# Buyer Journey
The API for interacting with the buyer journey.
### Block progress and show error for a checkout UI field

```jsx
import {
  reactExtension,
  useBuyerJourneyIntercept,
  useShippingAddress,
} from '@shopify/ui-extensions-react/checkout';

export default reactExtension(
  'purchase.checkout.delivery-address.render-before',
  () => <Extension />,
);

function Extension() {
  const address = useShippingAddress();

  useBuyerJourneyIntercept(
    ({canBlockProgress}) => {
      return canBlockProgress &&
        address?.countryCode &&
        address.countryCode !== 'CA'
        ? {
            behavior: 'block',
            reason: 'Invalid shipping country',
            errors: [
              {
                message:
                  'Sorry, we can only ship to Canada',
                // Show an error underneath the country code field
                target:
                  '$.cart.deliveryGroups[0].deliveryAddress.countryCode',
              },
              {
                // In addition, show an error at the page level
                message:
                  'Please use a different address.',
              },
            ],
          }
        : {
            behavior: 'allow',
          };
    },
  );

  return null;
}

```

```js
import {extension} from '@shopify/ui-extensions/checkout';

export default extension(
  'purchase.checkout.delivery-address.render-before',
  (root, {buyerJourney, shippingAddress}) => {
    let address = shippingAddress?.current;
    shippingAddress?.subscribe((newAddress) => {
      address = newAddress;
    });

    buyerJourney.intercept(
      ({canBlockProgress}) => {
        return canBlockProgress &&
          address?.countryCode &&
          address.countryCode !== 'CA'
          ? {
              behavior: 'block',
              reason: 'Invalid shipping country',
              errors: [
                {
                  message:
                    'Sorry, we can only ship to Canada',
                  // Show an error underneath the country code field
                  target:
                    '$.cart.deliveryGroups[0].deliveryAddress.countryCode',
                },
                {
                  // In addition, show an error at the page level
                  message:
                    'Please use a different address.',
                },
              ],
            }
          : {
              behavior: 'allow',
            };
      },
    );
  },
);

```


## StandardApi
The base API object provided to `purchase`, and `customer-account.order-status` extension targets.
### Docs_Standard_BuyerJourneyApi

### buyerJourney
Provides details on the buyer's progression through the checkout.

See [buyer journey](/docs/api/checkout-ui-extensions/apis/buyer-journey#examples)
examples for more information.
### BuyerJourney
Provides details on the buyer's progression through the checkout.
### intercept
Installs a function for intercepting and preventing progress on checkout.

This returns a promise that resolves to a teardown function. Calling the
teardown function will remove the interceptor.

To block checkout progress, you must set the [block_progress](/docs/api/checkout-ui-extensions/configuration#block-progress)
capability in your extension's configuration.
### completed
This subscribable value will be true if the buyer completed submitting their order.

For example, when viewing the **Order status** page after submitting payment, the buyer will have completed their order.
### Interceptor
A function for intercepting and preventing navigation on checkout. You can block
navigation by returning an object with
`{behavior: 'block', reason: InvalidResultReason.InvalidExtensionState, errors?: ValidationErrors[]}`.
If you do, then you're expected to also update some part of your UI to reflect the reason why navigation
was blocked, either by targeting checkout UI fields, passing errors to the page level or rendering the errors in your extension.
(
  interceptorProps: InterceptorProps,
) => InterceptorRequest | Promise<InterceptorRequest>
### InterceptorProps

### canBlockProgress
Whether the interceptor has the capability to block a buyer's progress through
checkout. This ability might be granted by a merchant in differing checkout contexts.
### InterceptorRequest

InterceptorRequestAllow | InterceptorRequestBlock
### InterceptorRequestAllow

### behavior
Indicates that the interceptor will allow the buyer's journey to continue.
### perform
This callback is called when all interceptors finish. We recommend
setting errors or reasons for blocking at this stage, so that all the errors in
the UI show up at once.
### InterceptorResult

InterceptorResultAllow | InterceptorResultBlock
### InterceptorResultAllow

### behavior
Indicates that the buyer was allowed to progress through checkout.
### InterceptorResultBlock

### behavior
Indicates that some part of the checkout UI intercepted and prevented
the buyer’s progress. The buyer typically needs to take some action
to resolve this issue and to move on to the next step.
### InterceptorRequestBlock

### behavior
Indicates that the interceptor will block the buyer's journey from continuing.
### reason
The reason for blocking the interceptor request. This value isn't presented to
the buyer, so it doesn't need to be localized. The value is used only for Shopify’s
own internal debugging and metrics.
### errors
Used to pass errors to the checkout UI, outside your extension's UI boundaries.
### perform
This callback is called when all interceptors finish. We recommend
setting errors or reasons for blocking at this stage, so that all the errors in
the UI show up at once.
### ValidationError

### message
Error message to be displayed to the buyer.
### target
The checkout UI field that the error is associated with.

