Using multiple leaves with one proof in MerkleTreeJS and Solidity requires you to aggregate the proofs for multiple leaves into a single proof. This can be achieved through a technique known as "proof aggregation" or "batch verification." The process involves combining multiple proofs into a single compact proof, reducing gas costs and optimizing verification on the Solidity smart contract side.

Here's a step-by-step guide on how to use multiple leaves with one proof in MerkleTreeJS and Solidity:

**Create the Merkle Tree in MerkleTreeJS**:First, create the Merkle Tree in MerkleTreeJS by adding all the leaves (data elements) and computing the root hash. You can do this using the

`MerkleTree`

class provided by MerkleTreeJS.

`javascript````
const { MerkleTree } = require('merkletreejs');
const keccak256 = require('keccak256');
// Sample leaves (data elements)
const leaves = ['Leaf1', 'Leaf2', 'Leaf3'];
// Hash function to use (keccak256 in this example)
const hashFunc = (data) => keccak256(data);
// Create the Merkle Tree
const merkleTree = new MerkleTree(leaves, hashFunc);
const root = merkleTree.getHexRoot(); // Get the root hash
```

**Compute the Individual Proofs**:Next, compute the individual proofs for each leaf you want to use in the batch verification. In MerkleTreeJS, you can use the

`getHexProof()`

method to obtain the proof for a specific leaf.

`javascript````
// Get the proof for a specific leaf
const leafToProve = 'Leaf2'; // Replace with the leaf you want to prove
const proof = merkleTree.getHexProof(leafToProve);
```

**Aggregate Proofs**:Now, aggregate the individual proofs into a single compact proof. One common method to do this is to concatenate all the proofs into a single array. Ensure you maintain the order of the proofs and include any necessary information to reconstruct the tree in Solidity.

`javascript````
// Aggregate proofs into a single array
const aggregatedProof = [proof1, proof2, proof3]; // Replace with your actual proofs
```

**Send Data to Solidity**:Send the aggregated proof, the root hash, and any other required information to the Solidity smart contract.

**Implement Batch Verification in Solidity**:On the Solidity side, you'll need to implement a batch verification function that can verify multiple proofs at once. You can use the

`bytes32[]`

array to pass the compact proofs to the smart contract.

solidity`pragma solidity ^0.8.0; contract MerkleTreeVerifier { // Check if the leaf is included in the Merkle Tree function verifyMerkleProof(bytes32[] memory proof, bytes32 root, bytes32 leaf) public pure returns (bool) { bytes32 computedHash = leaf; for (uint256 i = 0; i < proof.length; i++) { bytes32 proofElement = proof[i]; if (computedHash < proofElement) { // Hash the concatenation of the proof element and computed hash computedHash = keccak256(abi.encodePacked(proofElement, computedHash)); } else { // Hash the concatenation of the computed hash and the proof element computedHash = keccak256(abi.encodePacked(computedHash, proofElement)); } } return computedHash == root; } // Batch verification for multiple proofs function verifyBatchProof(bytes32[] memory proofs, bytes32 root) public pure returns (bool) { // Loop through each proof and verify individually for (uint256 i = 0; i < proofs.length; i++) { if (!verifyMerkleProof(proofs[i], root, leafToProve[i])) { return false; } } return true; } }`

Please note that the code provided above is for demonstration purposes and may require adjustments based on your specific use case and data types.

By following these steps, you can use multiple leaves with one proof in MerkleTreeJS and Solidity, reducing gas costs and optimizing proof verification on the blockchain.