openzeppelin-contracts/test/token/ERC721/ERC721.test.js

const { accounts, contract } = require('@openzeppelin/test-environment');

const { BN, constants, expectEvent, expectRevert } = require('@openzeppelin/test-helpers');
const { ZERO_ADDRESS } = constants;

const { expect } = require('chai');

const { shouldSupportInterfaces } = require('../../introspection/SupportsInterface.behavior');

const ERC721Mock = contract.fromArtifact('ERC721Mock');
const ERC721ReceiverMock = contract.fromArtifact('ERC721ReceiverMock');

describe('ERC721', function () {
  const [owner, newOwner, approved, anotherApproved, operator, other] = accounts;

  const name = 'Non Fungible Token';
  const symbol = 'NFT';

  const firstTokenId = new BN('5042');
  const secondTokenId = new BN('79217');
  const nonExistentTokenId = new BN('13');

  const RECEIVER_MAGIC_VALUE = '0x150b7a02';

  beforeEach(async function () {
    this.token = await ERC721Mock.new(name, symbol);
  });

  shouldSupportInterfaces([
    'ERC165',
    'ERC721',
    'ERC721Enumerable',
    'ERC721Metadata',
  ]);

  describe('metadata', function () {
    it('has a name', async function () {
      expect(await this.token.name()).to.be.equal(name);
    });

    it('has a symbol', async function () {
      expect(await this.token.symbol()).to.be.equal(symbol);
    });

    describe('token URI', function () {
      beforeEach(async function () {
        await this.token.mint(owner, firstTokenId);
      });

      const baseURI = 'https://api.com/v1/';
      const sampleUri = 'mock://mytoken';

      it('it is empty by default', async function () {
        expect(await this.token.tokenURI(firstTokenId)).to.be.equal('');
      });

      it('reverts when queried for non existent token id', async function () {
        await expectRevert(
          this.token.tokenURI(nonExistentTokenId), 'ERC721Metadata: URI query for nonexistent token'
        );
      });

      it('can be set for a token id', async function () {
        await this.token.setTokenURI(firstTokenId, sampleUri);
        expect(await this.token.tokenURI(firstTokenId)).to.be.equal(sampleUri);
      });

      it('reverts when setting for non existent token id', async function () {
        await expectRevert(
          this.token.setTokenURI(nonExistentTokenId, sampleUri), 'ERC721Metadata: URI set of nonexistent token'
        );
      });

      it('base URI can be set', async function () {
        await this.token.setBaseURI(baseURI);
        expect(await this.token.baseURI()).to.equal(baseURI);
      });

      it('base URI is added as a prefix to the token URI', async function () {
        await this.token.setBaseURI(baseURI);
        await this.token.setTokenURI(firstTokenId, sampleUri);

        expect(await this.token.tokenURI(firstTokenId)).to.be.equal(baseURI + sampleUri);
      });

      it('token URI can be changed by changing the base URI', async function () {
        await this.token.setBaseURI(baseURI);
        await this.token.setTokenURI(firstTokenId, sampleUri);

        const newBaseURI = 'https://api.com/v2/';
        await this.token.setBaseURI(newBaseURI);
        expect(await this.token.tokenURI(firstTokenId)).to.be.equal(newBaseURI + sampleUri);
      });

      it('token URI is empty for tokens with no URI but with base URI', async function () {
        await this.token.setBaseURI(baseURI);

        expect(await this.token.tokenURI(firstTokenId)).to.be.equal('');
      });

      it('tokens with URI can be burnt ', async function () {
        await this.token.setTokenURI(firstTokenId, sampleUri);

        await this.token.burn(firstTokenId, { from: owner });

        expect(await this.token.exists(firstTokenId)).to.equal(false);
        await expectRevert(
          this.token.tokenURI(firstTokenId), 'ERC721Metadata: URI query for nonexistent token'
        );
      });
    });
  });

  context('with minted tokens', function () {
    beforeEach(async function () {
      await this.token.mint(owner, firstTokenId);
      await this.token.mint(owner, secondTokenId);
      this.toWhom = other; // default to other for toWhom in context-dependent tests
    });

