let addresses: string[]不初始化addresses为任何东西，所以除非有东西分配一个数组给变量，它将只是undefined . addresses.push(await _signer.getAddress())不分配一个新的数组给变量，如果它是undefined .你需要手动地做.只是尝试做以下在你的浏览器控制台：

let a
a.push(1) // will error since a is undefined

相反，您应该：

let signersWithAddress: SignerWithAddress[] = [];

感谢您的指导。以下是代码的最终工作版本：

import { SignerWithAddress } from "@nomiclabs/hardhat-ethers/signers"
import { ethers } from "hardhat"
import { beforeEach, describe, it } from "mocha"

let signersWithAddress: SignerWithAddress[] = []
let addresses: string[] = []

describe("Basic Test", () => {
    beforeEach(async () => {
        const signersArray = await ethers.getSigners()
        await Promise.all(
            signersArray.map(async (i) => {
                addresses.push(await i.getAddress())
            })
        )
    })
    it("Prints every address that we have", () => {
        console.log(addresses)
    })
})