Accessing Solidity contract member objects from Javascript

Question

I have the following contract defined:

pragma solidity >=0.7.0 < 0.9.0;

contract LoanApplication {

     struct Applicant {
        bytes32 ssn;
    bool isTenant;
    
     }

     struct Loan {
        string loanID;
    uint applicantCount;
    uint256 rate;
    uint term;
      }

     mapping (address => Applicant) public applicants;

     Loan[] public loans;



     constructor(string[] memory loanIDs) {

       for (uint i=0; i < loanIDs.length; i++) {
            // Create Proposal object and append to proposals array
            loans.push(Loan({
                loanID: loanIDs[i],
                applicantCount:0,
        rate: 10,
        term: 30
                }));
        }
     
     }

}

I have written a Javascript test contract

const LoanAppContract = artifacts.require("LoanApplication");

contract("LoanApplicationTest", accounts => {


const loanIDs = ["Loan1", "Loan2", "Loan3"];

const banker = accounts[0];


beforeEach (async () =>  {
    loanApp = await LoanAppContract.new(loanIDs);
});
describe("initialization", () => {

it("gets the loan IDs", async () => {
    let loans = await loanApp.loans;

    //console.log("loan ID=" + loans[0].loanID); 1 - Unsuccessful

    let loanID = await loanApp.getLoanIDByIndex(0);

    console.log("loan ID=" + loanID);    // 2 - Succeeds

});



    it("Gets the applicants", async () => {
        let applicants = await loanApp.applicants;
        });

        });

});

Why can't I access the loans member array directly ? In order to make it work I have to define the following getter function in the contract:

 function  getLoanIDByIndex(uint index) external view returns (string memory){
       return loans[index].loanID;
    }

This line fails:

 //console.log("loan ID=" + loans[0].loanID); // 1 - Unsuccessful`

while this succeeds:

    let loanID = await loanApp.getLoanIDByIndex(0);

    console.log("loan ID=" + loanID);    // 2 - Succeeds```

Can someone explain why I need to create a getter ?

Answer 1

There is a getter automatically generated, but it is not the js array accessor.

If you look at the generated ABI you can see the folowing method:

        {
            "inputs": [
                {
                    "internalType": "uint256",
                    "name": "",
                    "type": "uint256"
                }
            ],
            "name": "loans",
            "outputs": [
                {
                    "internalType": "string",
                    "name": "loanID",
                    "type": "string"
                },
                {
                    "internalType": "uint256",
                    "name": "applicantCount",
                    "type": "uint256"
                },
                {
                    "internalType": "uint256",
                    "name": "rate",
                    "type": "uint256"
                },
                {
                    "internalType": "uint256",
                    "name": "term",
                    "type": "uint256"
                }
            ],
            "stateMutability": "view",
            "type": "function"
        }

This is automatically generated for public arrays. You can therefore load the information from the loans by doing:

const loan = await loanApp.loans(0);
console.log("loan ID=" + loan.loanID);