Accessing to a contract's public array from javascript

Question

Look at this basic Solidity contract:

// SPDX-License-Identifier: MIT

pragma solidity 0.6.11;

contract MyContract {
   string[] public tmp;
   ...

   function read_tmp() public view returns(string[] memory){
      return tmp;
   }
}

I have create a scaffolded truffle react project.

First, i get an error when trying to deploy this contract with this command:

truffle migrate --network ropsten --reset

The error is:

TypeError: This type is only supported in ABIEncoderV2. Use "pragma experimental ABIEncoderV2;" to enable the feature.

This error is due to string[] return type of read_tmp. I am not happy do to that but i have add this line at the top of my contract and the deploy works now:

pragma experimental ABIEncoderV2;

My first question is: Why should i enable experimental features ? My 0.6.11 compiler version should allow this feature natively...

My second question is i have tried this in App.js file:

await this.state.contract.methods.tmp().call();

Solidity should create automatically a getter called tmp() for tmp public field.

I do not understand why but i have this javascript error in my browser:

Uncaught (in promise) Error: Invalid number of parameters for "tmp". Got 0 expected 1!

Everything works if i call read_tmp(). So my question is why tmp() needs an argument ? (i have tried to pass an integer but it does not work)

Thanks a lot

Answer 1

You cannot retrieve whole array from solidity. You have to choose the index, that is why it is expecting 1 argument. If you want to get whole array elements in javascript, in solidity define a function to get the length of the array.

//Contract Code

function getTmpCount() public view returns(uint){
    return tmp.length;
}

then in javascript code:

// you get the lenght of the array
const tmpCount = await contract.methods.getTmpCount().call()

const tmps = await Promise.all(
    Array(tmpCount)
      .fill()  
      .map((element, index) => {
        return campaign.methods.tmp(index).call();
      })
  );

Answer 2

For type[] public arr, the compiler auto-generates the following function:

function arr(uint256 index) public view returns (type) {
    return arr[index];
}

It is not added to the source code of course, because that would yield a naming collision.

But it is added to the byte-code itself, essentially becoming a part of the contract.

If you want a function which returns the entire array, then you need to implement it yourself:

function getArr() public view returns (type[] memory) {
    return arr;
}

---

With regards to:

My 0.6.11 compiler version should allow this feature natively...

That's wrong; this feature is support under ABI encoder V2, which is activated by default only starting from solc 0.8.0 (as stated in the Solidity 0.8.0 Release Announcement).