is not provided by Ember }}
<button onclick={{fn (mut count) (inc count)}}>
  Increment count
</button>

You can also use the value option:

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<input value={{name}} oninput={{fn (mut name) value="target.value"}}>
Module: ember

Available since v3.11.0

The {{on}} modifier lets you easily add event listeners (it uses EventTarget.addEventListener internally).

For example, if you'd like to run a function on your component when a <button> in the components template is clicked you might do something like:

post.hbs
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<button {{on 'click' this.saveLike}}>Like this post!</button>
post.js
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import Component from '@glimmer/component';
import { action } from '@ember/object';

export default class LikePostComponent extends Component {
  @action saveLike() {
    // someone likes your post!
    // better send a request off to your server...
  }
}

Arguments

{{on}} accepts two positional arguments, and a few named arguments.

The positional arguments are:

  • event -- the name to use when calling addEventListener
  • callback -- the function to be passed to addEventListener

The named arguments are:

  • capture -- a true value indicates that events of this type will be dispatched to the registered listener before being dispatched to any EventTarget beneath it in the DOM tree.
  • once -- indicates that the listener should be invoked at most once after being added. If true, the listener would be automatically removed when invoked.
  • passive -- if true, indicates that the function specified by listener will never call preventDefault(). If a passive listener does call preventDefault(), the user agent will do nothing other than generate a console warning. See Improving scrolling performance with passive listeners to learn more.

The callback function passed to {{on}} will receive any arguments that are passed to the event handler. Most commonly this would be the event itself.

If you would like to pass additional arguments to the function you should use the {{fn}} helper.

For example, in our example case above if you'd like to pass in the post that was being liked when the button is clicked you could do something like:

post.js
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<button {{on 'click' (fn this.saveLike @post)}}>Like this post!</button>

In this case, the saveLike function will receive two arguments: the click event and the value of @post.

Function Context

In the example above, we used @action to ensure that likePost is properly bound to the items-list, but let's explore what happens if we left out @action:

post.js
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import Component from '@glimmer/component';

export default class LikePostComponent extends Component {
  saveLike() {
    // ...snip...
  }
}

In this example, when the button is clicked saveLike will be invoked, it will not have access to the component instance. In other words, it will have no this context, so please make sure your functions are bound (via @action or other means) before passing into on!

Module: ember
name
String

The {{outlet}} helper lets you specify where a child route will render in your template. An important use of the {{outlet}} helper is in your application's application.hbs file:

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{{! app/templates/application.hbs }}
<!-- header content goes here, and will always display -->
<MyHeader />
<div class="my-dynamic-content">
  <!-- this content will change based on the current route, which depends on the current URL -->
  {{outlet}}
</div>
<!-- footer content goes here, and will always display -->
<MyFooter />

You may also specify a name for the {{outlet}}, which is useful when using more than one {{outlet}} in a template:

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{{outlet "menu"}}
{{outlet "sidebar"}}
{{outlet "main"}}

Your routes can then render into a specific one of these outlets by specifying the outlet attribute in your renderTemplate function:

app/routes/menu.js
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import Route from '@ember/routing/route';

export default Route.extend({
  renderTemplate() {
    this.render({ outlet: 'menu' });
  }
});

See the routing guide for more information on how your route interacts with the {{outlet}} helper. Note: Your content will not render if there isn't an {{outlet}} for it.

Module: ember
partialName
String
The name of the template to render minus the leading underscore.

The partial helper renders another template without changing the template context:

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{{foo}}
{{partial "nav"}}

The above example template will render a template named "-nav", which has the same context as the parent template it's rendered into, so if the "-nav" template also referenced {{foo}}, it would print the same thing as the {{foo}} in the above example.

If a "-nav" template isn't found, the partial helper will fall back to a template named "nav".

Bound template names

The parameter supplied to partial can also be a path to a property containing a template name, e.g.:

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{{partial someTemplateName}}

The above example will look up the value of someTemplateName on the template context (e.g. a controller) and use that value as the name of the template to render. If the resolved value is falsy, nothing will be rendered. If someTemplateName changes, the partial will be re-rendered using the new template name.

