Writing Helpers

Unlike components, which require a dash in the name to follow the Custom Element spec, helper names can be single or multi-word. If your helper's name is multi-word, it should be dasherized as the examples in this page.

You can pass one or more arguments to be used inside the function. In the above example, we passed the amount in cents as the first and only argument.

To pass multiple arguments to a helper, add them as a space-separated list after the helper name:

{{my-helper "hello" "world"}}

An array of these arguments is passed to the helper function:

export default Ember.Helper.helper(function(params) {
  let [arg1, arg2] = params;

  console.log(arg1); // => "hello"
  console.log(arg2); // => "world"
});

You can use JavaScript's destructuring assignment shorthand to clean up the code. This example is equivalent to the above example (note the function signature):

export default Ember.Helper.helper(function([arg1, arg2]) {
  console.log(arg1); // => "hello"
  console.log(arg2); // => "world"
});

Normal arguments are useful for passing data to be transformed into helper functions. However, because the order in which you pass arguments matters, it is usually best not to have helpers take more than one or two of them.

That said, sometimes you may want to make behavior of helpers configurable by the developers that call them from their templates. For example, let's abandon our Americentric ways and update our format-currency helper to take an optional configuration for which currency symbol to display.

Helpers allow you to pass named arguments as a JavaScript object that contains the name of the argument along with an associated value. The order in which named arguments are supplied does not affect functionality.

In this example, we can pass a sign argument to our format-currency helper:

{{format-currency 350 sign="£"}}

We'd like our helper to print pounds sterling rather than US dollars:

£3.50

The object containing named arguments is passed as the second argument to the helper function. Here is our example from above, updated to support the optional sign option:

export default Ember.Helper.helper(function([value, ...rest], namedArgs) {
  let dollars = Math.floor(value / 100);
  let cents = value % 100;
  let sign = namedArgs.sign === undefined ? '$' : namedArgs.sign;

  if (cents.toString().length === 1) { cents = '0' + cents; }
  return `${sign}${dollars}.${cents}`;
});

You can pass as many named arguments as you'd like. They get added to the namedArgs argument passed to the function:

{{my-helper option1="hello" option2="world" option3="goodbye cruel world"}}

export default Ember.Helper.helper(function(params, namedArgs) {
  console.log(namedArgs.option1); // => "hello"
  console.log(namedArgs.option2); // => "world"
  console.log(namedArgs.option3); // => "goodbye cruel world"
});

You can use JavaScript's destructuring assignment shorthand in this case as well to clean up the above code:

export default Ember.Helper.helper(function(params, { option1, option2, option3 }) {
  console.log(option1); // => "hello"
  console.log(option2); // => "world"
  console.log(option3); // => "goodbye cruel world"
});

In sum, arguments are good for passing values:

{{format-date currentDate}}

Hashes are useful for configuring the behavior of a helper:

{{print-current-date format="YYYY MM DD"}}

You can have as many of both as you want, so long as the parameters come first:

{{format-date-and-time date time format="YYYY MM DD h:mm" locale="en"}}

The above example contains two arguments:

• date
• time

And two named arguments:

• format="YYY MM DD h:mm"
• locale="en"

By default, helpers are stateless. They are passed inputs (parameters and a hash), they perform an operation on those inputs, and return a single output. They have no side-effects and don't save any information that is used on subsequent runs of the function.

In some situations, however, you may need to write a helper that interacts with the rest of your application. You can create class-based helpers that have access to services in your application, and can optionally save state as well, although this is usually unnecessary and error-prone.

To create a class-based helper, rather than exporting a simple function, you should export a subclass of Ember.Helper. Helper classes must contain a compute method that behaves the same as the function passed to Ember.Helper.helper. In order to access a service, you must first inject it into the class-based helper. Once added, you can call the service's methods or access its properties from within the compute() method.

As an exercise, here is the above format-currency helper re-factored into a class-based helper:

export default Ember.Helper.extend({
  compute([value, ...rest], hash) {
    let dollars = Math.floor(value / 100);
    let cents = value % 100;
    let sign = hash.sign === undefined ? '$' : hash.sign;

    if (cents.toString().length === 1) { cents = '0' + cents; }
    return `${sign}${dollars}.${cents}`;
  }
});

This is exactly equivalent to the format-currency example above. You can think of the function version as a shorthand for the longer class form if it does not require dependency injection.

As another example, let's make a helper utilizing an authentication service that welcomes users by their name if they're logged in:

export default Ember.Helper.extend({
  authentication: Ember.inject.service(),
  compute() {
    let authentication = this.get('authentication');

    if (authentication.get('isAuthenticated')) {
      return 'Welcome back, ' + authentication.get('username');
    } else {
      return 'Not logged in';
    }
  }
});

To protect your application from cross-site scripting attacks (XSS), Ember automatically escapes any value you return from a helper so that the browser will not interpret it as HTML.

For example, here's a make-bold helper that returns a string containing HTML:

export default Ember.Helper.helper(function([param, ...rest]) {
  return `<b>${param}</b>`;
});

You can invoke it like this:

{{make-bold "Hello world"}}

Ember will escape the HTML tags, like this:

<b>Hello world</b>

This shows the literal string <b>Hello world</b> to the user, rather than the text in bold as you probably intended. We can tell Ember not to escape the return value (that is, that it is safe) by using the htmlSafe string utility:

export default Ember.Helper.helper(function([param, ...rest]) {
  return Ember.String.htmlSafe(`<b>${param}</b>`);
});

If you return a SafeString (a string that has been wrapped in a call to htmlSafe), Ember knows that you have vouched on its behalf that it contains no malicious HTML.

However, note that in the above code we may have inadvertently introduced an XSS vulnerability into our application! By blindly marking the string as safe, a malicious user could get their own HTML into our app, allowing them to do things like access sensitive customer data.

For example, imagine that we have a chat app and use our make-bold helper to welcome the new users into the channel:

Welcome back! {{make-bold model.firstName}} has joined the channel.

Now a malicious user simply needs to set their firstName to a string containing HTML (like a <script> tag that sends private customer data to their server, for example) and every user in that chat room has been compromised.

In general, you should prefer using components if you are wrapping content in HTML. However, if you really want to include a mix of HTML and values from models in what you return from the helper, make sure you escape anything that may have come from an untrusted user with the escapeExpression utility:

export default Ember.Helper.helper(function([param, ...rest]) {
  let value = Ember.Handlebars.Utils.escapeExpression(param);
  return Ember.String.htmlSafe(`<b>${value}</b>`);
});

Now the value passed into the helper has its HTML escaped, but the trusted <b> tags that we want to wrap the value in are not escaped. A malicious user setting their firstName to something containing HTML would see this:

Welcome back! <b>&lt;script
type="javascript"&gt;alert('pwned!');&lt;/script&gt;</b> has joined the channel.