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

## Overview

The `determineLocale` function determines the best matching locale from 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 first exact or language match from approved list
- Respects preference order in input array
- Returns undefined when no match found
- Essential for web application locale negotiation

## Next steps

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