feat: add util for querying token balance

packages/rln/src/contract/token.ts

@@ -0,0 +1,98 @@
+import { type Address, formatUnits, PublicClient, WalletClient } from "viem";
+
+/**
+ * Minimal ERC20 ABI containing only the functions we need
+ */
+export const erc20Abi = [
+  {
+    type: "function",
+    inputs: [{ name: "account", internalType: "address", type: "address" }],
+    name: "balanceOf",
+    outputs: [{ name: "", internalType: "uint256", type: "uint256" }],
+    stateMutability: "view"
+  },
+  {
+    type: "function",
+    inputs: [],
+    name: "decimals",
+    outputs: [{ name: "", internalType: "uint8", type: "uint8" }],
+    stateMutability: "view"
+  },
+  {
+    type: "function",
+    inputs: [],
+    name: "symbol",
+    outputs: [{ name: "", internalType: "string", type: "string" }],
+    stateMutability: "view"
+  }
+] as const;
+
+export interface TokenBalance {
+  /** Raw balance as bigint */
+  raw: bigint;
+  /** Formatted balance as string (e.g., "100.5") */
+  formatted: string;
+  /** Token decimals */
+  decimals: number;
+  /** Token symbol (if available) */
+  symbol?: string;
+  /** User's wallet address */
+  userAddress: Address;
+}
+
+/**
+ * Gets the token balance for the connected wallet
+ * @param walletClient The viem wallet client with a connected account
+ * @param publicClient The viem public client for reading contract state
+ * @param tokenAddress The ERC20 token contract address
+ * @returns Promise<TokenBalance> The token balance information
+ * @throws Error if no account is connected to the wallet client
+ *
+ * @example
+ * ```typescript
+ * const { walletClient, publicClient } = await createViemClientsFromWindow();
+ * const balance = await getTokenBalance(walletClient, publicClient, "0x...");
+ * console.log(`Balance: ${balance.formatted} ${balance.symbol}`);
+ * ```
+ */
+export async function getTokenBalance(
+  walletClient: WalletClient,
+  publicClient: PublicClient,
+  tokenAddress: Address
+): Promise<TokenBalance> {
+  if (!walletClient.account) {
+    throw new Error("No account connected to wallet client");
+  }
+
+  const userAddress = walletClient.account.address;
+
+  // Read balance, decimals, and symbol in parallel
+  const [balance, decimals, symbol] = await Promise.all([
+    publicClient.readContract({
+      address: tokenAddress,
+      abi: erc20Abi,
+      functionName: "balanceOf",
+      args: [userAddress]
+    }),
+    publicClient.readContract({
+      address: tokenAddress,
+      abi: erc20Abi,
+      functionName: "decimals"
+    }),
+    publicClient
+      .readContract({
+        address: tokenAddress,
+        abi: erc20Abi,
+        functionName: "symbol"
+      })
+      .catch(() => undefined) // Symbol is optional, some tokens don't have it
+  ]);
+
+  return {
+    raw: balance,
+    formatted: formatUnits(balance, decimals),
+    decimals,
+    symbol,
+    userAddress
+  };
+}

packages/rln/src/index.ts

@@ -31,6 +31,10 @@ export {
   membershipUpgradeableAbi
 } from "./contract/wagmi/generated.js";
 
+// Export token utilities
+export { getTokenBalance, erc20Abi } from "./contract/token.js";
+export type { TokenBalance } from "./contract/token.js";
+
 export type {
   DecryptedCredentials,
   EncryptedCredentials,