在React中使用MetaMask中的web3

我试图在React js app中使用MetaMask中的web3,如下所示:

import Web3 from 'web3';

    componentDidMount(){
            if (typeof web3 !== 'undefined') {
                console.log(web3.currentProvider);
                    // Use Mist/MetaMask's provider
                    var web3js = new Web3(web3.currentProvider);

                    web3.version.getNetwork((err, netId) => {
                    switch (netId) {
                        case "1":
                            console.log('This is mainnet')
                            break
                        case "2":
                            console.log('This is the deprecated Morden test network.')
                            break
                        case "3":
                            console.log('This is the ropsten test network.')
                            break
                        case "4":
                            console.log('This is the Rinkeby test network.')
                            break
                        case "42":
                            console.log('This is the Kovan test network.')
                            break
                        default:
                            console.log('This is an unknown network.')
                    }
                })
            } else {
                    console.log('No web3? You should consider trying MetaMask!')
            }           
        }

这是我在chrome控制台中获得的输出:

很明显,在某些时候,MetaMask正在基于前两行正确定义web3,但是然后反应会抛出一个错误,说明web3没有为它出现的实例定义if(typeof web3!=='undefined').我尝试过的所有内容都会导致相同的错误或web3无法加载.

您应该在浏览器中使用MetaMask等web3提供程序.这是我用于web3检测的脚本:

window.addEventListener('load', function () {
    if (typeof web3 !== 'undefined') {        
        window.web3 = new Web3(window.web3.currentProvider)

        if (window.web3.currentProvider.isMetaMask === true) {
            window.web3.eth.getAccounts((error, accounts) => {
                if (accounts.length == 0) {
                    // there is no active accounts in MetaMask
                }
                else {
                    // It's ok
                }
            });
        } else {
            // Another web3 provider
        }
    } else {
        // No web 3 provider
    }    
});