Module: ember
hash
Object
takes a hash of query parameters
returns
Object
A `QueryParams` object for `{{link-to}}`

This is a helper to be used in conjunction with the link-to helper. It will supply url query parameters to the target route.

Module: ember

See Ember.Templates.components.Textarea.

Module: ember

The {{unbound}} helper disconnects the one-way binding of a property, essentially freezing its value at the moment of rendering. For example, in this example the display of the variable name will not change even if it is set with a new value:

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{{unbound name}}

Like any helper, the unbound helper can accept a nested helper expression. This allows for custom helpers to be rendered unbound:

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{{unbound (some-custom-helper)}}
{{unbound (capitalize name)}}
{{! You can use any helper, including unbound, in a nested expression }}
{{capitalize (unbound name)}}

The unbound helper only accepts a single argument, and it return an unbound value.

Module: ember

The unless helper is the inverse of the if helper. It displays if a value is falsey ("not true" or "is false"). Example values that will display with unless: false, undefined, null, "", 0, NaN or an empty array.

Inline form

The inline unless helper conditionally renders a single property or string. This helper acts like a ternary operator. If the first property is falsy, the second argument will be displayed, otherwise, the third argument will be displayed

For example, if useLongGreeting is false below:

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{{unless useLongGreeting "Hi" "Hello"}} Ben

Then it will display:

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Hi

You can use the unless helper inside another helper as a subexpression. If isBig is not true, it will set the height to 10:

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{{! If isBig is not true, it will set the height to 10.}}
<SomeComponent @height={{unless isBig "10" "100"}} />

or

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{{some-component height=(unless isBig "10" "100")}}

Block form

Like the if helper, unless helper also has a block form.

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{{! If greetings are found, the text below will not render.}}
{{#unless greetings}}
  No greetings were found. Why not set one?
{{/unless}}

You can also use an else helper with the unless block. The else will display if the value is truthy.

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{{! Is the user logged in?}}
{{#unless userData}}
  Please login.
{{else}}
  Welcome back!
{{/unless}}

If userData is false, undefined, null, or empty in the above example, then it will render:

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Please login.
Module: ember
options
Object
returns
String
HTML string

Use the {{with}} helper when you want to alias a property to a new name. This is helpful for semantic clarity as it allows you to retain default scope or to reference a property from another {{with}} block.

If the aliased property is "falsey", for example: false, undefined null, "", 0, NaN or an empty array, the block will not be rendered.

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{{! Will only render if user.posts contains items}}
{{#with user.posts as |blogPosts|}}
  <div class="notice">
    There are {{blogPosts.length}} blog posts written by {{user.name}}.
  </div>
  {{#each blogPosts as |post|}}
    <li>{{post.title}}</li>
  {{/each}}
{{/with}}

NOTE: The alias should not reuse a name from the bound property path.

For example: {{#with foo.bar as |foo|}} is not supported because it attempts to alias using the first part of the property path, foo. Instead, use {{#with foo.bar as |baz|}}.

Module: ember
options
Hash
returns
String
HTML string

{{yield}} denotes an area of a template that will be rendered inside of another template.

Use with Component

When designing components {{yield}} is used to denote where, inside the component's template, an optional block passed to the component should render:

app/templates/application.hbs
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{{#labeled-textfield value=someProperty}}
  First name:
{{/labeled-textfield}}
textfield.hbs
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<label>
  {{yield}} {{input value=value}}
</label>

Result:

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<label>
  First name: <input type="text" />
</label>

Additionally you can yield properties into the context for use by the consumer:

app/templates/application.hbs
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{{#labeled-textfield value=someProperty validator=(action 'firstNameValidator') as |validationError|}}
  {{#if validationError}}
    <p class="error">{{validationError}}</p>
  {{/if}}
  First name:
{{/labeled-textfield}}
textfield.hbs
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<label>
  {{yield validationError}} {{input value=value}}
</label>

Result:

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<label>
  <p class="error">First Name must be at least 3 characters long.</p>
  First name: <input type="text" />
</label>