docs: tx-helper

Author: ryanleecode

.changeset/small-shirts-press.md

@@ -0,0 +1,5 @@
+---
+"@substrate/light-client-extension-helpers": patch
+---
+
+add docs for tx-helper

packages/light-client-extension-helpers/README.md

@@ -211,4 +211,41 @@ client.supervise()
 
 ### tx-helper
 
-TODO: write usage guide
+The tx-helper package allows you to easily sign transactions. You just need to calldata and an implementation of the
+`@polkadot-api/signer` interface.
+
+#### PJS Example Implementation
+
+```ts
+import { getLightClientProvider } from '@substrate/light-client-extension-helpers/web-page';
+import { connectInjectedExtension } from '@polkadot-api/pjs-signer';
+import { fromHex, toHex } from '@polkadot-api/utils';
+import { createTx } from '@substrate/light-client-extension-helpers/tx-helper'; // 👈 create-tx import
+
+const CHANNEL_ID = "..."
+const lightClientProvider = await getLightClientProvider(CHANNEL_ID)
+
+const createTx = async (chainId: string, from: string, callData: string) => {
+  const chains = Object.values(lightClientProvider.getChains());
+  const chain = chains.find(({ genesisHash }) => genesisHash === chainId);
+
+  if (!chain) {
+    throw new Error('unknown chain');
+  }
+
+  const injectedExt = await connectInjectedExtension('polkadot-js');
+
+  const account = injectedExt.getAccounts()
+    .find((account) => toHex(account.polkadotSigner.publicKey) === from);
+
+  if (!account) {
+    throw new Error('no account');
+  }
+
+  const signer = account.polkadotSigner; // 👈 @polkadot-api/signer implementation
+
+  const tx = await createTx(chain.connect)({ callData: fromHex(callData), signer });
+
+  return toHex(tx);
+}
+```