使用JavaScript测试Solidity代码遇TypeError：无法读取undefined的address属性

问题原因 #

错误的核心是变量作用域冲突：你在before块内重新声明了const swapExamples，这会覆盖外层已经声明的let swapExamples变量，导致测试用例中的swapExamples始终处于undefined状态，访问其address属性时自然抛出错误。

修复方案 #

去掉before块中声明合约实例时的const关键字，直接赋值给外层已定义的swapExamples变量：

修改后的完整测试代码 #

const { expect } = require("chai");
const { ethers } = require("hardhat");

const DAI = "0x6B175474E89094C44Da98b954EedeAC495271d0F";
const WETH9 = "0xC02aaA39b223FE8D0A0e5C4F27eAD9083C756Cc2";

describe("SwapExamples", function () {
    let swapExamples;
    let accounts;
    let weth;
    let dai;

    before(async () => {
        accounts = await ethers.getSigners();

        weth = await ethers.getContractAt("IWETH", WETH9);
        dai = await ethers.getContractAt("IERC20", DAI);

        const SwapExamples = await ethers.getContractFactory("SwapExamples");
        // 移除const，直接赋值给外层变量
        swapExamples = await SwapExamples.deploy();
        await swapExamples.deployed();
    })

    it("swapExactInputSingle", async function () {
        const amountIn = 10n ** 18n;

        await weth.connect(accounts[0]).deposit({ value: amountIn });
        await weth.connect(accounts[0]).approve(swapExamples.address, amountIn);

        await swapExamples.swapExactInputSingle(amountIn);

        console.log("DAI balance", await dai.balanceOf(accounts[0].address));
    });

    it("swapExactOutputSingle", async function () {
        const wethAmountInMax = 10n ** 18n;
        const daiAmountOut = 100n * 10n ** 18n;

        await weth.connect(accounts[0]).deposit({ value: wethAmountInMax });
        await weth.connect(accounts[0]).approve(swapExamples.address, wethAmountInMax);

        await swapExamples.swapExactOutputSingle(daiAmountOut, wethAmountInMax);

        console.log("DAI balance", await dai.balanceOf(accounts[0].address));
    });
});

额外注意事项 #

确保你的Hardhat环境配置了主网fork模式，因为测试中使用了主网的DAI和WETH合约地址，否则会出现合约不存在的异常。

内容的提问来源于stack exchange，提问作者Mav