    describe('balanceOf', function () {
      context('when the given address owns some tokens', function () {
        it('returns the amount of tokens owned by the given address', async function () {
          expect(await this.token.balanceOf(owner)).to.be.bignumber.equal('2');
        });
      });

      context('when the given address does not own any tokens', function () {
        it('returns 0', async function () {
          expect(await this.token.balanceOf(other)).to.be.bignumber.equal('0');
        });
      });

      context('when querying the zero address', function () {
        it('throws', async function () {
          await expectRevert(
            this.token.balanceOf(ZERO_ADDRESS), 'ERC721: balance query for the zero address'
          );
        });
      });
    });

    describe('ownerOf', function () {
      context('when the given token ID was tracked by this token', function () {
        const tokenId = firstTokenId;

        it('returns the owner of the given token ID', async function () {
          expect(await this.token.ownerOf(tokenId)).to.be.equal(owner);
        });
      });

      context('when the given token ID was not tracked by this token', function () {
        const tokenId = nonExistentTokenId;

        it('reverts', async function () {
          await expectRevert(
            this.token.ownerOf(tokenId), 'ERC721: owner query for nonexistent token'
          );
        });
      });
    });

    describe('transfers', function () {
      const tokenId = firstTokenId;
      const data = '0x42';

      let logs = null;

      beforeEach(async function () {
        await this.token.approve(approved, tokenId, { from: owner });
        await this.token.setApprovalForAll(operator, true, { from: owner });
      });

      const transferWasSuccessful = function ({ owner, tokenId, approved }) {
        it('transfers the ownership of the given token ID to the given address', async function () {
          expect(await this.token.ownerOf(tokenId)).to.be.equal(this.toWhom);
        });

        it('clears the approval for the token ID', async function () {
          expect(await this.token.getApproved(tokenId)).to.be.equal(ZERO_ADDRESS);
        });

        if (approved) {
          it('emit only a transfer event', async function () {
            expectEvent.inLogs(logs, 'Transfer', {
              from: owner,
              to: this.toWhom,
              tokenId: tokenId,
            });
          });
        } else {
          it('emits only a transfer event', async function () {
            expectEvent.inLogs(logs, 'Transfer', {
              from: owner,
              to: this.toWhom,
              tokenId: tokenId,
            });
          });
        }

        it('adjusts owners balances', async function () {
          expect(await this.token.balanceOf(owner)).to.be.bignumber.equal('1');
        });

        it('adjusts owners tokens by index', async function () {
          if (!this.token.tokenOfOwnerByIndex) return;

          expect(await this.token.tokenOfOwnerByIndex(this.toWhom, 0)).to.be.bignumber.equal(tokenId);

          expect(await this.token.tokenOfOwnerByIndex(owner, 0)).to.be.bignumber.not.equal(tokenId);
        });
      };

      const shouldTransferTokensByUsers = function (transferFunction) {
        context('when called by the owner', function () {
          beforeEach(async function () {
            ({ logs } = await transferFunction.call(this, owner, this.toWhom, tokenId, { from: owner }));
          });
          transferWasSuccessful({ owner, tokenId, approved });
        });

        context('when called by the approved individual', function () {
          beforeEach(async function () {
            ({ logs } = await transferFunction.call(this, owner, this.toWhom, tokenId, { from: approved }));
          });
          transferWasSuccessful({ owner, tokenId, approved });
        });

        context('when called by the operator', function () {
          beforeEach(async function () {
            ({ logs } = await transferFunction.call(this, owner, this.toWhom, tokenId, { from: operator }));
          });
          transferWasSuccessful({ owner, tokenId, approved });
        });

        context('when called by the owner without an approved user', function () {
          beforeEach(async function () {
            await this.token.approve(ZERO_ADDRESS, tokenId, { from: owner });
            ({ logs } = await transferFunction.call(this, owner, this.toWhom, tokenId, { from: operator }));
          });
          transferWasSuccessful({ owner, tokenId, approved: null });
        });

        context('when sent to the owner', function () {
          beforeEach(async function () {
            ({ logs } = await transferFunction.call(this, owner, owner, tokenId, { from: owner }));
          });

          it('keeps ownership of the token', async function () {
            expect(await this.token.ownerOf(tokenId)).to.be.equal(owner);
          });

