# Use FECL

Create custom criteria for dynamic page rules with Frontend Entity Criterion Language.

Frontend Entity Criterion Language (FECL) lets you write expressions to define criteria for [dynamic page rules](/frontend-studio/dynamic-pages.md#dynamic-page-rules). In a FECL expression, a criterion consists of a field, a [conditional operator](/frontend-studio/using-fecl.md#conditional-operators), and a value. The conditional operator compares the field value with the specified value.
In a FECL expression, you can also use [logical operators](/frontend-studio/using-fecl.md#logical-operators) to combine more criteria.

For example, you can define a FECL expression for a product details page with criteria to display only if the requested product is from the specified category.

```text title="FECL expression with criteria to check the product category ID"
(categoryId = "2074" and categoryId = "3392") or (categoryId < "2000")
```

## FECL syntax

### Conditional operators

In your FECL expression, you can use the following conditional operators to define dynamic page rule criteria:

- `=`: checks if the field value and the specified value are equal.
- `!=`: checks if the field value and the specified value are unequal.
- `>`: checks if the field value is greater than the specified value.
- `<`: checks if the field value is lesser than the specified value.
- `<=`: checks if the field value is less than or equal to the specified value.
- `>=`: checks if the field value is greater than or equal to the specified value.
- `in`: checks if the specified value is present in the selected field, if the field is an array.

With FECL you cannot define an all elements in a list expression. The workaround is to pre-calculate a resulting property in the `pageMatchingPayload` and then use the property in a FECL expression.

### Logical operators

In your FECL expression, you can use the following logical operators to combine dynamic page rule criteria:

- `and`: the rule applies if all criteria are all met.
- `or`: the rule applies if at least one criterion is met.
- `not`: the rule applies if the criterion is not met.

You should use [parentheses](https://en.wikipedia.org/wiki/Glossary_of_mathematical_symbols#Parentheses) to indicate the precedence for sub-expression evaluation. For example:

```text title="FECL expression combining criteria with the 'and' and 'or' operators"
(categoryId = "2074" and categoryId = "3392") or (categoryId < "2000")
```

You must add a space before and after an operator. Otherwise, the dynamic page rule criteria will not be applied.

## FECL on dynamic pages

The FECL expression is evaluated based on the data returned in the `pageMatchingPayload` field in the [dynamic page handler](/frontend-development/developing-a-dynamic-page-extension.md) response. To use FECL, you must return the `pageMatchingPayload` from the dynamic page handler. The payload can have nested structures that you can traverse like JavaScript objects by separating levels using a `.`.

For example, for this dynamic page handler implementation you can use the following FECL expression.

```typescript title="Dynamic page handler returning pageMatchingPayload"
export default {
  'dynamic-page-handler': async (
    request: Request,
    context: DynamicPageContext
  ): Promise<DynamicPageSuccessResult | DynamicPageRedirectResult | null> => {
    // Code of the dynamic page logic.
    return {
      dynamicPageType: 'example/dynamic-page',
      dataSourcePayload: {
        // ...
      },
      pageMatchingPayload: {
        productId: 3020,
        categoryIds: [2020, 2040, 2060],
        name: 'top-shirt',
        user: {
          id: 50020,
        },
      },
    };
  },

  // More code.
};
```

```text title="FECL expression that would result false and true based on the pageMatchingPayload"
# This expression would result false.
(productId = 2000) and (user.id = 50020 and name = 'top-pant')

# This expression would result true.
(productId = 2000) or (user.id = 50020 and name = 'top-shirt')
```

### FECL for user personalization

You can use FECL to customize dynamic pages based on the information of the logged user. For example, you can show a special discount banner to users from a particular postal code.

For example, for the following code this FECL expression will result `true` for users with `postalCode` between `122000` and `123000`.

```typescript title="Return user personalization information in the pageMatchingPayload"
export default {
  'dynamic-page-handler': async (
    request: Request,
    context: DynamicPageContext
  ): Promise<DynamicPageSuccessResult | DynamicPageRedirectResult | null> => {
    // Identify Product
    if (ProductRouter.identifyFrom(request)) {
      return ProductRouter.loadFor(request, context.frontasticContext).then(
        (product: Product) => {
          if (product) {
            return {
              dynamicPageType: 'frontastic/product-detail-page',
              dataSourcePayload: {
                product: product,
              },
              pageMatchingPayload: {
                product: product,
                user: {
                  postalCode:
                    request?.sessionData?.account?.addresses[0].postalCode,
                },
              },
            };
          }
          return null;
        }
      );
    }
    // more code...
  },
};
```

```text title="FECL expression to check if the user's postalCode is between 122000 and 123000"
user.postalCode > 122000 and user.postalCode < 123000
```

## Related pages

- [Area overview page with navigation](/frontend-studio.md)
- [Previous page: Use Dynamic pages](/frontend-studio/using-dynamic-pages.md)
- [Search documentation and API specs](/search.md)