# Indexed Merkle Map

> A comprehensive guide on how to use Indexed Merkle Map to reference off-chain data in zkApps on Mina. - zkapp - o1js - merkle map - merkle tree - zkapps - mina blockchain - storage - off-chain data - blockchain technology - data structures

Canonical URL: https://docs.minaprotocol.com/zkapps/o1js/indexed-merkle-map

:::experimental

The Indexed Merkle Map API is currently an experimental feature.

:::

# Indexed Merkle Map

Similar to a Merkle Tree, a Merkle Map allows referencing off-chain data by storing a single hash, also known as the root.

A Merkle Map is a wrapper around a [Merkle Tree](https://docs.o1labs.org/o1js/basic-types/merkle-trees). Both data structures are analogous, but instead of using an index to set a leaf in a tree, a Merkle Map uses a key in a map.

## Design

The Indexed Merkle Map is an improved version of the [MerkleMap](/zkapps/tutorials/common-types-and-functions#merkle-map), offering enhanced efficiency and usability:

- **Reduced Constraints:** Uses 4-8x fewer constraints than `MerkleMap`.
- **Provable Code Integration:** Unlike `MerkleTree` and `MerkleMap`, the high-level API of `IndexedMerkleMap` is usable within provable code.

:::note

The `Indexed Merkle Map` can have a height of at most `52`, whereas the `Merkle Map` has a larger height fixed at `256`.

:::

## Utilizing Indexed Merkle Map

### Prerequisites

The `IndexedMerkleMap` API is accessible within the `Experimental` namespace. To use the API, import `Experimental` from o1js version 1.5.0 or higher.

```ts


const { IndexedMerkleMap } = Experimental;
```

### Instantiating an Indexed Merkle Map

Given a height, you can instantiate an Indexed Merkle Map by extending the base class.
The height determines the capacity of the map; the maximum number of leaf nodes it can contain.

```ts
const height = 31;
class IndexedMerkleMap31 extends IndexedMerkleMap(height) {}
```

In this example, `IndexedMerkleMap31` is a Merkle map capable of holding up to 2<sup>(31−1)</sup> leaves; approximately 1 billion entries.

### Indexed Merkle Map - API reference

For an example, see the `IndexedMerkleMap` [API reference](https://docs.o1labs.org/o1js/api-reference/variables/IndexedMerkleMap) in o1js.

## Additional Resources

For more details and examples, please refer to the following GitHub resources:

- [Indexed Merkle Tree: o1js PR#1666](https://github.com/o1-labs/o1js/pull/1666)
- [IndexedMerkleMap: Support 0 and -1 Keys: o1js PR#1671](https://github.com/o1-labs/o1js/pull/1671)
- [Mastermind zkApp Example Using Indexed Merkle Map](https://github.com/o1-labs-XT/mastermind-zkApp/tree/level3)