How to retrieve length of array stored in mapping on frontend?

Question

I have a contract with the following mapping:

mapping (uint16 => uint256[]) public barLengthToBarIndexForSale;

What's the best way to retrieve the length of an array for a given key on the frontend using truffle?

Something like this...

barERC721Inst.barLengthToBarIndexForSale(5).length;

... seems to not work.

Using an additional function in my contract to retrieve the length...

    function getBarLengthToBarIndexForSaleLength(uint16 _length) external view returns (uint) {
    return barLengthToBarIndexForSale[_length].length;
}

...seems also not to work :(

I tried calling it on the frontend like this:

barERC721Inst.barLengthToBarAmtForSale.call(5)

and

barERC721Inst.barLengthToBarAmtForSale(5)

both throw errors.

Any help is appreciated.

Answer 1

You could just write a getter function in your contract that returns the length of the array. Be sure to make it view.

function getSize(uint16 key) public view returns (uint){
    return barLengthToBarAmtForSale[5].length
}

if not possible (the contract is already deployed) then you could "try" looping through the array (it cost no gas) until you get an error. This is a bit dirty, but should work.

Hope it helps.