Helpers in Ember are just functions or classes with a well-defined interface, which means they largely Just Work™ with TypeScript. However, there are a couple things you’ll want to watch out for.

As always, you should start by reading and understanding the Ember Guide on Helpers!

Function-based helpers

The basic type of a helper function in Ember is:

type FunctionBasedHelper =
  (positional: unknown[], named: Record<string, unknown>) => string | void;

This represents a function which may have an arbitrarily-long list of positional arguments, which may be followed by a single dictionary-style object containing any named arguments.

There are three important points about this definition:

  1. positional is an array of unknown, of unspecified length.

  2. named is a Record.

  3. Both arguments are always set, but may be empty.

Let’s walk through each of these.

Handling positional arguments

The type is an array of unknown because we don’t (yet!) have any way to make templates aware of the information in this definition—so users could pass in anything. We can work around this using type narrowing—TypeScript’s way of using runtime checks to inform the types at runtime.

function totalLength(positional: unknown[]) {
  // Account for case where user passes no arguments
    'all positional args to `total-length` must be strings',
    positional.every(arg => typeof arg === 'string')

  // safety: we can cast `positional as string[]` because we asserted above
  return (positional as string[]).reduce((sum, s) => sum + s.length, 0);

Handling named arguments

We specified the type of named as a Record<string, unknown>. Record is a built-in TypeScript type representing a fairly standard type in JavaScript: an object being used as a simple map of keys to values. Here we set the values to unknown and the keys to string, since that accurately represents what callers may actually pass to a helper.

(As with positional, we specify the type here as unknown to account for the fact that the template layer isn’t aware of types yet.)

positional and named presence

Note that even if the user passes no arguments, both positional and named are always present. They will just be empty in that case. For example:

import { helper } from '@ember/component/helper';

const describe = (entries: string): string => (entries.length > 0 ? entries : '(none)');

export function showAll(positional: unknown[], named: Record<string, unknown>) {
  // pretty print each item with its index, like `0: { neat: true }` or
  // `1: undefined`.
  const positionalEntries = positional
    .reduce<string[]>((items, arg, index) => items.concat(`${index}: ${JSON.stringify(arg)}`), [])
    .join(', ');

  // pretty print each item with its name, like `cool: beans` or
  // `answer: 42`.
  const namedEntries = Object.keys(named)
      (items, key) => items.concat(`${key}: ${JSON.stringify(named[key], undefined, 2)}`),
    .join(', ');

  return `positional: ${describe(positionalEntries)}\nnamed: ${describe(namedEntries)}`;

export default helper(showAll);

Putting it all together

Given those constraints, let’s see what a (very contrived) actual helper might look like in practice. Let’s imagine we want to take a pair of strings and join them with a required separator and optional prefix and postfixes:

import { helper } from '@ember/component/helper';
import { assert } from '@ember/debug';
import { is } from '../../type-utils'

export function join(positional: [unknown, unknown], named: Dict<unknown>) {
    `'join' requires two 'string' positional parameters`,
    is<[string, string]>(
      positional.length === 2 &&
      positional.every(el => typeof el === 'string')
  assert(`'join' requires argument 'separator'`, typeof named.separator === 'string');

  const joined = positional.join(named.separator);
  const prefix = typeof named.prefix === 'string' ? named.prefix : '';

  return `${prefix}${joined}`;

export default helper(join);

Class-based helpers

The basic type of a class-based helper function in Ember is:

interface ClassBasedHelper {
  compute(positional?: unknown[], named?: Record<string, unknown>): string | void;

Notice that the signature of compute is the same as the signature for the function-based helper! This means that everything we said above applies in exactly the same way here. The only differences are that we can have local state and, by extending from Ember’s Helper class, we can hook into the dependency injection system and use services.

import Helper from '@ember/component/helper';
import { inject as service } from '@ember/service';
import Authentication from 'my-app/services/authentication';

export default class Greet extends Helper {
  @service authentication: Authentication;

  compute() {
    return this.authentication.isAuthenticated
      ? `Welcome back, ${authentication.userName}!`
      : 'Sign in?';

For more details on using decorators, see our guide to using decorators. For details on using services, see our guide to services.

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