metamask环境中，使用metamask注入的 ethereum 对象来实例化

if (window.ethereum) {
    window.web3 = new Web3(ethereum);
    // 拿默认账户
    ethereum.request({ method: 'eth_requestAccounts' }).then((accounts) => {
        resolve(accounts);
        // Request account access if needed
    }).catch(() => {
        Toast.fail('User denied account access...')
        reject('User denied account access...');
    });
    } else {
    Toast.fail('Please install MetaMask!')
    }

也可以实例化一个provider对象:

import Web3 from 'web3';
    window.web3 = new Web3(Web3.givenProvider);
    // 拿默认账户
    resolve(await window.web3.eth.getAccounts())