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...


... 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:




both throw errors.

Any help is appreciated.

  • Your second approach should work just fine. The only think I would change is the name of the input argument - from _length to _index. Other than that, you'll need to explain what exactly "seems also not to work". Jul 27, 2018 at 11:29

1 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.

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