          it('clears the approval for the token ID', async function () {
            expect(await this.token.getApproved(tokenId)).to.be.equal(ZERO_ADDRESS);
          });

          it('emits only a transfer event', async function () {
            expectEvent.inLogs(logs, 'Transfer', {
              from: owner,
              to: owner,
              tokenId: tokenId,
            });
          });

          it('keeps the owner balance', async function () {
            expect(await this.token.balanceOf(owner)).to.be.bignumber.equal('2');
          });

          it('keeps same tokens by index', async function () {
            if (!this.token.tokenOfOwnerByIndex) return;
            const tokensListed = await Promise.all(
              [0, 1].map(i => this.token.tokenOfOwnerByIndex(owner, i))
            );
            expect(tokensListed.map(t => t.toNumber())).to.have.members(
              [firstTokenId.toNumber(), secondTokenId.toNumber()]
            );
          });
        });

        context('when the address of the previous owner is incorrect', function () {
          it('reverts', async function () {
            await expectRevert(
              transferFunction.call(this, other, other, tokenId, { from: owner }),
              'ERC721: transfer of token that is not own'
            );
          });
        });

        context('when the sender is not authorized for the token id', function () {
          it('reverts', async function () {
            await expectRevert(
              transferFunction.call(this, owner, other, tokenId, { from: other }),
              'ERC721: transfer caller is not owner nor approved'
            );
          });
        });

        context('when the given token ID does not exist', function () {
          it('reverts', async function () {
            await expectRevert(
              transferFunction.call(this, owner, other, nonExistentTokenId, { from: owner }),
              'ERC721: operator query for nonexistent token'
            );
          });
        });

        context('when the address to transfer the token to is the zero address', function () {
          it('reverts', async function () {
            await expectRevert(
              transferFunction.call(this, owner, ZERO_ADDRESS, tokenId, { from: owner }),
              'ERC721: transfer to the zero address'
            );
          });
        });
      };

      describe('via transferFrom', function () {
        shouldTransferTokensByUsers(function (from, to, tokenId, opts) {
          return this.token.transferFrom(from, to, tokenId, opts);
        });
      });

      describe('via safeTransferFrom', function () {
        const safeTransferFromWithData = function (from, to, tokenId, opts) {
          return this.token.methods['safeTransferFrom(address,address,uint256,bytes)'](from, to, tokenId, data, opts);
        };

        const safeTransferFromWithoutData = function (from, to, tokenId, opts) {
          return this.token.methods['safeTransferFrom(address,address,uint256)'](from, to, tokenId, opts);
        };

        const shouldTransferSafely = function (transferFun, data) {
          describe('to a user account', function () {
            shouldTransferTokensByUsers(transferFun);
          });

          describe('to a valid receiver contract', function () {
            beforeEach(async function () {
              this.receiver = await ERC721ReceiverMock.new(RECEIVER_MAGIC_VALUE, false);
              this.toWhom = this.receiver.address;
            });

            shouldTransferTokensByUsers(transferFun);

            it('should call onERC721Received', async function () {
              const receipt = await transferFun.call(this, owner, this.receiver.address, tokenId, { from: owner });

              await expectEvent.inTransaction(receipt.tx, ERC721ReceiverMock, 'Received', {
                operator: owner,
                from: owner,
                tokenId: tokenId,
                data: data,
              });
            });

            it('should call onERC721Received from approved', async function () {
              const receipt = await transferFun.call(this, owner, this.receiver.address, tokenId, { from: approved });

              await expectEvent.inTransaction(receipt.tx, ERC721ReceiverMock, 'Received', {
                operator: approved,
                from: owner,
                tokenId: tokenId,
                data: data,
              });
            });

            describe('with an invalid token id', function () {
              it('reverts', async function () {
                await expectRevert(
                  transferFun.call(
                    this,
                    owner,
                    this.receiver.address,
                    nonExistentTokenId,
                    { from: owner },
                  ),
                  'ERC721: operator query for nonexistent token'
                );
              });
            });
          });
        };

        describe('with data', function () {
          shouldTransferSafely(safeTransferFromWithData, data);
        });

        describe('without data', function () {
          shouldTransferSafely(safeTransferFromWithoutData, null);
        });