Example: `$.cart.deliveryGroups[0].deliveryAddress.countryCode`

See the [supported targets](/docs/api/functions/reference/cart-checkout-validation/graphql#supported-targets)
for more information.
## Related
- [Validating fields at checkout](/docs/apps/checkout/validation/fields)
- [Targets](/docs/api/checkout-ui-extensions/targets)
- [Components](/docs/api/checkout-ui-extensions/components)
- [Configuration](/docs/api/checkout-ui-extensions/configuration)
- [Tutorials](/apps/checkout)
## Examples
The API for interacting with the buyer journey.
### 
Intercept and prevent a buyer's progress through checkout while displaying an error message at the page level.
        See the [validation tutorial](/docs/apps/checkout/validation) for more examples and best practices.
### Block progress and show error at page level

```jsx
import {
  reactExtension,
  useBuyerJourneyIntercept,
  useShippingAddress,
} from '@shopify/ui-extensions-react/checkout';

export default reactExtension(
  'purchase.checkout.delivery-address.render-before',
  () =&gt; &lt;Extension /&gt;,
);

function Extension() {
  const address = useShippingAddress();

  useBuyerJourneyIntercept(
    ({canBlockProgress}) =&gt; {
      return canBlockProgress &&
        address?.countryCode &&
        address.countryCode !== 'CA'
        ? {
            behavior: 'block',
            reason: 'Invalid shipping country',
            errors: [
              {
                // An error without a `target` property is shown at page level
                message:
                  'Sorry, we can only ship to Canada',
              },
            ],
          }
        : {
            behavior: 'allow',
          };
    },
  );

  return null;
}

```

```js
import {extension} from '@shopify/ui-extensions/checkout';

export default extension(
  'purchase.checkout.delivery-address.render-before',
  (root, {buyerJourney, shippingAddress}) =&gt; {
    let address = shippingAddress?.current;
    shippingAddress?.subscribe((newAddress) =&gt; {
      address = newAddress;
    });

    buyerJourney.intercept(
      ({canBlockProgress}) =&gt; {
        return canBlockProgress &&
          address?.countryCode &&
          address.countryCode !== 'CA'
          ? {
              behavior: 'block',
              reason: 'Invalid shipping country',
              errors: [
                {
                  // An error without a `target` property is shown at page level
                  message:
                    'Sorry, we can only ship to Canada',
                },
              ],
            }
          : {
              behavior: 'allow',
            };
      },
    );
  },
);

```


### 
Intercept and prevent a buyer's progress through checkout while displaying an error message in your extension.
        See the [validation tutorial](/docs/apps/checkout/validation) for more examples and best practices.
### Block progress and show error in your extension

```jsx
import {useState} from 'react';
import {
  reactExtension,
  Banner,
  useBuyerJourneyIntercept,
  useCartLineTarget,
} from '@shopify/ui-extensions-react/checkout';

export default reactExtension(
  'purchase.checkout.cart-line-item.render-after',
  () =&gt; &lt;Extension /&gt;,
);

function Extension() {
  const [showError, setShowError] =
    useState(false);
  const {quantity} = useCartLineTarget();

  useBuyerJourneyIntercept(
    ({canBlockProgress}) =&gt; {
      return canBlockProgress && quantity &gt; 1
        ? {
            behavior: 'block',
            reason: 'limited stock',
            perform: (result) =&gt; {
              if (result.behavior === 'block') {
                setShowError(true);
              }
            },
          }
        : {
            behavior: 'allow',
            perform: () =&gt; {
              setShowError(false);
            },
          };
    },
  );

  return showError ? (
    &lt;Banner&gt;
      This item has a limit of one per customer.
    &lt;/Banner&gt;
  ) : null;
}

