允许跨域 解决’Access-Control-Allow-Origin’跨域问题

错误提示：Failed to load http://localhost:3000/users/validate: Response to preflight request doesn't pass access control check: No 'Access-Control-Allow-Origin' header is present on the requested resource. Origin 'http://localhost:8080' is therefore not allowed access.  

vue：lcalhsot:8080，nodejs：localhost:3000. 显示跨域。

这是因为http请求头部没有进行允许跨域导致的，打开后端服务的app.js文件，在路由配置前添加以下代码

app.all('*', function(req, res, next) {
    res.header("Access-Control-Allow-Origin", "*");
    res.header("Access-Control-Allow-Headers", "X-Requested-With,Content-Type");
    res.header("Access-Control-Allow-Methods","PUT,POST,GET,DELETE,OPTIONS");
    next();
});