Solidity: Mock function return value

Question

I'm writing an ERC-721 contract but I'm struggling to write a proper unit test when all token ids are minted.

Function I want to test:

    uint256 public constant MAX_SUPPLY = 10000;

    function safeMint() public payable {
        uint256 totalSupply = totalSupply();
        require(totalSupply <= MAX_SUPPLY, "Purchase would exceed max supply");

        //...
    }

In the code above I want to write a unit test so it would throw an error if total supply has reached its limit.

My first approach was to mock totalSupply() so it would return 10,000. Then I would call safeMint() and assert it throws an error with the correct error message.

So my question is how to mock the return value of a function? Or is there some other way to test this?

I tried something like this with hardhat, waffle and smock:

describe('MyContract', () => {
  describe('given mocked contract', () => {
    let mockInstance;
    let account;

    beforeEach(async () => {
      const myContractFactory = await smock.mock('MyContract');
      mockInstance = await myContractFactory.deploy();

      [account] = await ethers.getSigners();
    });

    it('given 10,000 tokens ids minted, when safe mint is called, throws', async () => {
      await mockInstance.totalSupply.returns(10000);

      await expect(
        mockInstance.connect(account).safeMint({
          value: ethers.utils.parseEther('0.123'),
        })
      ).to.be.revertedWith('Purchase would exceed max supply');
    });
  });
});

Above code sets the total supply to 10,000 but when safeMint() is called it does not throw an error.

Answer 1

The error is in the condition require(totalSupply <= MAX_SUPPLY,. If both totalSupply and MAX_SUPPLY are equal to 10,000 then the condition will be satisfied and it will not revert.

The solution is to only allow mint when totalSupply strictly less than MAX_SUPPLY:

require(totalSupply < MAX_SUPPLY, "Purchase would exceed max supply");