```

```js
import {
  extension,
  Banner,
} from '@shopify/ui-extensions/checkout';

export default extension(
  'purchase.checkout.cart-line-item.render-after',
  (root, {buyerJourney, target}) =&gt; {
    const banner = root.createComponent(Banner);

    let quantity = target.current.quantity;

    target?.subscribe((newTarget) =&gt; {
      quantity = newTarget.quantity;
    });

    buyerJourney.intercept(
      ({canBlockProgress}) =&gt; {
        return canBlockProgress && quantity &gt; 1
          ? {
              behavior: 'block',
              reason: 'limited stock',
              perform: (result) =&gt; {
                if (result.behavior === 'block') {
                  banner.appendChild(
                    'This item has a limit of one per customer.',
                  );
                  root.appendChild(banner);
                }
              },
            }
          : {
              behavior: 'allow',
              perform: () =&gt; {
                root.removeChild(banner);
              },
            };
      },
    );
  },
);

```


## useBuyerJourney
Returns the buyerJourney details on buyer progression in checkout.
### UseBuyerJourneyGeneratedType
Returns the `buyerJourney` details on buyer progression in checkout.
#### Returns: BuyerJourney

export function useBuyerJourney<
  Target extends RenderExtensionTarget = RenderExtensionTarget,
>(): BuyerJourney {
  const api = useApi<Target>();

  if ('buyerJourney' in api) {
    return api.buyerJourney;
  }

  throw new ExtensionHasNoMethodError(
    'applyAttributeChange',
    api.extension.target,
  );
}
### BuyerJourney
Provides details on the buyer's progression through the checkout.
### intercept
Installs a function for intercepting and preventing progress on checkout.

This returns a promise that resolves to a teardown function. Calling the
teardown function will remove the interceptor.

To block checkout progress, you must set the [block_progress](/docs/api/checkout-ui-extensions/configuration#block-progress)
capability in your extension's configuration.
### completed
This subscribable value will be true if the buyer completed submitting their order.

For example, when viewing the **Order status** page after submitting payment, the buyer will have completed their order.
### Interceptor
A function for intercepting and preventing navigation on checkout. You can block
navigation by returning an object with
`{behavior: 'block', reason: InvalidResultReason.InvalidExtensionState, errors?: ValidationErrors[]}`.
If you do, then you're expected to also update some part of your UI to reflect the reason why navigation
was blocked, either by targeting checkout UI fields, passing errors to the page level or rendering the errors in your extension.
(
  interceptorProps: InterceptorProps,
) => InterceptorRequest | Promise<InterceptorRequest>
### InterceptorProps

### canBlockProgress
Whether the interceptor has the capability to block a buyer's progress through
checkout. This ability might be granted by a merchant in differing checkout contexts.
### InterceptorRequest

InterceptorRequestAllow | InterceptorRequestBlock
### InterceptorRequestAllow

### behavior
Indicates that the interceptor will allow the buyer's journey to continue.
### perform
This callback is called when all interceptors finish. We recommend
setting errors or reasons for blocking at this stage, so that all the errors in
the UI show up at once.
### InterceptorResult

InterceptorResultAllow | InterceptorResultBlock
### InterceptorResultAllow

### behavior
Indicates that the buyer was allowed to progress through checkout.
### InterceptorResultBlock

### behavior
Indicates that some part of the checkout UI intercepted and prevented
the buyer’s progress. The buyer typically needs to take some action
to resolve this issue and to move on to the next step.
### InterceptorRequestBlock

### behavior
Indicates that the interceptor will block the buyer's journey from continuing.
### reason
The reason for blocking the interceptor request. This value isn't presented to
the buyer, so it doesn't need to be localized. The value is used only for Shopify’s
own internal debugging and metrics.
### errors
Used to pass errors to the checkout UI, outside your extension's UI boundaries.
### perform
This callback is called when all interceptors finish. We recommend
setting errors or reasons for blocking at this stage, so that all the errors in
the UI show up at once.
### ValidationError

### message
Error message to be displayed to the buyer.
### target
The checkout UI field that the error is associated with.