        describe('to a receiver contract returning unexpected value', function () {
          it('reverts', async function () {
            const invalidReceiver = await ERC721ReceiverMock.new('0x42', false);
            await expectRevert(
              this.token.safeTransferFrom(owner, invalidReceiver.address, tokenId, { from: owner }),
              'ERC721: transfer to non ERC721Receiver implementer'
            );
          });
        });

        describe('to a receiver contract that throws', function () {
          it('reverts', async function () {
            const revertingReceiver = await ERC721ReceiverMock.new(RECEIVER_MAGIC_VALUE, true);
            await expectRevert(
              this.token.safeTransferFrom(owner, revertingReceiver.address, tokenId, { from: owner }),
              'ERC721ReceiverMock: reverting'
            );
          });
        });

        describe('to a contract that does not implement the required function', function () {
          it('reverts', async function () {
            const nonReceiver = this.token;
            await expectRevert(
              this.token.safeTransferFrom(owner, nonReceiver.address, tokenId, { from: owner }),
              'ERC721: transfer to non ERC721Receiver implementer'
            );
          });
        });
      });
    });

    describe('safe mint', function () {
      const fourthTokenId = new BN(4);
      const tokenId = fourthTokenId;
      const data = '0x42';

      describe('via safeMint', function () { // regular minting is tested in ERC721Mintable.test.js and others
        it('should call onERC721Received — with data', async function () {
          this.receiver = await ERC721ReceiverMock.new(RECEIVER_MAGIC_VALUE, false);
          const receipt = await this.token.safeMint(this.receiver.address, tokenId, data);

          await expectEvent.inTransaction(receipt.tx, ERC721ReceiverMock, 'Received', {
            from: ZERO_ADDRESS,
            tokenId: tokenId,
            data: data,
          });
        });

        it('should call onERC721Received — without data', async function () {
          this.receiver = await ERC721ReceiverMock.new(RECEIVER_MAGIC_VALUE, false);
          const receipt = await this.token.safeMint(this.receiver.address, tokenId);

          await expectEvent.inTransaction(receipt.tx, ERC721ReceiverMock, 'Received', {
            from: ZERO_ADDRESS,
            tokenId: tokenId,
          });
        });

        context('to a receiver contract returning unexpected value', function () {
          it('reverts', async function () {
            const invalidReceiver = await ERC721ReceiverMock.new('0x42', false);
            await expectRevert(
              this.token.safeMint(invalidReceiver.address, tokenId),
              'ERC721: transfer to non ERC721Receiver implementer'
            );
          });
        });

        context('to a receiver contract that throws', function () {
          it('reverts', async function () {
            const revertingReceiver = await ERC721ReceiverMock.new(RECEIVER_MAGIC_VALUE, true);
            await expectRevert(
              this.token.safeMint(revertingReceiver.address, tokenId),
              'ERC721ReceiverMock: reverting'
            );
          });
        });

        context('to a contract that does not implement the required function', function () {
          it('reverts', async function () {
            const nonReceiver = this.token;
            await expectRevert(
              this.token.safeMint(nonReceiver.address, tokenId),
              'ERC721: transfer to non ERC721Receiver implementer'
            );
          });
        });
      });
    });

    describe('approve', function () {
      const tokenId = firstTokenId;

      let logs = null;

      const itClearsApproval = function () {
        it('clears approval for the token', async function () {
          expect(await this.token.getApproved(tokenId)).to.be.equal(ZERO_ADDRESS);
        });
      };

      const itApproves = function (address) {
        it('sets the approval for the target address', async function () {
          expect(await this.token.getApproved(tokenId)).to.be.equal(address);
        });
      };

      const itEmitsApprovalEvent = function (address) {
        it('emits an approval event', async function () {
          expectEvent.inLogs(logs, 'Approval', {
            owner: owner,
            approved: address,
            tokenId: tokenId,
          });
        });
      };

      context('when clearing approval', function () {
        context('when there was no prior approval', function () {
          beforeEach(async function () {
            ({ logs } = await this.token.approve(ZERO_ADDRESS, tokenId, { from: owner }));
          });

          itClearsApproval();
          itEmitsApprovalEvent(ZERO_ADDRESS);
        });

