ember-serviceable-helper

An API for writing functions that needs access to Ember services.

Motivation

Suppose you are using Ember 4.5+ (or installed the polyfill) and have been writing all your helpers as plain functions, like so:

// app/helpers/format-date.js

/**
 * Using the browser's default locale, return the formatted string for the
 * given date, optionally taking into account the given locale options.
 */
export default function formatDate(date, options = {}) {
  const userLocale = navigator.language;
  return new Intl.DateTimeFormat(userLocale, options).format(date);
}

That is great, until you realize your customers' operating system/browser settings do not always align with their preferred locale.

No problem, we will introduce a setting for this and expose it via the existing preferences service. But how do we access it from the formatDate function?

One option would be to accept it as a function argument. The downside is that you will have to pass that in wherever and whenever you call this function. The point of services in Ember is to consolidate these app-wide global dependencies and to automatically inject them into whenever it is needed in the app, it would be great if we can take advantage of that instead.

Another option would be to use the "class-based helper" API:

// app/helpers/format-date.js
import Helper from '@ember/component/helper';
import { service } from '@ember/service';

/**
 * Using the browser's default locale, return the formatted string for the
 * given date, optionally taking into account the given locale options.
 */
export default class FormatDate extends Helper {
  @service preferences;

  compute([date], options) {
    const userLocale = this.preferences.locale;
    return new Intl.DateTimeFormat(userLocale, options).format(date);
  }
}

This works, but the code is now vastly different from the simple function we had. This cliff is particular sharp when using TypeScript/Glint:

// before

export default function formatDate(
  date: Date,
  options: Intl.DateTimeFormatOptions = {},
): string {
  const userLocale = navigator.language;
  return new Intl.DateTimeFormat(userLocale, options).format(date);
}

declare module '@glint/environment-ember-loose/registry' {
  export default interface Registry {
    'format-date': typeof formatDate;
  }
}

// after

import Helper from '@ember/component/helper';
import { service } from '@ember/service';
import type PreferencesService from 'my-app/services/preferences';

interface FormatDateSignature {
  Args: {
    Positional: [value: Date];
    Named?: Intl.DateTimeFormatOptions;
  };
  Return: string;
}

export default class FormatDate extends Helper<FormatDateSignature> {
  @service declare preferences: PreferencesService;

  compute(
    [value]: FormatDateSignature['Args']['Positional'],
    options: FormatDateSignature['Args']['Named'] = {},
  ): string {
    const userLocale = this.preferences.locale;
    return new Intl.DateTimeFormat(userLocale, options).format(date);
  }
}

declare module '@glint/environment-ember-loose/registry' {
  export default interface Registry {
    'format-date': typeof FormatDate;
  }
}

Eh?

The "class-based helper" API is a bit of a legacy feature, it's yet another thing to learn, and it's really meant for helpers that needs to be "stateful", which is quite rare, and certainly not the case here.

It is a bit unfortunate that the extra stateful machinery and syntactic noise obfuscated the fact that this is still just a plain function that just happens to need access to some app-wide global state via services.

Usage

Enter ember-serviceable-helper:

// app/helpers/format-date.js
import inject from 'ember-serviceable-helper';

// The function will be called with a `this` that has the dependencies injected
function formatDate(date, options = {}) {
  const userLocale = this.preferences.locale;
  return new Intl.DateTimeFormat(userLocale, options).format(date);
}

export default inject.service('preferences').into(formatDate);

Alternatively:

// app/helpers/format-date.js
import inject from 'ember-serviceable-helper';

export default inject.service('preferences').build(
  ({ preferences }) =>
    // Alternatively, we can pass a callback that accepts the injections and
    // return our function, which now have access to the injections via the
    // closure variable.
    function formatDate(date, options = {}) {
      const userLocale = preferences.locale;
      return new Intl.DateTimeFormat(userLocale, options).format(date);
    },
);

Either way, the code for the function is pretty much identical to what it was before, its actual parameters still reflect what its consumers are expected to call it with, and the fact that it needs access to some services is hidden away as an internal implementation detail.

So what the value of this default export here? How do the consumers call this function?

import formatDate from 'my-app/helpers/format-date';

// It is still a function:
typeof formatDate; // "function"

// Using it as a template helper just works:
<template>
  Today's date is {{formatDate @date}}
  {{!-- so would {{format-date @date}} in .hbs --}}
</template>

// We can't just call it directly outside of templates, because the function
// has DI dependencies.

