Debug a Google Apps Script which is triggered by post request?

I have a simple script in Google Sheets which is trigger by a command in Slack and just adds the Slack message as a new row. It is quite a simple function and is deployed as a web app and does work:

function doPost(req) {
var sheet = SpreadsheetApp.openById('[My Sheet Id]');
var params = req.parameters;

Logger.log(params.text);

sheet.appendRow(params.text);

return ContentService.createTextOutput("Saved your entry:" + params.text);
}

However the Logger.log function never logs anything in the debugging logs. I expect it to be here:

Bizarrely the Executions lists is also empty:

But the script is being triggered and is appending the text message to the Google sheet.

So the question I suppose comes down to how exactly can I log from a script (deployed as a web app) when triggered by a post request and also how can I see its executions? In other words how do you debug such scripts?

When a doPost(e) is invoked remotely, it creates a server-side session whose logs you cannot access via Logger.log().

However, there is an alternative, ie. StackDriver Logging (accessible from the Apps Script editor menu via View -> StackDriver Logging ).

Once you have StackDriver Logging enabled, you'll need to replace all your Logger.log() calls with console.log().