        context('when there was a prior approval', function () {
          beforeEach(async function () {
            await this.token.approve(approved, tokenId, { from: owner });
            ({ logs } = await this.token.approve(ZERO_ADDRESS, tokenId, { from: owner }));
          });

          itClearsApproval();
          itEmitsApprovalEvent(ZERO_ADDRESS);
        });
      });

      context('when approving a non-zero address', function () {
        context('when there was no prior approval', function () {
          beforeEach(async function () {
            ({ logs } = await this.token.approve(approved, tokenId, { from: owner }));
          });

          itApproves(approved);
          itEmitsApprovalEvent(approved);
        });

        context('when there was a prior approval to the same address', function () {
          beforeEach(async function () {
            await this.token.approve(approved, tokenId, { from: owner });
            ({ logs } = await this.token.approve(approved, tokenId, { from: owner }));
          });

          itApproves(approved);
          itEmitsApprovalEvent(approved);
        });

        context('when there was a prior approval to a different address', function () {
          beforeEach(async function () {
            await this.token.approve(anotherApproved, tokenId, { from: owner });
            ({ logs } = await this.token.approve(anotherApproved, tokenId, { from: owner }));
          });

          itApproves(anotherApproved);
          itEmitsApprovalEvent(anotherApproved);
        });
      });

      context('when the address that receives the approval is the owner', function () {
        it('reverts', async function () {
          await expectRevert(
            this.token.approve(owner, tokenId, { from: owner }), 'ERC721: approval to current owner'
          );
        });
      });

      context('when the sender does not own the given token ID', function () {
        it('reverts', async function () {
          await expectRevert(this.token.approve(approved, tokenId, { from: other }),
            'ERC721: approve caller is not owner nor approved');
        });
      });

      context('when the sender is approved for the given token ID', function () {
        it('reverts', async function () {
          await this.token.approve(approved, tokenId, { from: owner });
          await expectRevert(this.token.approve(anotherApproved, tokenId, { from: approved }),
            'ERC721: approve caller is not owner nor approved for all');
        });
      });

      context('when the sender is an operator', function () {
        beforeEach(async function () {
          await this.token.setApprovalForAll(operator, true, { from: owner });
          ({ logs } = await this.token.approve(approved, tokenId, { from: operator }));
        });

        itApproves(approved);
        itEmitsApprovalEvent(approved);
      });

      context('when the given token ID does not exist', function () {
        it('reverts', async function () {
          await expectRevert(this.token.approve(approved, nonExistentTokenId, { from: operator }),
            'ERC721: owner query for nonexistent token');
        });
      });
    });

    describe('setApprovalForAll', function () {
      context('when the operator willing to approve is not the owner', function () {
        context('when there is no operator approval set by the sender', function () {
          it('approves the operator', async function () {
            await this.token.setApprovalForAll(operator, true, { from: owner });

            expect(await this.token.isApprovedForAll(owner, operator)).to.equal(true);
          });

          it('emits an approval event', async function () {
            const { logs } = await this.token.setApprovalForAll(operator, true, { from: owner });

            expectEvent.inLogs(logs, 'ApprovalForAll', {
              owner: owner,
              operator: operator,
              approved: true,
            });
          });
        });

        context('when the operator was set as not approved', function () {
          beforeEach(async function () {
            await this.token.setApprovalForAll(operator, false, { from: owner });
          });

          it('approves the operator', async function () {
            await this.token.setApprovalForAll(operator, true, { from: owner });

            expect(await this.token.isApprovedForAll(owner, operator)).to.equal(true);
          });

          it('emits an approval event', async function () {
            const { logs } = await this.token.setApprovalForAll(operator, true, { from: owner });

            expectEvent.inLogs(logs, 'ApprovalForAll', {
              owner: owner,
              operator: operator,
              approved: true,
            });
          });

          it('can unset the operator approval', async function () {
            await this.token.setApprovalForAll(operator, false, { from: owner });

            expect(await this.token.isApprovedForAll(owner, operator)).to.equal(false);
          });
        });

        context('when the operator was already approved', function () {
          beforeEach(async function () {
            await this.token.setApprovalForAll(operator, true, { from: owner });
          });

