How to decode extrinsic at json-RPC call's data(byte code)

Question

I call json-RPC through the terminal and get the data as below.

extrinsics is raw byte, but I'm using polkadot-js(SCALE codec?) to decode it.

But I don't know which method to call.

"block": {
"extrinsics": [
"0x280402000be1da78d37e01","0xd91f..(too long haha)..580"
],
"header": { "digest": { ... }

@polkadot/crypto-util? @polkadot/util? Which module and which method should I use? I want input string(raw data) and get string(json or human data)

please help..

Answer 1

As mentioned in the wiki page https://wiki.polkadot.network/docs/build-node-interaction, some return values (like extrinsics) are SCALE encoded and you need to decode them by using the metadata. You can retrieve the metadata :

• Through polkadot-js apps (https://polkadot.js.org/apps/#/explorer) under menu Developer > submenu RPC Calls > and there you choose the endpoint state and getMetadata(at).

• Through code you can check this link https://docs.substrate.io/v3/runtime/metadata/#get-the-metadata which shows in Rust and Javascript the corresponding functions to use, state_getMetadata and getMetadata respectively.

You can also use sidecar (as mentioned in the same wiki page) to get the information already decoded. The corresponding github page is https://github.com/paritytech/substrate-api-sidecar.

Answer 2

You can use @polkadot-js and do something like this:

const wsProvider = new WsProvider('wss://rpc.polkadot.io');
const api = await ApiPromise.create({ provider: wsProvider });
const exampleBlockNumber = 7200920
const blockHash = await api.rpc.chain.getBlockHash(exampleBlockNumber);
const signedBlock = await api.rpc.chain.getBlock(blockHash);

signedBlock.block.extrinsics.forEach((ex, index) => {
  console.log(index, ex.toHuman());
  const { method: { args, method, section } } = ex;
  console.log(`${section}.${method}(${args.map((a) => a.toString()).join(', ')})`);
});

which will give you output like this:

{isSigned: false, method: {…}}
timestamp.set(1633886766001)
{isSigned: true, method: {…}, era: {…}, nonce: '0', signature: '0x56344710ae648a1da40e2c1a1159c73fdbd7d522af01f1fd…7376ab59fd4c492a711cc64bba48d6ae23762d95176071a0c', …}
balances.transfer(13Hwdzi6jkXtBWTSU5de5gC93hDog23pdqd59SPZ7s9zXVNL, 395536044985)
{isSigned: true, method: {…}, era: {…}, nonce: '0', signature: '0x402484bf077d64a8018463a6ccb824f9487fdad8a186d50b…5ca230746fa10e793f5e42f027b53c065f9beb0472d1ca588', …}
balances.transfer(15YHa58TQ624RNd5agRP1uuaqf6fqjUY5n5wrd6V2Ziu3Qsg, 31835000000)
{isSigned: true, method: {…}, era: {…}, nonce: '6', signature: '0x158457697711485bc9b2b34fb1fbc0d4d2b0d91cf0de9502…eb85f4e03efef3fe58ccf174b827a3da05975af36d7687e07', …}
balances.transfer(15BFDbm97npEgs9g7Cfrn7qLeL1g212qRmB58qA7N5F32Ti9, 28665000000)
{isSigned: true, method: {…}, era: {…}, nonce: '1,697,264', signature: '0x928814bc12625c401f31df31cf3753c0045a770aac7e4d7f…5643cd9346bc07dd921f7c71605f311c783ba3d940d47a68d', …}
utility.batch([{"callIndex":"0x0500","args":{"dest":{"id":"16AhBPuZkS2Ugdx4G2VEBu6fJ734hr1Na3Qtn1PwiYrUdq6o"},"value":198803000000}}, {"callIndex":"0x0500","args":{"dest":{"id":"13QLYLKmVfJBkWVWMhJtnFtZBYFK6KEiGHEwrGVsSJyVRpjd"},"value":749000000000}}, {"callIndex":"0x0500","args":{"dest":{"id":"15aeeDCQrBnh5NhWxvkhQkZGQ6JH3HM3J8kTZBG8Pbmvj8mX"},"value":16600000000}}])

You can find more info here: https://polkadot.js.org/docs/api/cookbook/blocks#how-do-i-view-extrinsic-information

This code will work on the frontend or backend. You can see an example of running this code on node (backend) here: https://github.com/brunopgalvao/node-dot/blob/main/server.js