Example: `$.cart.deliveryGroups[0].deliveryAddress.countryCode`

See the [supported targets](/docs/api/functions/reference/cart-checkout-validation/graphql#supported-targets)
for more information.
## Related
- [Validating fields at checkout](/docs/apps/checkout/validation/fields)
- [Targets](/docs/api/checkout-ui-extensions/targets)
- [Components](/docs/api/checkout-ui-extensions/components)
- [Configuration](/docs/api/checkout-ui-extensions/configuration)
- [Tutorials](/apps/checkout)
## Examples
The API for interacting with the buyer journey.
### 
Intercept and prevent a buyer's progress through checkout while displaying an error message at the page level.
        See the [validation tutorial](/docs/apps/checkout/validation) for more examples and best practices.
### Block progress and show error at page level

```jsx
import {
  reactExtension,
  useBuyerJourneyIntercept,
  useShippingAddress,
} from '@shopify/ui-extensions-react/checkout';

export default reactExtension(
  'purchase.checkout.delivery-address.render-before',
  () =&gt; &lt;Extension /&gt;,
);

function Extension() {
  const address = useShippingAddress();

  useBuyerJourneyIntercept(
    ({canBlockProgress}) =&gt; {
      return canBlockProgress &&
        address?.countryCode &&
        address.countryCode !== 'CA'
        ? {
            behavior: 'block',
            reason: 'Invalid shipping country',
            errors: [
              {
                // An error without a `target` property is shown at page level
                message:
                  'Sorry, we can only ship to Canada',
              },
            ],
          }
        : {
            behavior: 'allow',
          };
    },
  );

  return null;
}

```

```js
import {extension} from '@shopify/ui-extensions/checkout';

export default extension(
  'purchase.checkout.delivery-address.render-before',
  (root, {buyerJourney, shippingAddress}) =&gt; {
    let address = shippingAddress?.current;
    shippingAddress?.subscribe((newAddress) =&gt; {
      address = newAddress;
    });

    buyerJourney.intercept(
      ({canBlockProgress}) =&gt; {
        return canBlockProgress &&
          address?.countryCode &&
          address.countryCode !== 'CA'
          ? {
              behavior: 'block',
              reason: 'Invalid shipping country',
              errors: [
                {
                  // An error without a `target` property is shown at page level
                  message:
                    'Sorry, we can only ship to Canada',
                },
              ],
            }
          : {
              behavior: 'allow',
            };
      },
    );
  },
);

```


### 
Intercept and prevent a buyer's progress through checkout while displaying an error message in your extension.
        See the [validation tutorial](/docs/apps/checkout/validation) for more examples and best practices.
### Block progress and show error in your extension

```jsx
import {useState} from 'react';
import {
  reactExtension,
  Banner,
  useBuyerJourneyIntercept,
  useCartLineTarget,
} from '@shopify/ui-extensions-react/checkout';

export default reactExtension(
  'purchase.checkout.cart-line-item.render-after',
  () =&gt; &lt;Extension /&gt;,
);

function Extension() {
  const [showError, setShowError] =
    useState(false);
  const {quantity} = useCartLineTarget();

  useBuyerJourneyIntercept(
    ({canBlockProgress}) =&gt; {
      return canBlockProgress && quantity &gt; 1
        ? {
            behavior: 'block',
            reason: 'limited stock',
            perform: (result) =&gt; {
              if (result.behavior === 'block') {
                setShowError(true);
              }
            },
          }
        : {
            behavior: 'allow',
            perform: () =&gt; {
              setShowError(false);
            },
          };
    },
  );

  return showError ? (
    &lt;Banner&gt;
      This item has a limit of one per customer.
    &lt;/Banner&gt;
  ) : null;
}

```

```js
import {
  extension,
  Banner,
} from '@shopify/ui-extensions/checkout';

export default extension(
  'purchase.checkout.cart-line-item.render-after',
  (root, {buyerJourney, target}) =&gt; {
    const banner = root.createComponent(Banner);

    let quantity = target.current.quantity;

    target?.subscribe((newTarget) =&gt; {
      quantity = newTarget.quantity;
    });

    buyerJourney.intercept(
      ({canBlockProgress}) =&gt; {
        return canBlockProgress && quantity &gt; 1
          ? {
              behavior: 'block',
              reason: 'limited stock',
              perform: (result) =&gt; {
                if (result.behavior === 'block') {
                  banner.appendChild(
                    'This item has a limit of one per customer.',
                  );
                  root.appendChild(banner);
                }
              },
            }
          : {
              behavior: 'allow',
              perform: () =&gt; {
                root.removeChild(banner);
              },
            };
      },
    );
  },
);

```


## useBuyerJourneyCompleted

        Returns `true` if the buyer completed submitting their order.
        For example, when viewing the **Order status** page after submitting payment, the buyer will have completed their order.
      
