firebase-js-sdk/docs-devsite/database.queryconstraint.md

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# QueryConstraint class
A `QueryConstraint` is used to narrow the set of documents returned by a Database query. `QueryConstraint`<!-- -->s are created by invoking [endAt()](./database.md#endat_51c2c8b)<!-- -->, [endBefore()](./database.md#endbefore_51c2c8b)<!-- -->, [startAt()](./database.md#startat_51c2c8b)<!-- -->, [startAfter()](./database.md#startafter_51c2c8b)<!-- -->, [limitToFirst()](./database.md#limittofirst_ec46c78)<!-- -->, [limitToLast()](./database.md#limittolast_ec46c78)<!-- -->, [orderByChild()](./database.md#orderbychild_fe1f8e4)<!-- -->, [orderByChild()](./database.md#orderbychild_fe1f8e4)<!-- -->, [orderByKey()](./database.md#orderbykey) , [orderByPriority()](./database.md#orderbypriority) , [orderByValue()](./database.md#orderbyvalue) or [equalTo()](./database.md#equalto_51c2c8b) and can then be passed to [query()](./database.md#query_870e07a) to create a new query instance that also contains this `QueryConstraint`<!-- -->.

<b>Signature:</b>

```typescript
export declare abstract class QueryConstraint 
```

## Properties

|  Property | Modifiers | Type | Description |
|  --- | --- | --- | --- |
|  [type](./database.queryconstraint.md#queryconstrainttype) |  | [QueryConstraintType](./database.md#queryconstrainttype) | The type of this query constraints |

## QueryConstraint.type

The type of this query constraints

<b>Signature:</b>

```typescript
abstract readonly type: QueryConstraintType;
```