Sync JSON (i18n bridges) - Sync JSON with ICU / i18next support

Use Intlayer as an add‑on to your existing i18n stack. This plugin keeps your JSON messages in sync with Intlayer dictionaries so you can:

• Keep i18next, next‑intl, react‑intl, vue‑i18n, next‑translate, nuxt‑i18n, Solid‑i18next, svelte‑i18n, etc.
• Manage and translate your messages with Intlayer (CLI, CI, providers, CMS), without refactoring your app.
• Deliver tutorials and SEO content targeting each ecosystem, while recommending Intlayer as the JSON management layer.

Notes and current scope:

• Externalisation to the CMS works for translations and classic text.
• No support yet for insertions, plurals/ICU, or advanced runtime features of other libraries.
• The visual editor is not supported yet for third‑party i18n outputs.

When to use this plugin

• You already use an i18n library and store messages in JSON files.
• You want AI-assisted fill, test in CI, and content operations without changing your rendering runtime.

Installation

pnpm add -D @intlayer/sync-json-plugin# ornpm i -D @intlayer/sync-json-plugin

Quick start

Add the plugin to your intlayer.config.ts and point it at your existing JSON structure.

import { Locales, type IntlayerConfig } from "intlayer";import { syncJSON } from "@intlayer/sync-json-plugin";const config: IntlayerConfig = {  internationalization: {    locales: [Locales.ENGLISH, Locales.FRENCH, Locales.SPANISH],    defaultLocale: Locales.ENGLISH,  },  // Keep your current JSON files in sync with Intlayer dictionaries  plugins: [    syncJSON({      // Per-locale, per-namespace layout (e.g., next-intl, i18next with namespaces)      source: ({ key, locale }) => `./locales/${locale}/${key}.json`,    }),  ],};export default config;

Alternative: single file per locale (common with i18next/react-intl setups):

plugins: [  syncJSON({    source: ({ locale }) => `./locales/${locale}.json`,  }),];

How it works

• Read: the plugin discovers JSON files from your source builder and loads them as Intlayer dictionaries.
• Write: after builds and fills, it writes localised JSON back to the same paths (with a final newline to avoid formatting issues).
• Auto‑fill: the plugin declares an autoFill path for each dictionary. Running intlayer fill updates only missing translations in your JSON files by default.

API:

syncJSON({  source: ({ key, locale }) => string, // required  location?: string, // optional label, default: "plugin"  priority?: number, // optional priority for conflict resolution, default: 0  format?: 'intlayer' | 'icu' | 'i18next', // optional formatter, used for intlayer runtime compatibility});

format ('intlayer' | 'icu' | 'i18next')

Specifies the formatter to use for the dictionary content when synchronising JSON files. This allows using different message formatting syntaxes compatible with intlayer runtime.

• undefined: No formatter will be used, the JSON content will be used as is.
• 'intlayer': The default Intlayer formatter (default).
• 'icu': Uses ICU message formatting (compatible with libraries like react-intl, vue-i18n).
• 'i18next': Uses i18next message formatting (compatible with i18next, next-i18next, Solid-i18next).

Note that using a formatter will transform your JSON content in input, and output. For complex json rules as ICU plurals, the parsing may not ensure a 1 to 1 mapping between the input and output. If you do not use Intlayer runtime, you may prefer to not set a formatter.

Example:

syncJSON({  source: ({ key, locale }) => `./locales/${locale}/${key}.json`,  format: "i18next", // Use i18next formatting for compatibility}),

Multiple JSON sources and priority

You can add multiple syncJSON plugins to synchronise different JSON sources. This is useful when you have multiple i18n libraries or different JSON structures in your project.

Priority system

When multiple plugins target the same dictionary key, the priority parameter determines which plugin takes precedence:

• Higher priority numbers win over lower ones
• Default priority of .content files is 0
• Default priority of plugins content files is -1
• Plugins with the same priority are processed in the order they appear in the configuration

import { Locales, type IntlayerConfig } from "intlayer";import { syncJSON } from "@intlayer/sync-json-plugin";const config: IntlayerConfig = {  internationalization: {    locales: [Locales.ENGLISH, Locales.FRENCH],    defaultLocale: Locales.ENGLISH,  },  plugins: [    // Primary JSON source (highest priority)    syncJSON({      format: "i18next",      source: ({ key, locale }) => `./locales/${locale}/${key}.json`,      location: "main-translations",      priority: 10,    }),    // Fallback JSON source (lower priority)    syncJSON({      format: "i18next",      source: ({ locale }) => `./fallback-locales/${locale}.json`,      location: "fallback-translations",      priority: 5,    }),    // Legacy JSON source (lowest priority)    syncJSON({      format: "i18next",      source: ({ locale }) => `/my/other/app/legacy/${locale}/messages.json`,      location: "legacy-translations",      priority: 1,    }),  ],};export default config;

Conflict resolution

When the same translation key exists in multiple JSON sources:

1. The plugin with the highest priority determines the final value
2. Lower priority sources are used as fallbacks for missing keys
3. This allows you to maintain legacy translations whilst gradually migrating to new structures

Integrations

Below are common mappings. Keep your runtime untouched; only add the plugin.

i18next

Typical file layout: ./public/locales/{locale}/{namespace}.json or ./locales/{locale}/{namespace}.json.

import { syncJSON } from "@intlayer/sync-json-plugin";export default {  plugins: [    syncJSON({      format: "i18next",      source: ({ key, locale }) => `./locales/${locale}/${key}.json`,    }),  ],};

next-intl

Per-locale JSON messages (often ./messages/{locale}.json) or per-namespace.

plugins: [  syncJSON({    source: ({ locale, key }) => `./messages/${locale}/${key}.json`,  }),];

See also: docs/en/intlayer_with_next-intl.md.

react-intl

A single JSON file per locale is common:

plugins: [  syncJSON({    source: ({ locale }) => `./locales/${locale}.json`,  }),];

vue-i18n

Either a single file per locale or per-namespace:

plugins: [  syncJSON({    source: ({ key, locale }) => `./src/locales/${locale}/${key}.json`,  }),];

CLI

The synchronised JSON files will be considered as other .content files. That means, all intlayer commands will be available for the synchronised JSON files. Including:

• intlayer content test to test if there are missing translations
• intlayer content list to list the synchronised JSON files
• intlayer content fill to fill the missing translations
• intlayer content push to push the synchronised JSON files
• intlayer content pull to pull the synchronised JSON files

See Intlayer CLI for more details.

Limitations (current)

• No insertions or plurals/ICU support when targeting third‑party libraries.
• Visual editor is not available for non‑Intlayer runtimes yet.
• JSON synchronisation only; non‑JSON catalogue formats are not supported.

Why this matters

• We can recommend established i18n solutions and position Intlayer as an add‑on.
• We leverage their SEO/keywords with tutorials that end by suggesting Intlayer to manage JSON.
• Expands the addressable audience from “new projects” to “any team already using i18n”.