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catch error for bad json format thrown by express.json() middleware

   February 24, 2021  1 Comment on catch error for bad json format thrown by express.json() middleware

I am getting SyntaxError: Unexpected string in JSON at position 59 error in html format when format of json data is not valid. I don’t know why it is giving me html instead of error object.
I have set my header like below.

//header middlewares
app.use((req, res, next) => {
    res.setHeader('Content-Type', 'application/json');
    res.setHeader("Access-Control-Allow-Origin", "*");
    next();
  });

I want to catch the error and send a message in below format.

{ 
  "status":404,
  "message":"Unexpected string in JSON at position 59"
}

Here is the error that I get.

<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="utf-8">
<title>Error</title>
</head>
<body>
<pre>SyntaxError: Unexpected string in JSON at position 59<br> &nbsp; &nbsp;at JSON.parse (&lt;anonymous&gt;)<br> &nbsp; &nbsp;at parse (C:\Users\mydirectory\auth\node_modules\body-parser\lib\types\json.js: 89: 19)<br> &nbsp; &nbsp;at C:\Users\mydirectory\auth\node_modules\body-parser\lib\read.js: 121: 18<br> &nbsp; &nbsp;at invokeCallback (C:\Users\mydirectory\auth\node_modules\raw-body\index.js: 224: 16)<br> &nbsp; &nbsp;at done (C:\Users\my-directory\auth\node_modules\raw-body\index.js: 213: 7)<br> &nbsp; &nbsp;at IncomingMessage.onEnd (C:\Users\mydirectory\auth\node_modules\raw-body\index.js: 273: 7)<br> &nbsp; &nbsp;at IncomingMessage.emit (events.js: 203: 15)<br> &nbsp; &nbsp;at endReadableNT (_stream_readable.js: 1145: 12)<br> &nbsp; &nbsp;at process._tickCallback (internal/process/next_tick.js: 63: 19)</pre>
</body>
</html>

I tried catching this error.

app.use((err, req, res, next) => {
    if (err instanceof SyntaxError && err.status === 400 && 'body' in err) {
        console.error(err);
        return res.status(400).send(err); // Bad request
    }
    next();
});

But the response that I get now is like below.

{
    "expose": true,
    "statusCode": 400,
    "status": 400,
    "body": "{\n\t\"username\":\n}",
    "type": "entity.parse.failed"
}

Author: Fantashit

1 thought on “catch error for bad json format thrown by express.json() middleware”

  1. Hi @Lordofcodes sorry you are having trouble. If you want that specific response, that is what you need to send in your error handler instead of the err object itself. Example:

    app.use((err, req, res, next) => {
    if (err instanceof SyntaxError && err.status === 400 && 'body' in err) {
        console.error(err);
        return res.status(400).send({ status: 404, message: err.message }); // Bad request
    }
    next();
});

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