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JavaScript Express Basics Parameters, Query Strings, and Modularizing Routes Linking Around the Application

Posted by Ismayil Aliyev
Ismayil Aliyev
18,560 Points

Error: Cannot set headers after they are sent to client (for redirecting when there is no side variable)

In the redirecting challenge, I wrote code exact as in video and it works. But command line gives me such kind of message:

Error [ERR_HTTP_HEADERS_SENT]: Cannot set headers after they are sent to the client

Please, help me to understand why I got such a message even if app works properly, and how can I solve it or should I?

Michael Hulet
Michael Hulet
47,913 Points

You've called res.set (or a related method) sometime after response.send, which isn't allowed. We'd need to know more about your code to point to a specific line

2 Answers

Ismayil Aliyev
Ismayil Aliyev
18,560 Points

Dear friends, I could figure out why cmd was giving me such kind of error. 

router.get('/:id', (req, res) => {
    const { side } = req.query;
    const { id } = req.params;
    if(!side){
      return res.redirect(`/cards/${id}?side=question`)
    }
    const text = cards[id][side];
    const { hint } = cards[id];

    const templateData = { id, text };

    if ( side === 'question' ) {
      templateData.hint = hint;
      templateData.sideToShow = 'answer';
      templateData.sideToShowDisplay = 'Answer';
    } else if ( side === 'answer' ) {
      templateData.sideToShow = 'question';
      templateData.sideToShowDisplay = 'Question';
    }

    res.render('card', templateData);
});

The main reason was router.get() method couldn't finish its work because of redirecting. I found solution in writing return value before redirecting. Thanks a lot for your attention :) .

Victoria Medley
Victoria Medley
17,085 Points

What do you mean by this? Where did you put the return value and what did you write exactly? Could you post a snippet of the completed code? I am stuck on the same issue, I tried what you said but cannot figure it out.

Gabbie Metheny
Gabbie Metheny
33,778 Points

Andrew addresses this a couple videos later, here. Basically, the rest of the router.get() function tries to execute, so we're trying to redirect and render simultaneously. Attempting to render after the redirect is what's causing the console error. Using the return keyword breaks us out of the router.get() function when we redirect, before the res.render().

Finley Williamson
Finley Williamson
11,375 Points

Thank you very much!

Tim Acker
.a{fill-rule:evenodd;}techdegree seal-36
Tim Acker
Front End Web Development Techdegree Graduate 31,247 Points

Nice catch!

Jaime Jesus Guerra Torres
Jaime Jesus Guerra Torres
16,954 Points
Jaime Jesus Guerra Torres
Jaime Jesus Guerra Torres
16,954 Points

I solved it, encasing the rest of the code in a "else" statement

router.get('/:id', (req, res) => {
    const { side } = req.query;
    const { id } = req.params;

    if(!side) {
        res.redirect(`/cards/${id}?side=question`);
    } else {
        const text = cards[id][side];
        const { hint } = cards[id];

        const templateData = { text, id };

        if (side === 'question') {
            templateData.hint = hint;
            templateData.sideToShow = 'answer';
            templateData.sideToShowDisplay = 'Answer';
        } else if ( side === 'answer' ) {
            templateData.sideToShow = 'question';
            templateData.sideToShowDisplay = 'Question';
        }

        res.render('card', templateData);
    }
});
Trevor Maltbie
.a{fill-rule:evenodd;}techdegree seal-36
Trevor Maltbie
Full Stack JavaScript Techdegree Graduate 17,021 Points

This solved the problem for me.

This is because the router was trying to render after a redirect?