### UseBuyerJourneyCompletedGeneratedType
Returns true if the buyer completed submitting their order.

For example, when viewing the **Order status** page after submitting payment, the buyer will have completed their order.
#### Returns: false | true

export function useBuyerJourneyCompleted<
  Target extends RenderExtensionTarget = RenderExtensionTarget,
>(): boolean {
  const api = useApi<Target>();

  if ('buyerJourney' in api) {
    return useSubscription(api.buyerJourney.completed);
  }

  throw new ExtensionHasNoMethodError('buyerJourney', api.extension.target);
}
## Related
- [Validating fields at checkout](/docs/apps/checkout/validation/fields)
- [Targets](/docs/api/checkout-ui-extensions/targets)
- [Components](/docs/api/checkout-ui-extensions/components)
- [Configuration](/docs/api/checkout-ui-extensions/configuration)
- [Tutorials](/apps/checkout)
## Examples
The API for interacting with the buyer journey.
### 
Intercept and prevent a buyer's progress through checkout while displaying an error message at the page level.
        See the [validation tutorial](/docs/apps/checkout/validation) for more examples and best practices.
### Block progress and show error at page level

```jsx
import {
  reactExtension,
  useBuyerJourneyIntercept,
  useShippingAddress,
} from '@shopify/ui-extensions-react/checkout';

export default reactExtension(
  'purchase.checkout.delivery-address.render-before',
  () =&gt; &lt;Extension /&gt;,
);

function Extension() {
  const address = useShippingAddress();

  useBuyerJourneyIntercept(
    ({canBlockProgress}) =&gt; {
      return canBlockProgress &&
        address?.countryCode &&
        address.countryCode !== 'CA'
        ? {
            behavior: 'block',
            reason: 'Invalid shipping country',
            errors: [
              {
                // An error without a `target` property is shown at page level
                message:
                  'Sorry, we can only ship to Canada',
              },
            ],
          }
        : {
            behavior: 'allow',
          };
    },
  );

  return null;
}

```

```js
import {extension} from '@shopify/ui-extensions/checkout';

export default extension(
  'purchase.checkout.delivery-address.render-before',
  (root, {buyerJourney, shippingAddress}) =&gt; {
    let address = shippingAddress?.current;
    shippingAddress?.subscribe((newAddress) =&gt; {
      address = newAddress;
    });

    buyerJourney.intercept(
      ({canBlockProgress}) =&gt; {
        return canBlockProgress &&
          address?.countryCode &&
          address.countryCode !== 'CA'
          ? {
              behavior: 'block',
              reason: 'Invalid shipping country',
              errors: [
                {
                  // An error without a `target` property is shown at page level
                  message:
                    'Sorry, we can only ship to Canada',
                },
              ],
            }
          : {
              behavior: 'allow',
            };
      },
    );
  },
);

```


### 
Intercept and prevent a buyer's progress through checkout while displaying an error message in your extension.
        See the [validation tutorial](/docs/apps/checkout/validation) for more examples and best practices.
### Block progress and show error in your extension

```jsx
import {useState} from 'react';
import {
  reactExtension,
  Banner,
  useBuyerJourneyIntercept,
  useCartLineTarget,
} from '@shopify/ui-extensions-react/checkout';

export default reactExtension(
  'purchase.checkout.cart-line-item.render-after',
  () =&gt; &lt;Extension /&gt;,
);

function Extension() {
  const [showError, setShowError] =
    useState(false);
  const {quantity} = useCartLineTarget();

  useBuyerJourneyIntercept(
    ({canBlockProgress}) =&gt; {
      return canBlockProgress && quantity &gt; 1
        ? {
            behavior: 'block',
            reason: 'limited stock',
            perform: (result) =&gt; {
              if (result.behavior === 'block') {
                setShowError(true);
              }
            },
          }
        : {
            behavior: 'allow',
            perform: () =&gt; {
              setShowError(false);
            },
          };
    },
  );

  return showError ? (
    &lt;Banner&gt;
      This item has a limit of one per customer.
    &lt;/Banner&gt;
  ) : null;
}

