# generaltranslation: General Translation Core SDK: determineLocale
URL: https://generaltranslation.com/en-GB/docs/core/functions/locales/determine-locale.mdx
---
title: determineLocale
description: API reference for the determineLocale function
---

## Overview

The `determineLocale` function finds the best matching locale from the approved locales based on user preferences, without requiring a GT class instance.

***

## Reference

### Parameters

<TypeTable
  type={{
  "locales": {
    type: 'string | string[]',
    optional: false,
  },
  "approvedLocales?": {
    type: 'string[]',
    optional: true,
    default: '[]'
  },
  "customMapping?": {
    type: 'CustomMapping',
    optional: true,
  }
}}
/>

### Returns

`string | undefined` - Best matching locale or undefined if no match

***

## Examples

### Content negotiation

```typescript
import { determineLocale } from 'generaltranslation';

const approvedLocales = ['en-US', 'es-ES', 'fr-FR', 'de-DE'];

// Exact match
console.log(determineLocale('en-US', approvedLocales)); // 'en-US'

// Language fallback
console.log(determineLocale('en-GB', approvedLocales)); // 'en-US'

// Multiple preferences
console.log(determineLocale(['fr-CA', 'es-MX'], approvedLocales)); // 'es-ES'

// No match
console.log(determineLocale('it-IT', approvedLocales)); // undefined
```

***


## Notes

* Implements intelligent locale negotiation
* Returns the first exact or language match from the approved list
* Respects preference order in the input array
* Returns undefined when no match is found
* Essential for web application locale negotiation

## Next steps

* Use the GT class method [`determineLocale`](/docs/core/class/methods/locales/determine-locale)
* Check whether translation is needed with [`requiresTranslation`](/docs/core/functions/locales/requires-translation)