          it('keeps the approval to the given address', async function () {
            await this.token.setApprovalForAll(operator, true, { from: owner });

            expect(await this.token.isApprovedForAll(owner, operator)).to.equal(true);
          });

          it('emits an approval event', async function () {
            const { logs } = await this.token.setApprovalForAll(operator, true, { from: owner });

            expectEvent.inLogs(logs, 'ApprovalForAll', {
              owner: owner,
              operator: operator,
              approved: true,
            });
          });
        });
      });

      context('when the operator is the owner', function () {
        it('reverts', async function () {
          await expectRevert(this.token.setApprovalForAll(owner, true, { from: owner }),
            'ERC721: approve to caller');
        });
      });
    });

    describe('getApproved', async function () {
      context('when token is not minted', async function () {
        it('reverts', async function () {
          await expectRevert(
            this.token.getApproved(nonExistentTokenId),
            'ERC721: approved query for nonexistent token'
          );
        });
      });

      context('when token has been minted ', async function () {
        it('should return the zero address', async function () {
          expect(await this.token.getApproved(firstTokenId)).to.be.equal(
            ZERO_ADDRESS
          );
        });

        context('when account has been approved', async function () {
          beforeEach(async function () {
            await this.token.approve(approved, firstTokenId, { from: owner });
          });

          it('should return approved account', async function () {
            expect(await this.token.getApproved(firstTokenId)).to.be.equal(approved);
          });
        });
      });
    });

    describe('tokensOfOwner', function () {
      it('returns total tokens of owner', async function () {
        const tokenIds = await this.token.tokensOfOwner(owner);
        expect(tokenIds.length).to.equal(2);
        expect(tokenIds[0]).to.be.bignumber.equal(firstTokenId);
        expect(tokenIds[1]).to.be.bignumber.equal(secondTokenId);
      });
    });

    describe('totalSupply', function () {
      it('returns total token supply', async function () {
        expect(await this.token.totalSupply()).to.be.bignumber.equal('2');
      });
    });

    describe('tokenOfOwnerByIndex', function () {
      describe('when the given index is lower than the amount of tokens owned by the given address', function () {
        it('returns the token ID placed at the given index', async function () {
          expect(await this.token.tokenOfOwnerByIndex(owner, 0)).to.be.bignumber.equal(firstTokenId);
        });
      });

      describe('when the index is greater than or equal to the total tokens owned by the given address', function () {
        it('reverts', async function () {
          await expectRevert(
            this.token.tokenOfOwnerByIndex(owner, 2), 'ERC721Enumerable: owner index out of bounds'
          );
        });
      });

      describe('when the given address does not own any token', function () {
        it('reverts', async function () {
          await expectRevert(
            this.token.tokenOfOwnerByIndex(other, 0), 'ERC721Enumerable: owner index out of bounds'
          );
        });
      });

      describe('after transferring all tokens to another user', function () {
        beforeEach(async function () {
          await this.token.transferFrom(owner, other, firstTokenId, { from: owner });
          await this.token.transferFrom(owner, other, secondTokenId, { from: owner });
        });

        it('returns correct token IDs for target', async function () {
          expect(await this.token.balanceOf(other)).to.be.bignumber.equal('2');
          const tokensListed = await Promise.all(
            [0, 1].map(i => this.token.tokenOfOwnerByIndex(other, i))
          );
          expect(tokensListed.map(t => t.toNumber())).to.have.members([firstTokenId.toNumber(),
            secondTokenId.toNumber()]);
        });

        it('returns empty collection for original owner', async function () {
          expect(await this.token.balanceOf(owner)).to.be.bignumber.equal('0');
          await expectRevert(
            this.token.tokenOfOwnerByIndex(owner, 0), 'ERC721Enumerable: owner index out of bounds'
          );
        });
      });
    });

    describe('tokenByIndex', function () {
      it('should return all tokens', async function () {
        const tokensListed = await Promise.all(
          [0, 1].map(i => this.token.tokenByIndex(i))
        );
        expect(tokensListed.map(t => t.toNumber())).to.have.members([firstTokenId.toNumber(),
          secondTokenId.toNumber()]);
      });

      it('should revert if index is greater than supply', async function () {
        await expectRevert(
          this.token.tokenByIndex(2), 'ERC721Enumerable: global index out of bounds'
        );
      });

