core-i18n provides core internationalization functionality for JavaScript. With first-class TypeScript support, it ensures type safety and simplifies managing translations across languages.

Common Functions

createTranslate

Creates a translation function that retrieves localized messages based on a specified locale. It supports placeholders in messages for dynamic content and handles plurals.

Parameters

• messages Record<string, Record<string, unknown>>

  Object with BCP 47 language tag keys and their corresponding messages: { "en-EN": {...}, "de-DE": {...} }.

  Note: Use as const assertion in TypeScript to ensure message values are inferred as literal types for better type safety: { "en-EN": {...}, "de-DE": {...} } as const.

• locale string

  key of messages

Basic usage

import { createTranslate } from "core-i18n"
// const { createTranslate } = require("core-i18n") // legacy way

// Define messages
const messages = {
  "en-EN": { email: "Email Address" },
  "de-DE": { email: "E-Mail-Adresse" },
} as const // TypeScript: "as const" assertion for better type safety

// Create translate function
const t = createTranslate(messages, "en-EN")

// Translate
const translation = t("email")
// Result: "Email Address"

Placeholders

Placeholders are variables for content, following the pattern {placeholder}, where:

1. Curly Braces {}: Mark the placeholder’s start and end to distinguish it from regular text.
2. Placeholder Name: A descriptive name inside the braces, e.g. {name} for a name.

const messages = {
  "en-EN": { farewell: "Goodbye, {city}!" },
  "de-DE": { farewell: "Auf Wiedersehen, {city}!" },
} as const // TypeScript: "as const" assertion for better type safety

const t = createTranslate(messages, "en-EN")

const translation = t("farewell", { city: "Berlin" })
// Result: "Goodbye, Berlin!"

Cardinal plurals (default)

Plural integration uses the Intl.PluralRules API:

• Check compatibility
• For detailed information about the rules and their usage, refer to the Plural Rules documentation.
• For a comprehensive list of rules and their application across different languages, see the LDML Language Plural Rules.

Declare plural translations by appending # followed by zero, one, two, few, many, or other:

const messages = {
  "en-EN": {
    "availability#zero": "Item currently unavailable",
    "availability#one": "Only one item available",
    "availability#other": "Many items available",
  },
} as const // TypeScript: "as const" assertion for better type safety

const t = createTranslate(messages, "en-EN")

const translation = t("availability", { count: 1 })
// Result: "Only one item available"

Special translations for { count: 0 } are allowed to enable more natural language. If a #zero entry exists, it replaces the default plural form:

const messages = {
  "en-EN": {
    "apple#zero": "You have no apples.",
    "apple#other": "You have {count} apples.",
  },
} as const // TypeScript: "as const" assertion for better type safety

const t = createTranslate(messages, "en-EN")

const translation = t("apple", { count: 0 })
// Result: "You have no apples."

Ordinal plurals

Ordinal numbers are also supported (e.g. “1st”, “2nd”, “3rd” in English). The ordinal option ensures the correct plural key is selected based on the ordinal value.

const messages = {
  "en-EN": {
    "direction#zero": "zero",
    "direction#one": "Take the {count}st right.",
    "direction#two": "Take the {count}nd right.",
    "direction#few": "Take the {count}rd right.",
    "direction#other": "Take the {count}th right.",
  },
} as const // TypeScript: "as const" assertion for better type safety

const t = createTranslate(messages, "en-EN")

const translation = t("direction", { count: 3, ordinal: true })
// Result: "Take the 3rd right."

Type safety

Type safety in i18n ensures that only valid translation keys are used, catching errors like missing keys or wrong placeholders during development. This improves developer productivity, reduces runtime bugs, and ensures a consistent, error-free user experience across all languages.

Locale

Locale validation ensures only predefined language keys, like en-EN or de-DE, are used to maintain consistency.

Translation keys

Strict key validation ensures only valid translation keys are used.

Placeholders & Pluralization

Supports placeholders and pluralization with type-safe suggestions for required properties.

Utility Functions

flattenObject

Flattens a nested object into a single-level object with dot-separated keys.

