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JavaScript Express Basics Middleware Handling 404 Errors

Posted by Daniel Stein
Daniel Stein
90 Points

hi I'm getting a Can't set headers after they are sent. trying to send a res.send

as above

1 Answer

Alexander La Bianca
Alexander La Bianca
15,959 Points

Hi Daniel,

Can you please post your code. In the meantime. Node throws this error when you are currently writing to the response and then try to write a second response right after it. Common places for this to show up is in loops or not correctly returning the request handler. 

app.get('/', (req,res)=>{
        //This can throw the error - as you are looping over all items and therefore writng multiple headers
         someArray.forEach((item)=>{
                res.end(item);
        });
});

app.get('/', (req,res)=>{
        //This will throw the error as well. As you are writing the header twice to fix it. put a return statement infront of the res.status(200).end('HI') in order to not execute the rest of the function. 
         if(true) {
              res.status(200).end("Hi");
         }
         res.status(400).end("Hi);
});

Again those are 2 common places in code that can throw this error, but if I saw your code I could help you out more.