# Add currency exchange rates and conversion to React

Call your own backend from React so the API key never reaches the browser. This example pairs a small Express proxy with a useEffect converter.

## Get a free API key

Sign up at https://currencyexchangerate-api.com/auth/login, then create a key under API keys in the console. The free plan includes 3,000 requests per month with no credit card. Store the secret as CXR_API_KEY in server-side environment configuration; never expose it in browser code.

File: `.env`

```shell
CXR_API_KEY=cxr_your_secret_key
```

## Create a reusable client and cache rates

Use the standard HTTP client. Cache every /latest response until nextUpdateAt: rates change at most once by then, and reusing the response saves quota. The helper also exposes /convert.

File: `server.js`

```javascript
import express from 'express';

const API_BASE_URL = 'https://api.currencyexchangerate-api.com/api/v1';
const app = express();

app.get('/api/convert', async (req, res) => {
  const query = new URLSearchParams({
    from: String(req.query.from),
    to: String(req.query.to),
    amount: String(req.query.amount),
  });
  const response = await fetch(API_BASE_URL + '/convert?' + query, {
    headers: { Authorization: 'Bearer ' + process.env.CXR_API_KEY },
  });
  res.status(response.status).json(await response.json());
});

app.listen(3000);
```

## Use it in React

Call the helper from a server route, controller, view, command, or script. Format converted values for the user’s locale.

File: `Converter.jsx`

```jsx
import { useEffect, useState } from 'react';

export function Converter() {
  const [result, setResult] = useState(null);

  useEffect(() => {
    fetch('/api/convert?from=EUR&to=GBP&amount=100')
      .then((response) => response.json())
      .then(setResult);
  }, []);

  if (!result) {
    return <p>Loading...</p>;
  }

  const formatted = new Intl.NumberFormat('en-GB', {
    style: 'currency',
    currency: result.to,
  }).format(result.result);
  return <p>{formatted}</p>;
}
```

## Handle API errors

A 401 means the key is invalid. A 429 can mean a rate limit or quota limit; read Retry-After in seconds before retrying. Other errors use application/problem+json.

File: `errors.js`

```javascript
// In get(), replace the generic throw with:
//   if (!response.ok) throw await apiError(response);
async function apiError(response) {
  const problem = await response.json().catch(() => ({}));
  if (response.status === 401) {
    return new Error('Check CXR_API_KEY');
  }
  if (response.status === 429) {
    const retryAfter = response.headers.get('Retry-After');
    return new Error('Retry after ' + retryAfter + ' seconds');
  }
  return new Error(problem.detail || 'Rates API status ' + response.status);
}
```

## Format the amount for display

Use locale-aware currency formatting instead of adding a symbol manually.

File: `format.js`

```javascript
const formatter = new Intl.NumberFormat('en-GB', {
  style: 'currency',
  currency: 'EUR',
});
const formatted = formatter.format(85.249105);
```

## Related links

- [API documentation](https://currencyexchangerate-api.com/docs.md)
- [All integration guides](https://currencyexchangerate-api.com/guides.md)
- [Get a free API key](https://currencyexchangerate-api.com/auth/login)