Cart buyer identity is used for associating customer information with a cart. You should update cart buyer identity when customer information is available:

- Update `countryCode` when a customer changes site currency or locale so the cart will return pricing in the correct currency.
- Update `customerAccessToken` when a customer logs into a customer account so the cart will return selected delivery options.

This guide shows you how to use the cart handler to update buyer identity.

## Requirements

- You've completed the [quickstart guide](/docs/storefronts/headless/hydrogen/getting-started).
- You've [set up a cart handler](/docs/storefronts/headless/hydrogen/cart/setup).

## Handle the buyer identity update

Handle the buyer identity update in an `action`. Use the `cart`, created from [`createCartHandler`](/docs/api/hydrogen/latest/utilities/createcarthandler), to handle cart mutation requests to the Storefront API.

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<script type="text/plain" data-language="jsx" data-title="JavaScript" data-filename="/app/routes/account.login.jsx">
export async function action({request, context}) {
  const {cart, session} = context;
  const formData = await request.formData();

  const email = formData.get('email');
  const password = formData.get('password');

  try {
    // Obtain a customer access token
    // https://shopify.dev/docs/custom-storefronts/building-with-the-storefront-api/customer-accounts#step-3-create-an-access-token
    const customerAccessToken = await doLogin(context, {email, password});
    session.set('customerAccessToken', customerAccessToken);

    // Sync customerAccessToken with existing cart
    const result = await cart.updateBuyerIdentity({customerAccessToken});

    // Update cart id in cookie
    const headers = cart.setCartId(result.cart.id);

    // Update session
    headers.append('Set-Cookie', await session.commit());

    return redirect(params.locale ? `/${params.locale}/account` : '/account', {
      headers,
    });
  } catch (error: any) {
    // handle error
  }
};
</script>
<script type="text/plain" data-language="jsx" data-title="TypeScript" data-filename="/app/routes/account.login.tsx">
export async function action({request, context}: ActionArgs) {
  const {cart, session} = context;
  const formData = await request.formData();

  const email = formData.get('email');
  const password = formData.get('password');

  try {
    // Obtain a customer access token
    // https://shopify.dev/docs/custom-storefronts/building-with-the-storefront-api/customer-accounts#step-3-create-an-access-token
    const customerAccessToken = await doLogin(context, {email, password});
    session.set('customerAccessToken', customerAccessToken);

    // Sync customerAccessToken with existing cart
    const result = await cart.updateBuyerIdentity({customerAccessToken});

    // Update cart id in cookie
    const headers = cart.setCartId(result.cart.id);

    // Update session
    headers.append('Set-Cookie', await session.commit());

    return redirect(params.locale ? `/${params.locale}/account` : '/account', {
      headers,
    });
  } catch (error: any) {
    // handle error
  }
};
</script>

</div>
</p>


## Next steps

- Learn how to [apply cart notes](/docs/storefronts/headless/hydrogen/cart/notes).