solidity mapping does not persist

Question

solidity mapping does not persist

I have this solidity code

contract Starting { //pragma solidity ^0.6.12

    struct itemStatus {
        bool created;
    }
    mapping (bytes => itemStatus) public item;
    
    bool[] public boolArray;    

    function start(bytes memory itemId) public returns (bool[] memory) {
        boolArray.push(item[itemId].created);
        if (item[itemId].created) {
            boolArray.push(item[itemId].created);
            return boolArray;
        } else {
            item[itemId] = itemStatus(true);
            boolArray.push(item[itemId].created);
            return boolArray;
        }
    }
}

frontend javascript code is

  toStart: function () {
    let Instance;
    App.contracts.Starting.deployed().then(function (instance) {
      Instance = instance;
      return Instance.start.call(itemId);
    }).then(function (result) {
      console.log(result);
    }).catch(function (err) {
      console.log(err.message);
    });
  },

The first time I call toStart, I would expect [false, true] and the second time I call with the same itemId [false, true, true, true] and third time [false, true, true, true, true, true]. However, the result is always [false, true] irregard how many times I call toStart. It seems item[itemId] = itemStatus(true) and boolArray.push(item[itemId].created) changes are never stored/persisted. Why is it so? How to change the code to get the expected result? Many thanks in advance.

Answer 1

Change this:

Instance.start.call(itemId)

To this:

Instance.start.send(itemId)

Answer 2

Assuming the ethers version you're using is the older version, the error arises because ethers.js makes a distinction between constant calls and non constant(state writing) calls using call and send. When trying to call a constant method, call is used and when calling a non constant method, send is used.

Because your contract changes the state in boolArray.push(item[itemId].created);, the function is non constant and you need to use the send.

Ethers v5 : Ethers v5 does not make this distinction. You can call both methods the same way but the return values would be Promise<any> for constant and Promise<TransactionResponse> for non constant functions.