      [firstTokenId, secondTokenId].forEach(function (tokenId) {
        it(`should return all tokens after burning token ${tokenId} and minting new tokens`, async function () {
          const newTokenId = new BN(300);
          const anotherNewTokenId = new BN(400);

          await this.token.burn(tokenId, { from: owner });
          await this.token.mint(newOwner, newTokenId);
          await this.token.mint(newOwner, anotherNewTokenId);

          expect(await this.token.totalSupply()).to.be.bignumber.equal('3');

          const tokensListed = await Promise.all(
            [0, 1, 2].map(i => this.token.tokenByIndex(i))
          );
          const expectedTokens = [firstTokenId, secondTokenId, newTokenId, anotherNewTokenId].filter(
            x => (x !== tokenId)
          );
          expect(tokensListed.map(t => t.toNumber())).to.have.members(expectedTokens.map(t => t.toNumber()));
        });
      });
    });
  });

  describe('_mint(address, uint256)', function () {
    it('reverts with a null destination address', async function () {
      await expectRevert(
        this.token.mint(ZERO_ADDRESS, firstTokenId), 'ERC721: mint to the zero address'
      );
    });

    context('with minted token', async function () {
      beforeEach(async function () {
        ({ logs: this.logs } = await this.token.mint(owner, firstTokenId));
      });

      it('emits a Transfer event', function () {
        expectEvent.inLogs(this.logs, 'Transfer', { from: ZERO_ADDRESS, to: owner, tokenId: firstTokenId });
      });

      it('creates the token', async function () {
        expect(await this.token.balanceOf(owner)).to.be.bignumber.equal('1');
        expect(await this.token.ownerOf(firstTokenId)).to.equal(owner);
      });

      it('adjusts owner tokens by index', async function () {
        expect(await this.token.tokenOfOwnerByIndex(owner, 0)).to.be.bignumber.equal(firstTokenId);
      });

      it('adjusts all tokens list', async function () {
        expect(await this.token.tokenByIndex(0)).to.be.bignumber.equal(firstTokenId);
      });

      it('reverts when adding a token id that already exists', async function () {
        await expectRevert(this.token.mint(owner, firstTokenId), 'ERC721: token already minted');
      });
    });
  });

  describe('_burn', function () {
    it('reverts when burning a non-existent token id', async function () {
      await expectRevert(
        this.token.burn(firstTokenId), 'ERC721: owner query for nonexistent token'
      );
    });

    context('with minted tokens', function () {
      beforeEach(async function () {
        await this.token.mint(owner, firstTokenId);
        await this.token.mint(owner, secondTokenId);
      });

      context('with burnt token', function () {
        beforeEach(async function () {
          ({ logs: this.logs } = await this.token.burn(firstTokenId));
        });

        it('emits a Transfer event', function () {
          expectEvent.inLogs(this.logs, 'Transfer', { from: owner, to: ZERO_ADDRESS, tokenId: firstTokenId });
        });

        it('deletes the token', async function () {
          expect(await this.token.balanceOf(owner)).to.be.bignumber.equal('1');
          await expectRevert(
            this.token.ownerOf(firstTokenId), 'ERC721: owner query for nonexistent token'
          );
        });

        it('removes that token from the token list of the owner', async function () {
          expect(await this.token.tokenOfOwnerByIndex(owner, 0)).to.be.bignumber.equal(secondTokenId);
        });

        it('adjusts all tokens list', async function () {
          expect(await this.token.tokenByIndex(0)).to.be.bignumber.equal(secondTokenId);
        });

        it('burns all tokens', async function () {
          await this.token.burn(secondTokenId, { from: owner });
          expect(await this.token.totalSupply()).to.be.bignumber.equal('0');
          await expectRevert(
            this.token.tokenByIndex(0), 'ERC721Enumerable: global index out of bounds'
          );
        });

        it('reverts when burning a token id that has been deleted', async function () {
          await expectRevert(
            this.token.burn(firstTokenId), 'ERC721: owner query for nonexistent token'
          );
        });
      });
    });
  });
});