How to get Ethers.js response data

(await contract.ping()).toString(); //-> 999999999 ( correct )

ping() is a view function - you can just call it without creating a transaction. So ethers.js doesn't create a transaction and just returns result of the call.

(await contract.ping2()).toString(); //-> [object Object] ( ?? )

Why does using sendTransaction also return [object Object] ?

ping2() is a regular public function. Which suggests that you need to create a transaction to execute it (even though in this case it doesn't make any state changes so it could be a view function as well).

Ethers.js returns the transaction data instead of the contract function return value, when you're creating a transaction.

There are few ways to read values that the transaction produced using Ethers.js.

• In this case, ping2() doesn't make any state changes and doesn't even read any blockchain data, so it could be a pure function. If it were reading blockchain data, it would be a view function... In both cases, ethers.js returns the result of the function call (not tx).

• Transaction to a setter and calling a getter.

  contract MyContract {
      uint256 value;
  
      function setValue(uint256 _value) public {
          value = _value;
      }
  
      function getValue() public view returns (uint256) {
          return value;
      }
  }
  

  First you create a transaction that executes the setValue() function, and then you make a call to getValue() (without transaction, so it returns the value in your JS).

• Reading event logs that your transaction produced

  event Transfer(address from, address to, uint256 amount);
  
  function transfer(address _to, uint256 _amount) public {
      emit Transfer(msg.sender, _to, _amount);
  }
  

  You can get the transaction receipt that also contains the event logs (in this case, the Transfer event and its values).

Here is a great solution from Force Hero using Events.

In a nutshell:

const tx = await contract.transfer(...args); // 100ms
const rc = await tx.wait(); // 0ms, as tx is already confirmed
const event = rc.events.find(event => event.event === 'Transfer');
const [from, to, value] = event.args;
console.log(from, to, value);