How to get array elements in JavaScript from contract call with JSON-RPC returning an array?

Question

I have a function in my Solidity contract whose prototype is:

function checkLastCustomerInvoices() public view returns (bytes32 [] memory)

When I call it from the Truffle console it returns an array of elements, namely :

truffle(development)> myContractInstance.checkLastCustomerInvoices(); [ '0x0000000000000000000000000000000000000000000000000000657567656f74', '0x0000000000000000000000000000000000000000000000000000656e61756c74', '0x0000000000000000000000000000000000000000000000000065726365646573' ]

When I call it from the browser with JSON-RPC, with the JavaScript code :

function checkLastCustomerInvoices()
{
    myContractInstance.checkLastCustomerInvoices(function(err,res){
           if (err)
                   console.log(err);
                else
                     {  
                        console.log(res);
                     }
       }
     );
    return false;
}

It returns in my browser's console:

function checkLastCustomerInvoices()
Array [ "0x0000000000000000000000000000000000000000000000000000000000000020" ]
{"id":1,"jsonrpc":"2.0","result":"0x000000000000000000000000000000000000000000000000000000000000002000000000000000000000000000000000000000000000000000000000000000030000000000000000000000000000000000000000000000000000657567656f740000000000000000000000000000000000000000000000000000656e61756c740000000000000000000000000000000000000000000000000065726365646573"}

So the result returned by JSON-RPC seems to be a string that concatenates in some way :

0x
0000000000000000000000000000000000000000000000000000000000000020
0000000000000000000000000000000000000000000000000000000000000003
0000000000000000000000000000000000000000000000000000657567656f74
0000000000000000000000000000000000000000000000000000656e61756c74
0000000000000000000000000000000000000000000000000065726365646573

Besides the 0x, The first line is 32 (what use ?) The second line the number of elements and the 3 other lines my array elements.

Where is described this encoding ? Is there a standard JavaScript method used to parse this string to a JavaScript array ?

Thanks

Answer 1

Not sure how to do this from a browser, but from a (client-side) node script, you can do:

const web3_eth_abi = require("web3-eth-abi");
const array = web3_eth_abi.decodeParameter("uint256[]", yourReturnedObject.result);

Answer 2

Arrays are encoded with a 32 bytes denoting a position (offset) from the beginning of the data. Then at that offset you will find another 32 bytes showing the length. After that you will have all the array elements.

In your case the first 32 bytes show an offset of 0x20 = 32, then at position 32 we have another 32 bytes showing the length of 1, and then 1 record of 32 bytes encode the actual element.

Find a detailed example below to understand this better.

The following article explains this in more detail: https://medium.com/@hayeah/how-to-decipher-a-smart-contract-method-call-8ee980311603

> encode_abi(
  ["uint256[]", "uint256[]", "uint256[]"],
  [[0xa1, 0xa2, 0xa3], [0xb1, 0xb2, 0xb3], [0xc1, 0xc2, 0xc3]]
).hex()
/************* HEAD (32*3 bytes) *************/
// arg1: look at position 0x60 for array data
0000000000000000000000000000000000000000000000000000000000000060
// arg2: look at position 0xe0 for array data
00000000000000000000000000000000000000000000000000000000000000e0
// arg3: look at position 0x160 for array data
0000000000000000000000000000000000000000000000000000000000000160
/************* TAIL (128**3 bytes) *************/
// position 0x60. Data for arg1.
// Length followed by elements.
0000000000000000000000000000000000000000000000000000000000000003
00000000000000000000000000000000000000000000000000000000000000a1
00000000000000000000000000000000000000000000000000000000000000a2
00000000000000000000000000000000000000000000000000000000000000a3
// position 0xe0. Data for arg2.
0000000000000000000000000000000000000000000000000000000000000003
00000000000000000000000000000000000000000000000000000000000000b1
00000000000000000000000000000000000000000000000000000000000000b2
00000000000000000000000000000000000000000000000000000000000000b3
// position 0x160. Data for arg3.
0000000000000000000000000000000000000000000000000000000000000003
00000000000000000000000000000000000000000000000000000000000000c1
00000000000000000000000000000000000000000000000000000000000000c2
00000000000000000000000000000000000000000000000000000000000000c3