> Documentation index: https://www.gooddata.ai/docs/gooddata-ui/llms.txt lists every page of these docs as Markdown.

# Header predicates

> Match visualization headers for drilling with custom predicate functions or the HeaderPredicates helpers.

API reference: [HeaderPredicates](https://www.gooddata.ai/docs/gooddata-ui/latest/api-reference/sdk-ui/sdk-ui.headerpredicates/index.md) (variable), [IHeaderPredicate](https://www.gooddata.ai/docs/gooddata-ui/latest/api-reference/sdk-ui/sdk-ui.iheaderpredicate/index.md) (type-alias)

A header predicate matches elements of a visualization, such as a measure header item or an attribute header item, with logic as simple or complex as you need. [Drilling](https://www.gooddata.ai/docs/gooddata-ui/latest/integrations/interactivity/drillable-items/index.md) uses predicates to decide which parts of a visualization the user can drill into.

## [Structure](https://www.gooddata.ai/docs/gooddata-ui/latest/integrations/interactivity/predicates/#structure)

A header predicate ([`IHeaderPredicate`](https://www.gooddata.ai/docs/gooddata-ui/latest/api-reference/sdk-ui/sdk-ui.iheaderpredicate/index.md)) is a function that takes two arguments and returns a boolean:

```ts
type IHeaderPredicate = (header: IMappingHeader, context: IHeaderPredicateContext) => boolean;
```

- `header` ([`IMappingHeader`](https://www.gooddata.ai/docs/gooddata-ui/latest/api-reference/sdk-ui/sdk-ui.imappingheader/index.md)) is an object that describes the item whose match is being tested: an attribute descriptor, an attribute result header, a measure descriptor, a total descriptor, or a color descriptor.

- `context` ([`IHeaderPredicateContext`](https://www.gooddata.ai/docs/gooddata-ui/latest/api-reference/sdk-ui/sdk-ui.iheaderpredicatecontext/index.md)) is additional data that describes the context in which the match is being tested. Its `dv` property holds the [`DataViewFacade`](https://www.gooddata.ai/docs/gooddata-ui/latest/api-reference/sdk-ui/sdk-ui.dataviewfacade/index.md) of the data view that produced the header passed as the first argument.

If the predicate returns `true` for an item, the item is matched.

The most common predicates are predefined in [`HeaderPredicates`](https://www.gooddata.ai/docs/gooddata-ui/latest/api-reference/sdk-ui/sdk-ui.headerpredicates/index.md), exported from `@gooddata/sdk-ui`:

- `identifierMatch(identifier, type?)` matches an attribute or measure header by identifier, including measures derived from a master measure with that identifier. `uriMatch(uri)` does the same by URI, which only GoodData Legacy uses.
- `localIdentifierMatch(localIdOrMeasure)` matches a header by the local identifier of its attribute or measure.
- `objRefMatch(objRef)` and `objMatch(obj)` match a header by an [`ObjRef`](https://www.gooddata.ai/docs/gooddata-ui/latest/api-reference/sdk-model/sdk-model.objref/index.md), or by an attribute or simple measure object from the generated catalog.
- `attributeItemNameMatch(name)` matches an attribute result header by its displayed value.
- `composedFromIdentifier(identifier, type?)` matches arithmetic measures that use a measure with that identifier as an operand. `composedFromUri(uri)` does the same by URI.

## [Example](https://www.gooddata.ai/docs/gooddata-ui/latest/integrations/interactivity/predicates/#example)

You can also write your own predicates. This one matches all attribute values that start with a capital `A`:

```ts
import { type IHeaderPredicate } from "@gooddata/sdk-ui";
import { isResultAttributeHeader } from "@gooddata/sdk-model";

const startsWithA: IHeaderPredicate = (header) => {
    return isResultAttributeHeader(header) && !!header.attributeHeaderItem.name?.startsWith("A");
};
```

This one matches only attribute headers, and only when the visualization has at most three attributes:

```ts
import { type IHeaderPredicate } from "@gooddata/sdk-ui";
import { isAttributeDescriptor } from "@gooddata/sdk-model";

const attributesIfNoMoreThanThree: IHeaderPredicate = (header, context) => {
    if (context.dv.def().attributes().length > 3) {
        return false;
    }

    return isAttributeDescriptor(header);
};
```