Handling require/assert failure errors correctly from a Node.JS app?

Question

I have a getter function in my smart contract that returns one of the string values in statically defined string array. Upon entry to the function, it does a bounds check on the incoming index using require(). If the bounds check fails, what will happen in my Node.JS app that used Web3.js to make the call? Will a Javascript error be thrown? Will I get NULL back?

If anyone has a good article or web resource that shows the proper way to handle errors from a Web3.JS call that resulted in an assert or require operation being triggered on the smart contract side, please share.

Here is my smart contract code:

pragma solidity ^0.4.23;

import "github.com/OpenZeppelin/zeppelin-solidity/contracts/ownership/Ownable.sol";

contract VideoDummyData is Ownable{

    // Constructor, MUST be public.
    constructor() public {
        // Unused.
    }

    // Array of sample video IDs.
    string[3] m_aryVideoIds = 
        ["ZUSPD9zOyJs", "4nqJiBRNQuw", "PLcxE4UkJt0"];


    // Return one of the sample video IDs.
    function getVideoIdAt (uint ndx) public view returns(string)
    {
        // Bounds check on the desired index.
        require(ndx < m_aryVideoIds.length);

        return m_aryVideoIds[ndx];
    }
}

Answer 1

Since you are calling a view function, the first callback will error, more or less immediately.

It's currently possible to return errors with

require(expression, "error msg");

The challenge is accessing this message further up the stack in Web3 and Node.

In the meantime, you can use try and .catch() in promisified code such as a truffle contract abstraction.

You won't know the exact reason but you'll be able to infer it given the contract's requirements.

Hope it helps.

Answer 2

Once you call/send methods get reverted with what so ever reason you be getting the below response in you express/nodejs code.

   {
    "data": {
        "0x926192b712005c4eab709b7f7e1718435b6f2117580360b73e7601a439feffcd": {
            "error": "revert",
            "program_counter": 383,
            "return": "0x08c379a0000000000000000000000000000000000000000000000000000000000000002000000000000000000000000000000000000000000000000000000000000000165468697320496420616c72656164792065786973747300000000000000000000",
            "reason": "This Id already exists"
        },
        "stack": "RuntimeError: VM Exception while processing transaction: revert This Id already exists\n    at Function.RuntimeError.fromResults (C:\\Program Files\\WindowsApps\\GanacheUI_2.5.4.0_x64__5dg5pnz03psnj\\app\\resources\\static\\node\\node_modules\\ganache-core\\lib\\utils\\runtimeerror.js:94:13)\n    at BlockchainDouble.processBlock (C:\\Program Files\\WindowsApps\\GanacheUI_2.5.4.0_x64__5dg5pnz03psnj\\app\\resources\\static\\node\\node_modules\\ganache-core\\lib\\blockchain_double.js:627:24)\n    at processTicksAndRejections (internal/process/task_queues.js:93:5)",
        "name": "RuntimeError"
    }
}

Now here we need to show the user a appropriate message so the user can understand for that we can use below code. To get the reason from the reverted response. use the below code in nodeJS/expressjs code....

try {
    await myContract.methods.foo().send();
} catch (e) {
    const data = e.data;
    const txHash = Object.keys(data)[0]; 
    const reason = data[txHash].reason;

    console.log(reason); 
}