I am creating a Nodejs and express based backend application and trying to handle error in a manner which is suitable for production systems.
I use async await to handle all synchronous operations in the code.
Here is a code snippet of router end points
app.get("/demo",async (req, res, next) => {
await helper().catch(e => return next(e))
console.log("After helper is called")
res.json(1)
})
function helper(){ //helper function that throws an exception
return new Promise((resolve, reject)=> reject(new Error("Demo Error")))
}
After all routes are defined I have added a common error handler that catches exceptions. To simplify it I am adding a simple function
routes.use( (err, req, res, next) => {
console.log("missed all", err)
return res.status(500).json({error:err.name, message: err.message});
});
I expect that the code after await helper() should not execute since the exception has been handled and response sent to frontend. Instead what I get is this error.
After helper is called
(node:46) UnhandledPromiseRejectionWarning: Error
[ERR_HTTP_HEADERS_SENT]: Cannot set headers after they are sent to the
client
What is the correct way to handle error with async await?
you can use try
catch
to handle the situation
app.get("/demo",async (req, res, next) => { try { await helper() console.log("After helper is called") res.json(1) } catch(err) { next(err) } }) function helper(){ //helper function that throws an exception return new Promise((resolve, reject)=> reject(new Error("Demo Error"))) }
You get After helper is called
, because your code continues to execute
since it did not return
Don't chain catch
with async/await
. You do that with Promise
.
helper()
.then(data => console.log(data))
.catch(e => console.log(e))
You can handle error like:
app.get("/demo",async (req, res, next) => {
try {
await helper();
// respond sent if all went well
res.json(something)
catch(e) {
// don't need to respond as you're doing that with catch all error handler
next(e)
}
})
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