Lesson 12: Testing error Property '_mint' does not exist | Typescript

[sanjayojha]

While writing test for ourToken-unit.test.ts. I am getting following error for the testing mint function. Note I am using TypeScript and the typescript repo does not have all the test covered as the main repo. I am using main repo test to create my typescript version.

error TS2339: Property '_mint' does not exist on type 'OurToken'.

The test block causing error is.

describe("minting", () => {
        it("user can not mint", async () => {
            try {
                await ourToken._mint(deployer, 100); //this line, `_mint` does not exist.
                assert(false);
            } catch (e) {
                assert(e);
            }
        });
});

OurToken.sol

import "@openzeppelin/contracts/token/ERC20/ERC20.sol";

contract OurToken is ERC20 {
    constructor(uint256 initialSupply) ERC20("OurToken", "OT") {
        _mint(msg.sender, initialSupply);
    }
}

ERC20.sol _mint function

function _mint(address account, uint256 amount) internal virtual {
        require(account != address(0), "ERC20: mint to the zero address");

        _beforeTokenTransfer(address(0), account, amount);

        _totalSupply += amount;
        _balances[account] += amount;
        emit Transfer(address(0), account, amount);

        _afterTokenTransfer(address(0), account, amount);
}

All other function of ERC20.sol is available in our test but the above _mint function is not. What could be the reason?
@PatrickAlphaC

[krakxn]

Do you get the contract instance of ourToken in your test file? If so, could you send that code here ((full test file works too). Thank you

[sanjayojha]

@krakxn
Sure

import { SignerWithAddress } from "@nomiclabs/hardhat-ethers/signers";
import { assert, expect } from "chai";
import { deployments, ethers } from "hardhat";
import { OurToken } from "../../typechain-types";
import { INITIAL_SUPPLY } from "../../helper-hardhat-config";

describe("OurToken Unit Test", function () {
    //Multipler is used to make reading the math easier because of the 18 decimal points
    const multiplier = 10 ** 18;
    let ourToken: OurToken;
    let deployer: SignerWithAddress;
    let user: SignerWithAddress;
    this.beforeEach(async function () {
        const accounts = await ethers.getSigners();
        deployer = accounts[0];
        user = accounts[1];
        await deployments.fixture("all");
        ourToken = await ethers.getContract("OurToken", deployer);
    });
    ......

[sanjayojha]

All other tests are working and passing. Only the above mention test has the problem as described.

[krakxn]

Got it - If you want to be able to access _mint then you must define it explicitly; you can not access indexed properties using the dot notation because Typescript has no way of knowing whether or not the specific object has that property.
However, when you specifically define a property then the compiler knows that it's there (or not), whether it's optional or not and what's the type. Hope this answers your query.

[sanjayojha]

Got it - If you want to be able to access _mint then you must define it explicitly; you can not access indexed properties using the dot notation because Typescript has no way of knowing whether or not the specific object has that property. However, when you specifically define a property then the compiler knows that it's there (or not), whether it's optional or not and what's the type. Hope this answers your query.

How _mint function is indexed property? While transfer, approve and other methods define in ERC20.sol is not ?

[krakxn]

Sorry, no clue about this question. But explicitly calling it should fix the actual issue.

[sanjayojha]

Look like all the methods with modifier internal in ERC20.sol can not be accessed using dot notation.

[krakxn]

This is a good read for more information: Link to Solidity docs regarding internal

[mattjaf]

It looks like your trying to call an internal function externally and its not visible.

[sanjayojha]

Ya, I read it somewhere, internal functions is not part of abi