// Error: cannot call formatDate without a context object; it must be called
// with a `this` object with an owner
formatDate(new Date());

// Here are ways some ways to satisfy the DI requirement. We are using a
// component as an example, but any object with an owner (i.e. `getOwner(...)`
// is not `undefined`) would work.
import Component from '@glimmer/component';
import { getOwner } from '@ember/owner';

class MyComponent extends Component {
  // Again, using it as a template helper just works:
  <template>
    Today's date is {{formatDate @date}}
  </template>

  // This assigns the function to `this.formatDate`
  formatDate = formatDate;

  someMethod() {
    // Now this works just fine
    this.formatDate(new Date());

    // Alternatively, it's the same as...
    formatDate.call(this, new Date());

    // Or...
    const formatDate2 = formatDate.bind(this);

    // Now this value can be passed around and works everywhere
    formatDate2(new Date());

    // Finally, if you have the owner available, you can instantiate the
    // injected version of the function directly:
    const owner = getOwner(this);
    const formatDate3 = new formatDate(owner);

    // Same as above, this value can be passed around and works anywhere
    formatDate3(new Date());
  }
}

Conclusions:

1. Write normal plain functions
2. If/when your function need access to services, ember-serviceable-helper offers an API that lets you inject these dependencies into the function and returns an InjectableConstructor
3. The InjectableConstructor...
   1. Can be used as template helpers directly
   2. Can instantiate the fully injected function using the new keyword and passing the owner
   3. As a convenience, can be instantiated and called directly with the method call syntax on any this object with an owner

Builder API

The main entrypoint is a builder API for enumerating the required dependencies and then applying them to a function.

The API is designed to be chained:

inject
  .service('config')
  .service('store')
  // ...and more...
  .into(function () {
    this.config; // ConfigService here
    this.store; // StoreService here
  });

If you need to the property to be different from the name of the service:

inject.service('someProperty', 'my-service').into(function () {
  this.someProperty; // MyService here
});

You can also inject the owner directly:

inject
  .owner()
  // ...can still chain on .service('config') etc
  .into(function () {
    this.owner; // Owner here
  });

Just like @service, we create lazy getters on the injections object (the this argument), so that service lookup happens lazily on first access and is cached thereafter.

Furthermore, the injections object is itself created lazily and cached/reused in subsequent invocations of the same function. Do not abuse this and use it to store random properties! If you need your helper to be stateful, just use the class-based helper API instead. The injections object is frozen in debug builds to prevent this.

However, it is sometimes useful to compute some derived state with the same lazy and once-per-owner semantics. For this purpose, there is this additional API on the builder:

inject
  .property('router', function (owner) {
    // `this` is the injections object
    return owner.lookup('service:router') ?? owner.lookup('router:main');
  })
  // ...can still chain on .service('config') etc
  .into(function () {
    this.router; // either RouterService or Router
  });

For finalization, either into() or build() is available on the builder. They do the same thing, calling either of them will "terminate" the method chain, finalize the injections and return an InjectableConstructor.

The difference lies in how they provide the injections to the function.

As shown above, into() takes the "injectable" function and provides the injections object via the this argument. This makes it possible to write code like this:

// app/helpers/format-date.js
import inject from 'ember-serviceable-helper';

// This function can be imported in tests and unit tested without going through
// the DI system
export function formatDate(date, options = {}) {
  const userLocale = this.preferences.locale;
  return new Intl.DateTimeFormat(userLocale, options).format(date);
}

export default inject.service('preferences').into(formatDate);

On the other hand, build() takes a callback and provides the injections object via a closure argument. This makes it possible to write code like this:

// app/helpers/random.js
import inject from 'ember-serviceable-helper';
import SeededRandom from 'seeded-random';

export default inject.service('config').build((injections) => {
  // The initialization code here runs once-per-owner and lazily on the first
  // invocation of the helper/function; don't abuse this to store state!
  const { config } = injections;
  const RNG = new SeededRandom(config.seed ?? Date.now());

  return function random() {
    return RNG.next();
  };
});

This particular example is probably better modelled as a service anyway, the same can be accomplished with the property() method on the builder, but nevertheless it is sometimes useful for integrating/initializing third-party libraries, registering destructors against the owner, etc.