Parameters

• obj Record<string, unknown>

  object to flatten

• prefix string

  optional key prefix

Example

import { flattenObject } from "core-i18n"
// const { flattenObject } = require("core-i18n") // legacy way

const nestedObject = {
  car: {
    brand: "BMW",
    model: "M5",
    features: { autopilot: true, color: "red" },
  },
}

const flattenedObject = flattenObject(nestedObject)
// Result: { "car.brand": "BMW", "car.model": "M5", "car.features.autopilot": true, "car.features.color": "red" }

// Example with prefix

const nestedObject = {
  car: {
    brand: "BMW",
  },
}

const flattenedObject = flattenObject(nestedObject, "myPrefix")
// Result: { "myPrefix.car.brand": "BMW" }

getPluralKey

Returns the appropriate plural key including support for zero based on the count using the Intl.PluralRules API.

For detailed information about the rules and their usage, refer to the Plural Rules documentation. For a comprehensive list of rules and their application across different languages, see the LDML Language Plural Rules.

Parameters

• count number

  used to determine the plural form key

• pluralRules Intl.PluralRules

  pluralization rules for a specific locale

Example

import { getPluralKey } from "core-i18n"
// const { getPluralKey } = require("core-i18n") // legacy way

const pluralRulesEnUS = new Intl.PluralRules("en-US")

const pluralKey = getPluralKey(0, pluralRulesEnUS)
// Result: "zero"

const pluralKey = getPluralKey(2, pluralRulesEnUS)
// Result: "other"

replacePlaceholders

Replaces placeholders in a string.

Parameters

• value string

  string containing placeholders in the {placeholder} pattern, where:

  1. Curly Braces {}: Mark the placeholder’s start and end to distinguish it from regular text.
  2. Placeholder Name: A descriptive name inside the braces, e.g. {name} for a name.

• placeholders Record<string, string>

  object with key-value pairs for placeholder replacement

Example

import { replacePlaceholders } from "core-i18n"
// const { replacePlaceholders } = require("core-i18n") // legacy way

const message = replacePlaceholders("ID: {id}, Price: {price}", {
  id: 123,
  price: 19.99,
})
// Result: "ID: 123, Price: 19.99"

Utility Types

ExtractPrefix<Value extends string>

Extracts the prefix from a string separated by #. If no # is present, the entire string is returned:

import type { ExtractPrefix } from "core-i18n"

ExtractPrefix<"prefix#suffix">
// Type Result: "prefix"

ExtractPrefix<"noSeparator">
// Type Result: "noSeparator"

FlattenObjectKeys<Obj, Prefix extends string = "">

Flattens object keys into dot notation, supporting union objects.

import type { FlattenObjectKeys } from "core-i18n"

type Account = {
  user: {
    name: string
    age: number
  }
  active: boolean
}

FlattenObjectKeys<Account>
// Type Result: "user.name" | "user.age" | "active"

FlattenObjectKeys<Account, "myPrefix.">
// Type Result: "myPrefix.user.name" | "myPrefix.user.age" | "myPrefix.active"

// Union object example

type FailedState = {
  state: "failed"
  code: number
}

type SuccessState = {
  state: "success"
  response: {
    title: string
  }
}

FlattenObjectKeys<FailedState | SuccessState>
// Type Result: "state" | "code" | "response.title"

GetObjectKeyValue<Obj, Key extends ExtractPrefix<FlattenObjectKeys<Obj>>>

Retrieves the type of a value for a flattened key in an object. Supports union objects and pluralized keys separated by a #.

import type { GetObjectKeyValue } from "core-i18n"

type TestType = {
  car: {
    manufacturer: "BMW"
  }
}

GetObjectKeyValue<TestType, "car.manufacturer">
// Type Result: "BMW"

// Example with pluralized key

type TestType = {
  "apple#zero": "You have no apples."
  "apple#other": "You have {count} apples."
}

GetObjectKeyValue<TestType, "apple">
// Type Result: "You have no apples." | "You have {count} apples."

IsPlural<Value extends string, Prefix extends ExtractPrefix<Value>>

Checks if a string matches the pattern Prefix#Suffix.

import type { IsPlural } from "core-i18n"

IsPlural<"user#name" | "user#age", "user">
// Type Result: true -> "user" is plural

// multiple strings match the pattern 'Prefix#Suffix' including non matching strings
IsPlural<"order#amount" | "order#date" | "avatar", "order">
// Type Result: true -> "order" is plural

// no string matches the pattern 'Prefix#string'
IsPlural<"username" | "avatar", "username">
// Type Result: never

ExtractPlaceholders<Value>

Extracts placeholders from a string that match the {placeholder} pattern.

import type { ExtractPlaceholders } from "core-i18n"

ExtractPlaceholders<"Hello, {firstname} {lastname}">
// Type Result: "firstname" | "lastname"