Custom faker instance with en locale cannot generate system variables

[terrymun]

Pre-Checks

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• The provided reproduction is a minimal reproducible example of the bug.
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Describe the bug

I am using a custom faker instance in my app, i.e.:

import { faker, Faker, en } from '@faker-js/faker';

const customFaker = new Faker({
  locale: en,
  seed: 1337,
});
console.log(customFaker.system.fileName());

However, I get an error in the console with this message:

"The locale data for 'system.mime_type' are missing in this locale"

However, en is the locale with the widest / largest dataset and I would expect it to have system.mine_type implemented. Using the default faker instance does not give me any issues, but I assume that the default faker is also using the "en" locale.

Minimal reproduction code

See example here: https://stackblitz.com/edit/vitejs-vite-iwf3husr

To reproduce this code independently, you can do this:

import { faker, Faker, en } from '@faker-js/faker';

const customFaker = new Faker({
  locale: en,
  seed: 1337,
});

// NOTE: This works!
console.log(faker.system.fileName());

// NOTE: This will throw an error in the console
//       "The locale data for 'system.mime_type' are missing in this locale"
console.log(customFaker.system.fileName());

Additional Context

No response

Environment Info

System:
  OS: Linux 5.0 undefined
  CPU: (8) x64 Intel(R) Core(TM) i9-9880H CPU @ 2.30GHz
  Memory: 0 Bytes / 0 Bytes
  Shell: 1.0 - /bin/jsh
Binaries:
  Node: 18.20.3 - /usr/local/bin/node
  Yarn: 1.22.19 - /usr/local/bin/yarn
  npm: 10.2.3 - /usr/local/bin/npm
  pnpm: 8.15.6 - /usr/local/bin/pnpm
npmPackages:
  @faker-js/faker: ^9.5.0 => 9.5.0

Which module system do you use?

• CJS
• ESM

Used Package Manager

npm

[terrymun]

Realized that I need to extend locale with base as well, my bad.

[matthewmayer]

I wonder if there is a way we could emit a warning if someone makes a custom faker without extending base? As I feel this is likely to be not intentional most of the time.

[ST-DDT]

Maybe we should add a hint to the constructor like

/**
 * ...
 * Please make sure that all required locales including parent locales are present e.g. `[de_AT, de, en, base]`
 * ...
 */

And extend the error message a bit:

    throw new FakerError(
      `The locale data for '${path.join('.')}' are missing in this locale.
  If this is a custom Faker instance, please make sure all required locales are used e.g. '[de_AT, de, en, base]'.
  Please contribute missing data to the project or use a locale/Faker instance that has these data.
  For more information see https://fakerjs.dev/guide/localization.html`
    );

?

[terrymun]

@ST-DDT That's a great idea. I have to admit though the error I encountered is a user error coming from myself because I totally missed the part of extending off base in the docs...

[matthewmayer]

Still we can always improve error messages etc to reduce human error 😀

By the way why did you feel you had to make a custom faker instance - if you just need a custom seed you don't need one? Or was this a simplified example just to explain the bug and you actually need a custom Faker instance?

[terrymun]

@matthewmayer Your second guess is correct: we needed to create a custom faker instance with additional fake data, and the example I shown is simply a contrived one but minimal enough to reproduce the "issue" (which turns out to be a user error, hehe)

[matthewmayer]

@ST-DDT what do you think about adding a check like this to the constructor?

    if (locale && (typeof locale === 'string' || locale.length === 1)) {
      // eslint-disable-next-line no-undef
      console.warn(
        '[@faker-js/faker] only a single locale was provided. Was this intentional? In general, pass an array of locales ending with the `base` locale, for example [customLocale, de_CH, de, en, base]. See https://fakerjs.dev/guide/localization.html#custom-locales-and-fallbacks'
      );
    }

[ST-DDT]

If you really want to save some bundle size, you might want to create a locale instance that only contains the data you need prepackaged.
So IMO the single locale usecase is too useful to interfere with it using a warning.

[ST-DDT]

I created PR to improve the error message: #3406

• docs: improve missing data error #3406