AWS Lambda & JSON.stringify \\n -> \n - not working with Slack API

Question

We're using a lambda function (node js) to send a message into our slack channel which works. However, when aws is getting a new line its in the format \n. JSON.stringify then encodes this into \\n - and message in slack is like "this is \n a new line". I already tried to replace \\n by \n but this wont work (no msg posted into slack). If i'm checking the console output its:

{"text":"\"This is a \

 new line\""}

So it really replaces it with a new line. But what I actually need is a replacement from "This is a \\n new line" into "This is a \n newline" where \n is considered as text so the API of slack is aware of it...??

req.write(  JSON.stringify({text: JSON.stringify(rec.Sns.Message, null, '  ')}).replace('\\n', '\n')  );

As requested more information - its a webhook in Slack, yes :)

To be honest - i'm a bit lost as I have no idea how to debug directly in AWS. I just assume i have something like this:

console.log(     JSON.stringify({text: JSON.stringify("Thats a new \n line", null, '  ')})   );

So i'm ending up with \\n in the console. Then I tried

console.log(     JSON.stringify({text: JSON.stringify("Thats a new \n line", null, '  ')}).replace("\\n", "\n")   );

Which actually works but then the console output is

Thats a new
line

which makes sense but breaks my API call. So I have no idea how to end up with This is a new \n line encoding..

Maybe it makes sense to post the full lambda function:

console.log('Loading function');

const https = require('https');
const url = require('url');
const slack_url = 'https://hooks.slack.cosm/etc';
const slack_req_opts = url.parse(slack_url);
slack_req_opts.method = 'POST';
slack_req_opts.headers = {'Content-Type': 'application/json'};

exports.handler = function(event, context) {
  (event.Records || []).forEach(function (rec) {
    if (rec.Sns) {
      var req = https.request(slack_req_opts, function (res) {
        if (res.statusCode === 200) {
          context.succeed('sns posted to slack');
        } else {
          context.fail('status code: ' + res.statusCode);
        }
      });

      req.on('error', function(e) {
        console.log('problem with request: ' + e.message);
        context.fail(e.message);
      });

      req.write(  JSON.stringify({text: JSON.stringify(rec.Sns.Message, null, '  ')})  ); 

      req.end();
    }
  });
};

This can not work. You are JSON encoding the text string twice. — Erik Kalkoken
Ha! You're right - it works now. Got the code from a documentation and thought its correct...thanks! — AndTa

Erik Kalkoken Erik Kalkoken · Accepted Answer · 2019-10-10T11:46:15

You did not say, so I am assuming you are sending you message to an incoming webhook.

The correct JSON to send to Slack would be this:

{"text": "\"This is a \n newline\""}

The reason it does not work for you is that you are JSON encoding the text string twice. Just remove the inner JSON.stringify and it should work.

req.write(  JSON.stringify({text: rec.Sns.Message})  );

Btw. manually replacing parts of a string should be a last resort, e.g. when your programming lanuguage has no support for a specific encoding. It should not be needed in your case.

AWS Lambda & JSON.stringify \\n -> \n - not working with Slack API

1 Answers