Class Ember.Helper

public
Defined in: packages/ember-htmlbars/lib/helper.js:8
Module: ember
Since: v1.13.0

Ember Helpers are functions that can compute values, and are used in templates. For example, this code calls a helper named format-currency:

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<div>{{format-currency cents currency="$"}}</div>

Additionally, a helper can be called as a nested helper (sometimes called a subexpression). In this example, the computed value of a helper is passed to a component named show-money:

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{{show-money amount=(format-currency cents currency="$")}}

Helpers defined using a class must provide a compute function. For example:

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export default Ember.Helper.extend({
  compute(params, hash) {
    let cents = params[0];
    let currency = hash.currency;
    return `${currency}${cents * 0.01}`;
  }
});

Each time the input to a helper changes, the compute function will be called again.

As instances, these helpers also have access to the container and will accept injected dependencies.

Additionally, class helpers can call recompute to force a new computation.

If the output of your helper is only dependent on the current input, then you can use the Helper.helper function. See Ember.Helper.helper.

In this form the example above becomes:

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export default Ember.Helper.helper((params, hash) => {
  let cents = params[0];
  let currency = hash.currency;
  return `${currency}${cents * 0.01}`;
});

Show:

compute (params, hash) public

Module: ember

Defined in packages/ember-htmlbars/lib/helper.js:94

Available since v1.13.0

params
Array
The positional arguments to the helper
hash
Object
The named arguments to the helper

Override this function when writing a class-based helper.

helper (helper) public

Module: ember

Defined in packages/ember-htmlbars/lib/helper.js:109

Available since v1.13.0

helper
Function
The helper function

In many cases, the ceremony of a full Ember.Helper class is not required. The helper method creates pure-function helpers without instances. For example:

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// app/helpers/format-currency.js
export function formatCurrency([cents], hash) {
  let currency = hash.currency;
  return `${currency}${cents * 0.01}`;
});

export default Ember.Helper.helper(formatCurrency);

// tests/myhelper.js
import { formatCurrency } from ..../helpers/myhelper
// add some tests

This form is more efficient at run time and results in smaller compiled js. It is also easier to test by using the following structure and importing the formatCurrency function into a test.

recompute public

Module: ember

Defined in packages/ember-htmlbars/lib/helper.js:66

Available since v1.13.0

On a class-based helper, it may be useful to force a recomputation of that helpers value. This is akin to rerender on a component.

For example, this component will rerender when the currentUser on a session service changes:

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// app/helpers/current-user-email.js
export default Ember.Helper.extend({
  session: Ember.inject.service(),
  onNewUser: Ember.observer('session.currentUser', function() {
    this.recompute();
  }),
  compute() {
    return this.get('session.currentUser.email');
  }
});