node.js - 为什么重定向后无法设置 header ？

Question

这是我在expressjs(node)中发出json请求后的回调

function question(req, res){
    request(req, res, function(response){
        //redirect to the right url if page doesn't exist.
        var params = app.req.params;
        if(params.questionId && encodeURIComponent(params.slug) !== response.question.slug){
            app.res.redirect(response.question.url);
        }
        //Render question
        app.res.render('master',response);
    });
}

这是请求:

function request(req, res, callback){
    app.req = req;
    app.res = res;
    http.get({
        hostname: 'localhost',
        port: 80,
        path: req.url,
        agent: false  // create a new agent just for this one request
    }, function(response){
        response.setEncoding('utf8');
        response.on('data', function(json){
            json = JSON.parse(json);
            console.log(json)
            callback(json);
        });
    });
}

如果我触发 res.redirect 我会收到以下错误。我的期望是我被重定向到新的 url 并且 app.res.render 应该被渲染。无论我访问哪个网址，此错误都会持续触发一段时间。

_http_outgoing.js:344

throw new Error('Can\'t set headers after they are sent.');
^

Error: Can't set headers after they are sent.
    at ServerResponse.OutgoingMessage.setHeader (_http_outgoing.js:344:11)
    at ServerResponse.header (/Users/kristoffer/web/zenqa2.1/node_modules/express/lib/response.js:718:10)
    at ServerResponse.send (/Users/kristoffer/web/zenqa2.1/node_modules/express/lib/response.js:163:12)
    at done (/Users/kristoffer/web/zenqa2.1/node_modules/express/lib/response.js:957:10)
    at /Users/kristoffer/web/zenqa2.1/node_modules/mustache-express/mustache-express.js:168:5
    at /Users/kristoffer/web/zenqa2.1/node_modules/mustache-express/mustache-express.js:131:10
    at /Users/kristoffer/web/zenqa2.1/node_modules/mustache-express/mustache-express.js:118:11
    at loadAllPartials (/Users/kristoffer/web/zenqa2.1/node_modules/mustache-express/mustache-express.js:79:10)
    at /Users/kristoffer/web/zenqa2.1/node_modules/mustache-express/mustache-express.js:102:10
    at /Users/kristoffer/web/zenqa2.1/node_modules/async/lib/async.js:232:13

Answer 1

对于单个请求，只会发送一个响应，并且您必须在发送响应之前设置 header 。 app.res.redirect 和 app.res.render 用于传递响应。所以实际上你只能使用其中之一。使用return退出代码或使用else条件。

function question(req, res){
    request(req, res, function(response){
        //redirect to the right url if page doesn't exist.
        var params = app.req.params;
        if(params.questionId && encodeURIComponent(params.slug) !== response.question.slug){
            return app.res.redirect(response.question.url); // return
        }
        // also you can use else condition here
        app.res.render('master',response);
    });
}