Chain Reorganization Guide

1// Monitor for reorgs
2const monitorReorgs = async (callback, checkInterval = 5000) => {
3	let lastBlock = await web3.eth.getBlockNumber();
4	let lastBlockHash = (await web3.eth.getBlock(lastBlock)).hash;
5 
6	return setInterval(async () => {
7		const currentBlock = await web3.eth.getBlockNumber();
8		const currentBlockHash = (await web3.eth.getBlock(lastBlock)).hash;
9 
10		if (currentBlockHash !== lastBlockHash) {
11			// Reorg detected
12			callback({
13				oldBlock: lastBlock,
14				oldHash: lastBlockHash,
15				newBlock: currentBlock,
16				newHash: currentBlockHash,
17			});
18		}
19 
20		lastBlock = currentBlock;
21		lastBlockHash = currentBlockHash;
22	}, checkInterval);
23};
24 
25// Get common ancestor block
26const findCommonAncestor = async (hash1, hash2) => {
27	let block1 = await web3.eth.getBlock(hash1);
28	let block2 = await web3.eth.getBlock(hash2);
29 
30	while (block1.number > block2.number) {
31		block1 = await web3.eth.getBlock(block1.parentHash);
32	}
33	while (block2.number > block1.number) {
34		block2 = await web3.eth.getBlock(block2.parentHash);
35	}
36	while (block1.hash !== block2.hash) {
37		block1 = await web3.eth.getBlock(block1.parentHash);
38		block2 = await web3.eth.getBlock(block2.parentHash);
39	}
40	return block1;
41};