Helper

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

<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:

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

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

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 an will accept injected dependencies.

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

Override this function when writing a class-based helper.

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

// app/helpers/format-currency.js
export default Ember.Helper.helper(function(params, hash) {
  let cents = params[0];
  let currency = hash.currency;
  return `${currency}${cents * 0.01}`;
});

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:

// 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');
  }
});