Just keep in mind that this library is really just meant for "upgrading" simple plain functions to have access to services, and if you find yourself doing very elaborate things with it, consider whether a service or a stateful class-based helper would be more appropriate.

Another reason (and the original reason) this build() API exists is that it can sometimes provide better type inference for TypeScript. See the next section for details.

TypeScript/Glint Support

This addon comes with types for TypeScript. For the most part, all the code shown in this README so far will work with TypeScript with little to no manual type annotation required.

For example:

// app/helpers/format-date.ts
import inject from 'ember-serviceable-helper';

export default inject.service('preferences').into(function formatDate(
  date: Date,
  options: Intl.DateTimeFormatOptions = {},
) {
  // TypeScript can infer the `this` type from the builder DSL.  Furthermore,
  // it uses the `Registry` type in `@ember/service` to infer the correct
  // type for the `preferences` service
  const userLocale = this.preferences.locale;
  return new Intl.DateTimeFormat(userLocale, options).format(date);
});

Of course, if you prefer, you can type the injections explicitly too, and TypeScript will make sure it is typed correctly (matching the builder DSL):

// app/helpers/format-date.ts
import inject from 'ember-serviceable-helper';
import PreferencesService from 'my-app/services/preferences';

// Explicitly typing the `this` argument
export interface FormatDateInjections {
  preferences: PreferencesService;
}

// This function can be imported in tests and unit tested without going through
// the DI system
export function formatDate(
  this: FormatDateInjections,
  date: Date,
  options: Intl.DateTimeFormatOptions = {},
) {
  const userLocale = this.preferences.locale;
  return new Intl.DateTimeFormat(userLocale, options).format(date);
}

// If the inferred type from the builder DSL differs from your provided `this`
// type, you will get a type error here
export default inject.service('preferences').into(formatDate);

Arguably, this is good documentation for future human readers of the code, but it is not necessarily unsafe to let TypeScript infer it.

If you are using Glint and @glint/environment-ember-loose, you may also want to add it to your template registry:

// app/helpers/format-date.ts
import inject from 'ember-serviceable-helper';
import PreferencesService from 'my-app/services/preferences';

export interface FormatDateInjections {
  preferences: PreferencesService;
}

export function formatDate(
  this: FormatDateInjections,
  date: Date,
  options: Intl.DateTimeFormatOptions = {},
) {
  const userLocale = this.preferences.locale;
  return new Intl.DateTimeFormat(userLocale, options).format(date);
}

// We need to name this return value so we can reference it for the registry
const formatDateHelper = inject.service('preferences').into(formatDate);
export default formatDateHelper;

declare module '@glint/environment-ember-loose/registry' {
  export default interface Registry {
    // Note: we want `typeof formatDateHelper`, not `typeof formatDate` here,
    // see discussion below
    'format-date': typeof formatDateHelper;
  }
}

Unfortunately, in JavaScript/TypeScript, there isn't a syntax that names the result of an expression as a local (let/const) variable and making it the default export at the same time, so this adds a bit of boilerplate, but it is very much necessary.

From the perspective of your function's implementation, the signature of the function is (this: Injections, ...args), but your consumers should not see that this parameter, as it is not an argument that they need to provide when calling your function. In fact, Glint will not let you invoke a function as a template helper unless it matches (this: void, ...args).

The types in the builder DSL takes care of that for you and returns the correct InjectableConstructor type (which also adds the new (owner: Owner) union), it is important that that's the type we export to our consumers.

To do this, it essentially uses the same technique as TypeScript's built-in OmitThisParameter type. Unfortunately, there is an notable documented limitation/caveat here:

Generics are erased and only the last overload signature is propagated into the new function type.

This is not a problem in most cases, but consider this example:

// app/helpers/get-config.ts
import inject from 'ember-serviceable-helper';
import ConfigService from 'my-app/services/config';

export interface GetConfigInjections {
  config: ConfigService;
}

export function getConfig<K extends keyof ConfigService>(
  this: GetConfigInjections,
  key: K,
): ConfigService[K] {
  return this.config.get(key);
}

const getConfigHelper = inject.service('config').into(getConfig);
export default getConfigHelper;

declare module '@glint/environment-ember-loose/registry' {
  export default interface Registry {
    'get-config': typeof getConfigHelper;
  }
}

Suppose the config service has an interface like this:

interface ConfigService {
  build: number;
  commit: string;
  environment: 'development' | 'production';
}

The getConfig function is typed generically such that getConfig('build') will have the return type number, getConfig('commit') returns string, etc.

