I am using truffle and ganache to make a token demo for myself. I'm following this guide then making my own changes according to my project.

When I try running tests to verify that my contract is running properly I am presented with an interesting return of data that I was not anticipating.

When I mint a token using this test:

describe("mint", () => {
    it("Created token with specified attributes", async () => {
        let instance = await Token.deployed();
        let owner = await instance.owner();

        let tokenId = await instance.mint(5, 2, 5, 10, 5);

        describe("Expected output: ", tokenId);

        let attributes = await instance.getToken(0);
        assert.equal(attributes, [5, 2, 5, 10, 5]);

I was expecting this function to return [5, 2, 5, 10, 5]:

function getToken( uint _TokenId ) public view returns(
    uint256 value1,
    uint256 value2,
    uint256 value3,
    uint256 value4,
    uint16 value5
    Token memory _token = tokens[_TokenId];
    value1 = _token.value1;
    value2 = _token.value2;
    value3 = _token.value3;
    value4 = _token.value4;
    value5 = _token.value5;

But instead the variable attributes reportedly returns this weird data structure which appears to be an array of 5 with 3 mappings of the characters "c", "e", "s" to a variable. "c" contains the value I was hoping for:

{"c" : [5]  "e" : 0 "s" : 1}
{"c" : [2]  "e" : 0 "s" : 1}
{"c" : [5]  "e" : 0 "s" : 1}
{"c" : [10] "e" : 1 "s" : 1}
{"c" : [5]  "e" : 0 "s" : 1}

My question is why I am I getting this data structure? Shouldn't I be receiving my expected results? I believe the "e"'s mapping is the number of digits - 1, but I don't know what "s" represents. I could go through and make an array with each of the "c" mappings, but I don't know if this is the best way of handling this.

1 Answer 1


TLDR; use tokenId.toString()

The array you are receiving is a representation of the BigNumber which is returned from the blockchain. Or actually the blockchain returns an uint256 (or however many bits you use) and it is then converted into a BigNumber.

The "problem" with BigNumber is that you can't display its value like that. I'm not exactly sure what the array values represent but the library itself is https://github.com/MikeMcl/bignumber.js/ I believe.

Anyway, to display a human-friendly version of the number, you can just use value.toString().


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