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

## Overview

The `determineLocale` method determines the best matching locale from a list of approved locales based on user preferences.
It implements locale negotiation to find the most suitable locale when exact matches aren't available.

```typescript
const gt = new GT({
  sourceLocale: 'en-US',
  locales: ['en-US', 'es-ES', 'fr-FR', 'de-DE']
});

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

// Language fallback
console.log(gt.determineLocale('en-GB')); // 'en-US' (closest English variant)

// Multiple preferences
console.log(gt.determineLocale(['fr-CA', 'es-MX', 'en-US'])); // 'es-ES' (closest Spanish)

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

---

## Reference

### Parameters

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

### Returns

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

---

## Examples

### User locale negotiation

```typescript
const gt = new GT({
  sourceLocale: 'en-US',
  locales: ['en-US', 'en-GB', 'es-ES', 'fr-FR']
});

// Simulate browser Accept-Language header
const userPreferences = ['fr-CA', 'en-GB', 'en'];
const bestMatch = gt.determineLocale(userPreferences);
console.log(bestMatch); // 'fr-FR' based on preference order

```

---

## Notes

- Returns the first exact match from approved locales
- Falls back to language matches when exact region isn't available  
- Respects preference order in input array
- Returns undefined when no suitable match is found
- Essential for implementing locale negotiation in web applications

## Next steps

- Check translation needs with [`requires-translation`](/docs/core/class/methods/locales/requires-translation)
- Compare languages with [`is-same-language`](/docs/core/class/methods/locales/is-same-language)