---
title: Usage
description: Basic operations for defining error classes, registering messages, switching languages, prefixes, and fallback.
---

## Basic flow [#basic-flow]

1. **Define an error class**

    Extend `I18nErrorBase<TMeta>` and define info for message generation in `TMeta`.

2. **Register per-language messages**

    Register the message for each class and language with `setErrorMessage`.

3. **Switch languages and read message**

    Switch with `setGlobalConfig({ lang })` and read `error.message`.

```ts
import { setGlobalConfig } from "valibot";
import { I18nErrorBase, setErrorMessage } from "i18n-error-base";

class BusinessRuleError extends I18nErrorBase<{ code: string; limit: number }> {
  public constructor(meta: { code: string; limit: number }) {
    super(meta, ({ code, limit }) => `Code: ${code}, Limit: ${limit}`);
  }
}

setErrorMessage(
  BusinessRuleError,
  ({ code, limit }) => `コード: ${code}、制限: ${limit}`,
  "ja",
);

const error = new BusinessRuleError({ code: "ERR", limit: 50 });

setGlobalConfig({ lang: "ja" });
console.log(error.message); // コード: ERR、制限: 50

setGlobalConfig({ lang: "en" });
console.log(error.message); // Code: ERR, Limit: 50
```

## Define an error class [#define-error-class]

`TMeta` is auxiliary info for message generation. It is kept in `error.meta` and passed to registered functions.

```ts
import { I18nErrorBase } from "i18n-error-base";

class NotFoundError extends I18nErrorBase<{ resource: string }> {}

const error = new NotFoundError({ resource: "user" }, "fallback message");
console.log(error.meta); // { resource: "user" }
```

The second argument is a string or a `(meta) => string` function. The third is `{ cause }` and other `Error` options.

```ts
class TestError extends I18nErrorBase<{ code: string }> {}

const cause = new Error("original");
const error = new TestError({ code: "ERR" }, "something went wrong", { cause });
console.log(error.cause === cause); // true
```

Setting `TMeta` to `undefined` lets you omit `meta`.

```ts
class NoMetaError extends I18nErrorBase<undefined> {}

const a = new NoMetaError("no meta");
const b = new NoMetaError(undefined, "no meta");
console.log(a.meta); // undefined
```

:::note
Do not use `new` directly; always inherit. Registrations are per-class, so parent registrations are not inherited by children.
:::

## Register messages [#register-message]

`setErrorMessage` registers per-class, per-language factory functions.

```ts
import { I18nErrorBase, setErrorMessage } from "i18n-error-base";

class TestError extends I18nErrorBase<{ code: string }> {}

setErrorMessage(TestError, ({ code }) => `エラーコード: ${code}`, "ja");
setErrorMessage(TestError, "something went wrong", "en");
```

Strings are also allowed. Languages can be specified as arrays.

```ts
const message = ({ code }: { code: string }) => `エラーコード: ${code}`;
setErrorMessage(TestError, message, ["ja", "ko", "zh"]);
```

Re-registering the same class and language overwrites. It applies only when the constructors at registration and lookup match, not via `instanceof`.

:::warning[Watch out for access before registration]
Reading `message` first fixes the default value. Register before creating instances.
:::

## Switch languages [#switch-language]

Switch with Valibot's global config. `message` refers to the current `lang` on each read.

```ts
import { setGlobalConfig } from "valibot";

setGlobalConfig({ lang: "ja" });
console.log(error.message); // 日本語の登録メッセージ

setGlobalConfig({ lang: "en" });
console.log(error.message); // 英語の登録メッセージ or 既定メッセージ
```

Unset `lang` is `"en"`. Unregistered languages use the constructor's default message.

```ts
setErrorMessage(BusinessRuleError, () => "日本語メッセージ", "ja");

setGlobalConfig({ lang: "fr" });
console.log(error.message); // "Code: ERR, Limit: 50"（既定メッセージ）
```

## Add a prefix [#prefix]

`static prefix` is prepended to the resolved message. It applies to both translated and default messages.

```ts
class PrefixedError extends I18nErrorBase<{ code: string }> {
  static {
    this.prefix = "[ERROR] ";
  }
}

setErrorMessage(PrefixedError, ({ code }) => `code is ${code}`, "ja");
const error = new PrefixedError({ code: "ERR" }, "fallback");

setGlobalConfig({ lang: "ja" });
console.log(error.message); // "[ERROR] code is ERR"

setGlobalConfig({ lang: "en" });
console.log(error.message); // "[ERROR] fallback"
```

When unset, it is returned as is.

## Behavior as an Error [#error-behavior]

- `error instanceof Error`, `instanceof I18nErrorBase`, and `instanceof <concrete class>` are all `true`
- `String(error)` / `error.toString()` include the resolved message (e.g. `"Error: ..."`)
- `cause` is kept via `{ cause }`

## Common patterns [#common-patterns]

### Fix the default message in the constructor [#fix-default-message-in-constructor]

```ts
class BusinessRuleError extends I18nErrorBase<{ code: string; limit: number }> {
  public constructor(meta: { code: string; limit: number }, options?: ErrorOptions) {
    super(meta, ({ code, limit }) => `Code: ${code}, Limit: ${limit}`, options);
  }
}
```

Callers only pass `meta`, avoiding duplication of the default text.

### Register multiple languages at once [#register-multiple-languages]

```ts
setErrorMessage(MyError, "エラーが発生しました", ["ja"]);
setErrorMessage(MyError, "An error occurred", ["en", "en-US"]);
```

Language codes are not normalized. `"en"` and `"en-US"` are separate keys, so register needed codes explicitly.