Custom Formatter

If your project requires a message catalog format that Lingui doesn't natively support, you can create a custom formatter to handle it. A custom formatter allows you to define how extracted strings are formatted into a custom catalog format, providing flexibility for specialized workflows and integration with unique file structures.

Overview

A formatter is an object with two main functions, parse and serialize, which define how catalogs are read from and written to your custom format.

The formatter can be configured directly in your lingui.config.{ts,js} file - no separate package is needed:

lingui.config.{ts,js}

import { defineConfig } from "@lingui/cli";
import { extractor } from "./my-custom-extractor.ts";

export default defineConfig({
  // [...]
  format: {
    catalogExtension: "json",
    parse: (content: string): CatalogType => JSON.parse(content),
    serialize: (catalog: CatalogType): string => JSON.stringify(catalog),
  },
});

Reference

The shape of formatter is the following:

export type CatalogFormatter = {
  catalogExtension: string;
  /**
   * Set extension used when extract to template
   * Omit if the extension is the same as catalogExtension
   */
  templateExtension?: string;
  parse(
    content: string,
    ctx: { locale: string | null; sourceLocale: string; filename: string }
  ): Promise<CatalogType> | CatalogType;
  serialize(
    catalog: CatalogType,
    ctx: { locale: string | null; sourceLocale: string; filename: string; existing: string | null }
  ): Promise<string> | string;
};

Lingui Catalog is an object with the following structure:

export type CatalogType = {
  [msgId: string]: MessageType;
};

type CatalogExtra = Record<string, unknown>;

export type MessageType<Extra = CatalogExtra> = {
  message?: string;
  origin?: MessageOrigin[];
  comments?: string[];
  obsolete?: boolean;
  context?: string;
  translation?: string;

  /**
   * the generic field where
   * formatters can store additional data
   */
  extra?: Extra;
};

Important

If you are using TypeScript to create your formatter, you should use the .ts extension for your Lingui configuration file.