使用ethers.js v6测试Solidity拍卖合约的1 wei舍入差异问题

问题场景 #

使用ethers.js v6进行Solidity拍卖合约测试时，验证交易后的账户ETH余额变化出现1 wei的舍入误差，导致测试不通过。

合约核心buy函数逻辑 #

function buy(uint index) external payable activeAuction(index) {
    Auction storage auction = auctions[index];
    require(block.timestamp < auction.endAt, "Auction has ended");
    uint price = getPriceFor(index);
    require(msg.value >= price, "Not enough money");
    auction.stopped = true;
    auction.finalPrice = price;

    //refund
    uint refund = msg.value - price;
    if(refund > 0) {
    payable(msg.sender).transfer(refund);
    }

    auction.seller.transfer(price - ((price * FEE)/100));

    emit AuctionEnded(index, price, msg.sender);
}

原测试代码（存在精度问题） #

it("Should allow to buy and refund", async () => {
        const FEE = 10; 
        
        const oneEtherInWei = ethers.parseEther("1");  // 1 ETH = 1e18 wei
        const twoEtherInWei = ethers.parseEther("2");  // 2 ETH = 2e18 wei

        await expect(smartContract.connect(seller).createAuction(
            oneEtherInWei,  
            FEE,
            "test item",
            60
        ));
        
        const tx = await smartContract.connect(buyer).buy(0, { value: twoEtherInWei });
        await tx.wait();
        
        const feeAmount = (parseFloat(oneEtherInWei.toString()) * FEE) / 100; 
        console.log("feeAmount :", feeAmount);
        
        const moneyForSeller = parseFloat(oneEtherInWei.toString()) - feeAmount;
        console.log("moneyForSeller :", moneyForSeller);
        
        const moneyLeftOnContract = parseFloat(oneEtherInWei.toString()) - moneyForSeller;
        console.log("moneyLeftOnContract :", moneyLeftOnContract);


        await expect(tx).to.changeEtherBalance(smartContract.target, moneyLeftOnContract.toString()) // 验证合约余额变化
        await expect(tx).to.changeEtherBalance(buyer.address, (parseFloat(twoEtherInWei) * -1).toString()) // 验证买家余额变化
        await expect(tx).to.changeEtherBalance(seller.address, moneyForSeller.toString()) // 验证卖家余额变化

        
        
        const price = await smartContract.auctions(0).finalPrice;
        expect(tx).to.emit(smartContract, "AuctionEnded")
            .withArgs(0, price, buyer);
    });

问题原因 #

核心问题是使用parseFloat处理wei级别的大整数导致精度丢失：

• wei是10^{18级别的超大整数，而JavaScript的`parseFloat`基于双精度浮点数，仅能精确表示2}53（约9×10^15）以内的整数。
• 当把1e18这样的大整数转成浮点数时，会被近似存储，后续计算（比如乘以10%手续费）会产生微小的精度偏差，最终导致1 wei的误差。

解决方案 #

全程使用ethers.js返回的BigInt类型进行整数运算，完全避免浮点数的精度问题，同时和Solidity中的uint整数除法行为保持一致（截断取整）。

修改后的测试代码 #

it("Should allow to buy and refund", async () => {
        const FEE = 10; 
        const FEE_BIGINT = BigInt(FEE);
        const HUNDRED = BigInt(100);
        
        const oneEtherInWei = ethers.parseEther("1");  // 返回BigInt类型
        const twoEtherInWei = ethers.parseEther("2");

        await smartContract.connect(seller).createAuction(
            oneEtherInWei,  
            FEE,
            "test item",
            60
        );
        
        const tx = await smartContract.connect(buyer).buy(0, { value: twoEtherInWei });
        await tx.wait();
        
        // 用BigInt做整数运算，和Solidity逻辑完全一致
        const feeAmount = oneEtherInWei * FEE_BIGINT / HUNDRED;
        const moneyForSeller = oneEtherInWei - feeAmount;
        // 合约留存的就是手续费，无需额外计算
        const moneyLeftOnContract = feeAmount;

        // 直接传入BigInt值，无需转字符串（ethers.js会自动处理）
        await expect(tx).to.changeEtherBalance(smartContract.target, moneyLeftOnContract)
        // 买家实际支出：付2ETH，退款1ETH，所以余额变化是-1ETH
        await expect(tx).to.changeEtherBalance(buyer.address, oneEtherInWei * BigInt(-1))
        await expect(tx).to.changeEtherBalance(seller.address, moneyForSeller)

        const price = await smartContract.auctions(0).finalPrice;
        await expect(tx).to.emit(smartContract, "AuctionEnded")
            .withArgs(0, price, buyer.address);
    });

关键修改点 #

1. 用BigInt替代parseFloat进行所有金额计算，确保整数精度
2. 买家余额变化直接计算为-oneEtherInWei（因为付2ETH退1ETH，实际支出1ETH），避免浮点数转换
3. 合约留存金额直接等于手续费，简化计算逻辑
4. 验证时直接传入BigInt值，无需转字符串（ethers.js的changeEtherBalance支持BigInt参数）

内容的提问来源于stack exchange，提问作者Vlad Tsykin