Unfortunately, because of the documented limitation in TypeScript, the function returned by the builder in this case will have its generics "erased", which is:

function getConfig(key: 'build' | 'commit' | 'environment'): number | string;

This is unfortunate, because now getConfig('build') (and everything else) will have the return type number | string, which is not really what you want.

Fortunately, for cases like this, the build() API does not suffer from the same limitation:

// app/helpers/get-config.ts
import inject from 'ember-serviceable-helper';
import ConfigService from 'my-app/services/config';

export interface GetConfigInjections {
  config: ConfigService;
}

const getConfig = inject.service('config').build(
  ({ config }: GetConfigInjections) =>
    function <K extends keyof ConfigService>(key: K): ConfigService[K] {
      return config.get(key);
    },
);
export default getConfig;

declare module '@glint/environment-ember-loose/registry' {
  export default interface Registry {
    'get-config': typeof getConfig;
  }
}

Because we did not have to erase the this type here, getConfig will have its generic signature left intact.

ember-polaris-service

If you are experimenting with ember-polaris-service, there is an alternative builder API that integrates with that addon instead:

import inject from 'ember-serviceable-helper/polaris-experimental';
import PreferencesService from 'my-app/services/preferences';

export default inject
  .service('preferences', PreferencesService)
  .into(function formatDate(date, options = {}) {
    const userLocale = this.preferences.locale;
    return new Intl.DateTimeFormat(userLocale, options).format(date);
  });

It works the same way as the main builder API, except that .service() takes both the property name and the service token/factory as arguments, and uses the lookup(owner, factory) mechanism from that addon rather than the traditional string-based lookup.

This is more or less a convenience over doing this yourself:

import inject from 'ember-serviceable-helper';
import { lookup } from 'ember-polaris-service';
import PreferencesService from 'my-app/services/preferences';

export default inject
  // This is how you would integrate it manually with the main builder API
  .property('preferences', (owner) => lookup(owner, PreferencesService))
  .into(function formatDate(date, options = {}) {
    const userLocale = this.preferences.locale;
    return new Intl.DateTimeFormat(userLocale, options).format(date);
  });

What about components?

It is certainly possible to make a version of this addon for components:

export default inject.service('config').into(
  <template>now available: {{this.config.foo}}</template>
);

Personally, I don't think the same problem exists and that the meta-programming "pays for itself" on components. With Glimmer components and <template>, the "upgrade path" into classes feels seamless, and the doesn't feel like a falling off an awkward cliff in the same way that helper -> class-based helper does to me.

In my opinion, for components, upgrading a <template> into the conventional class syntax when you need services is appropriate. You are very likely to need class-based components for other purposes in your app already, it's not worth adding another way to accomplish that same thing. (On the other hand, this was originally created so that we can eliminate usages of class-based helpers in the app, and that felt better motivated.)

What about modifiers?

Creating a version of this for function-based/stateless (other than cleanup) modifiers is certainly worth considering, largely for the same kind of reasons that motivated the creation of this addon for helpers. That being said, I don't currently have the use case for it and would prefer to see how things shake out for the helpers version before investing the effort, but if you are interested, certainly go for it. With the v2 addon blueprint, this set up as a monorepo and ready to accommodate additional sibling packages.

Compatibility

[!WARNING] This started as an extraction from work projects on a weekend, at the moment I am just relying on the work project test suite to confirm that it works as expected. I am a bit biased and thinks the test suite is pretty good, but it's just one app on one particular version of Ember/TypeScript/Glint, and we don't have many such helpers. So if you are going to use this, maybe consider helping out by adding some tests in the test app, it'll help me not break you in the future. Thank you and feel free to remove this warning in the PR.

• Ember.js v4.12 or above
• Embroider or ember-auto-import v2

Installation

ember install ember-serviceable-helper

Contributing

See the Contributing guide for details.

License

This project is licensed under the MIT License.