# ExpressionFunctionType

Canonical page: https://www.adaptabletools.com/docs/reference/expressionfunctiontype

The kinds of expression function.

Note: boolean predicates are `scalar` functions with
`returnType: 'boolean'`; the boolean aggregation operators (`WHERE`,
threshold comparators) are part of `aggregated`. `observable` are the
brace-only history readers (`CHANGE_COUNT{}`, `PREVIOUS{}`, `ROW_AGE{}`, …)
and their clause keywords (`ANY_COL`, `WITHIN`, `LAST_N`, `ALL_ROWS`, `TRIGGER`).

```ts title="TypeScript"
export type ExpressionFunctionType = 'scalar' | 'aggregated' | 'observable';
```