```

```js
import {
  extension,
  Banner,
} from '@shopify/ui-extensions/checkout';

export default extension(
  'purchase.checkout.cart-line-item.render-after',
  (root, {buyerJourney, target}) =&gt; {
    const banner = root.createComponent(Banner);

    let quantity = target.current.quantity;

    target?.subscribe((newTarget) =&gt; {
      quantity = newTarget.quantity;
    });

    buyerJourney.intercept(
      ({canBlockProgress}) =&gt; {
        return canBlockProgress && quantity &gt; 1
          ? {
              behavior: 'block',
              reason: 'limited stock',
              perform: (result) =&gt; {
                if (result.behavior === 'block') {
                  banner.appendChild(
                    'This item has a limit of one per customer.',
                  );
                  root.appendChild(banner);
                }
              },
            }
          : {
              behavior: 'allow',
              perform: () =&gt; {
                root.removeChild(banner);
              },
            };
      },
    );
  },
);

```


## useBuyerJourneyIntercept

        Installs a function for intercepting and preventing progress on checkout.
        To block checkout progress, you must set the [block_progress](/docs/api/checkout-ui-extensions/configuration#block-progress) capability in your extension's configuration.
        If you do, then you're expected to inform the buyer why navigation was blocked, either by passing validation errors to the checkout UI or rendering the errors in your extension.
      
### UseBuyerJourneyInterceptGeneratedType
Installs a function for intercepting and preventing progress on checkout.

To block checkout progress, you must set the [block_progress](/docs/api/checkout-ui-extensions/configuration#block-progress)
capability in your extension's configuration.

If you do, then you're expected to inform the buyer why navigation was blocked,
either by passing validation errors to the checkout UI or rendering the errors in your extension.
#### Returns: void

#### Params:
- interceptor: Interceptor
export function useBuyerJourneyIntercept<
  Target extends RenderExtensionTarget = RenderExtensionTarget,
>(interceptor: Interceptor): void {
  const api = useApi<Target>();

  if (!('buyerJourney' in api)) {
    throw new ExtensionHasNoMethodError('buyerJourney', api.extension.target);
  }

  const interceptorRef = useRef(interceptor);
  interceptorRef.current = interceptor;

  return useEffect(() => {
    const teardownPromise = api.buyerJourney.intercept((interceptorProps) =>
      interceptorRef.current(interceptorProps),
    );

    return () => {
      teardownPromise.then((teardown) => teardown()).catch(() => {});
    };
  }, [api.buyerJourney]);
}
### Interceptor
A function for intercepting and preventing navigation on checkout. You can block
navigation by returning an object with
`{behavior: 'block', reason: InvalidResultReason.InvalidExtensionState, errors?: ValidationErrors[]}`.
If you do, then you're expected to also update some part of your UI to reflect the reason why navigation
was blocked, either by targeting checkout UI fields, passing errors to the page level or rendering the errors in your extension.
(
  interceptorProps: InterceptorProps,
) => InterceptorRequest | Promise<InterceptorRequest>
### InterceptorProps

### canBlockProgress
Whether the interceptor has the capability to block a buyer's progress through
checkout. This ability might be granted by a merchant in differing checkout contexts.
### InterceptorRequest

InterceptorRequestAllow | InterceptorRequestBlock
### InterceptorRequestAllow

### behavior
Indicates that the interceptor will allow the buyer's journey to continue.
### perform
This callback is called when all interceptors finish. We recommend
setting errors or reasons for blocking at this stage, so that all the errors in
the UI show up at once.
### InterceptorResult

InterceptorResultAllow | InterceptorResultBlock
### InterceptorResultAllow

### behavior
Indicates that the buyer was allowed to progress through checkout.
### InterceptorResultBlock

### behavior
Indicates that some part of the checkout UI intercepted and prevented
the buyer’s progress. The buyer typically needs to take some action
to resolve this issue and to move on to the next step.
### InterceptorRequestBlock

### behavior
Indicates that the interceptor will block the buyer's journey from continuing.
### reason
The reason for blocking the interceptor request. This value isn't presented to
the buyer, so it doesn't need to be localized. The value is used only for Shopify’s
own internal debugging and metrics.
### errors
Used to pass errors to the checkout UI, outside your extension's UI boundaries.
### perform
This callback is called when all interceptors finish. We recommend
setting errors or reasons for blocking at this stage, so that all the errors in
the UI show up at once.
### ValidationError

### message
Error message to be displayed to the buyer.
### target
The checkout UI field that the error is associated with.

