Query ERC20 token balance on Elrond

convert.ts

export function convertBigEndianArray(arr: Uint8Array): number {
  let sum = 0;
  for (let i = 0; i < arr.length; i++) {
    sum *= 256;
    sum += arr[i];
  }
  return sum;
}

smartContractRequests.ts

export async function getErc20Balance({
  address,
  contractAddress,
  proxyProvider,
}: GetErc20BalanceInput) {
  const contract = new SmartContract({
    address: new Address(contractAddress),
  });
  const addressOfUser = new Address(address);

  const response = await contract.runQuery(proxyProvider, {
    func: new ContractFunction("balanceOf"),
    args: [new AddressValue(addressOfUser)],
  });
  const encoded = response.returnData[0];
  let byteArray: Uint8Array = [] as any;
  protobuf.util.base64.decode(encoded, byteArray, 0);
  const data = convertBigEndianArray(byteArray);

  return {
    data: data,
    success: response.isSuccess(),
  };
}