webpackdevserver 报错Something is already running on port 3000 不一定是因为端口被占用

webpackdevserver 报错Something is already running on port 3000 不一定是因为端口被占用
也有可能是你没有开localhost 绑定域名 导致域名解析失败

跟踪代码发现：

如果本地服务报错 会进入

listen(port, 'localhost', err => {
  if (err) {
    console.log('进入2',err)
    return handleError(err)
  }

  // 4. check current ip
  listen(port, address.ip(), (err, realPort) => {
    if (err) {
      console.log('进入1')
      return handleError(err)
    }

    callback(null, realPort)
  })
})

function handleError() {
  port++
  if (port >= maxPort) {
    debug(
      'port: %s >= maxPort: %s, give up and use random port',
      port,
      maxPort
    )
    port = 0
    maxPort = 0
  }
  tryListen(host, port, maxPort, callback)
}

而没进入一次这个函数 port都会累加一次  进入这个err的原因就是没有绑定host 

err信息打印出来是这样的：

Error: getaddrinfo ENOTFOUND localhost     at errnoException (dns.js:28:10)     at GetAddrInfoReqWrap.onlookup [as oncomplete] (dns.js:73:26)   code: 'ENOTFOUND',   errno: 'ENOTFOUND',   syscall: 'getaddrinfo',   hostname: 'localhost' 

翻译后大概意思就是没有绑定host 无法进行dns解析

 

导致在调用对比时候出现问题

devserver调用函数

function choosePort(host, defaultPort) {
  return detect(defaultPort, host).then(
    port =>
      new Promise(resolve => {
   // 这里做判断 此时port唯一 是你设置的值 相等直接resolve（port）不相等则会在控制台弹出选择随机port 意味着处于报错状态（可能是由于host 没有配置127.0.0.1）

        if (port === defaultPort) {
          return resolve(port);
        }
        const message =
          process.platform !== 'win32' && defaultPort < 1024 && !isRoot()
            ? `Admin permissions are required to run a server on a port below 1024.`
            : `Something is already running on port ${defaultPort}.`;
        if (isInteractive) {
          // clearConsole();
          const existingProcess = getProcessForPort(defaultPort);
          const question = {
            type: 'confirm',
            name: 'shouldChangePort',
            message:
              chalk.yellow(
                message +
                  `${existingProcess ? ` Probably:\n  ${existingProcess}` : ''}`
              ) + '\n\nWould you like to run the app on another port instead?',
            default: true,
          };
          inquirer.prompt(question).then(answer => {
            if (answer.shouldChangePort) {
              resolve(port);
            } else {
              resolve(null);
            }
          });
        } else {
          console.log(chalk.red(message));
          resolve(null);
        }
      }),
    err => {
      throw new Error(
        chalk.red(`Could not find an open port at ${chalk.bold(host)}.`) +
          '\n' +
          ('Network error message: ' + err.message || err) +
          '\n'
      );
    }
  );
}