Example: `$.cart.deliveryGroups[0].deliveryAddress.countryCode`

See the [supported targets](/docs/api/functions/reference/cart-checkout-validation/graphql#supported-targets)
for more information.
## Related
- [Validating fields at checkout](/docs/apps/checkout/validation/fields)
- [Targets](/docs/api/checkout-ui-extensions/targets)
- [Components](/docs/api/checkout-ui-extensions/components)
- [Configuration](/docs/api/checkout-ui-extensions/configuration)
- [Tutorials](/apps/checkout)
## Examples
The API for interacting with the buyer journey.
### 
Intercept and prevent a buyer's progress through checkout while displaying an error message at the page level.
        See the [validation tutorial](/docs/apps/checkout/validation) for more examples and best practices.
### Block progress and show error at page level

```jsx
import {
  reactExtension,
  useBuyerJourneyIntercept,
  useShippingAddress,
} from '@shopify/ui-extensions-react/checkout';

export default reactExtension(
  'purchase.checkout.delivery-address.render-before',
  () =&gt; &lt;Extension /&gt;,
);

function Extension() {
  const address = useShippingAddress();

  useBuyerJourneyIntercept(
    ({canBlockProgress}) =&gt; {
      return canBlockProgress &&
        address?.countryCode &&
        address.countryCode !== 'CA'
        ? {
            behavior: 'block',
            reason: 'Invalid shipping country',
            errors: [
              {
                // An error without a `target` property is shown at page level
                message:
                  'Sorry, we can only ship to Canada',
              },
            ],
          }
        : {
            behavior: 'allow',
          };
    },
  );

  return null;
}

```

```js
import {extension} from '@shopify/ui-extensions/checkout';

export default extension(
  'purchase.checkout.delivery-address.render-before',
  (root, {buyerJourney, shippingAddress}) =&gt; {
    let address = shippingAddress?.current;
    shippingAddress?.subscribe((newAddress) =&gt; {
      address = newAddress;
    });

    buyerJourney.intercept(
      ({canBlockProgress}) =&gt; {
        return canBlockProgress &&
          address?.countryCode &&
          address.countryCode !== 'CA'
          ? {
              behavior: 'block',
              reason: 'Invalid shipping country',
              errors: [
                {
                  // An error without a `target` property is shown at page level
                  message:
                    'Sorry, we can only ship to Canada',
                },
              ],
            }
          : {
              behavior: 'allow',
            };
      },
    );
  },
);

```


### 
Intercept and prevent a buyer's progress through checkout while displaying an error message in your extension.
        See the [validation tutorial](/docs/apps/checkout/validation) for more examples and best practices.
### Block progress and show error in your extension

```jsx
import {useState} from 'react';
import {
  reactExtension,
  Banner,
  useBuyerJourneyIntercept,
  useCartLineTarget,
} from '@shopify/ui-extensions-react/checkout';

export default reactExtension(
  'purchase.checkout.cart-line-item.render-after',
  () =&gt; &lt;Extension /&gt;,
);

function Extension() {
  const [showError, setShowError] =
    useState(false);
  const {quantity} = useCartLineTarget();

  useBuyerJourneyIntercept(
    ({canBlockProgress}) =&gt; {
      return canBlockProgress && quantity &gt; 1
        ? {
            behavior: 'block',
            reason: 'limited stock',
            perform: (result) =&gt; {
              if (result.behavior === 'block') {
                setShowError(true);
              }
            },
          }
        : {
            behavior: 'allow',
            perform: () =&gt; {
              setShowError(false);
            },
          };
    },
  );

  return showError ? (
    &lt;Banner&gt;
      This item has a limit of one per customer.
    &lt;/Banner&gt;
  ) : null;
}

```

```js
import {
  extension,
  Banner,
} from '@shopify/ui-extensions/checkout';

export default extension(
  'purchase.checkout.cart-line-item.render-after',
  (root, {buyerJourney, target}) =&gt; {
    const banner = root.createComponent(Banner);

    let quantity = target.current.quantity;

    target?.subscribe((newTarget) =&gt; {
      quantity = newTarget.quantity;
    });

    buyerJourney.intercept(
      ({canBlockProgress}) =&gt; {
        return canBlockProgress && quantity &gt; 1
          ? {
              behavior: 'block',
              reason: 'limited stock',
              perform: (result) =&gt; {
                if (result.behavior === 'block') {
                  banner.appendChild(
                    'This item has a limit of one per customer.',
                  );
                  root.appendChild(banner);
                }
              },
            }
          : {
              behavior: 'allow',
              perform: () =&gt; {
                root.removeChild(banner);
              },
            };
      },
    );
  },
);

```