Add option to allow logging to standard error

Sometimes you'd like to be able to influence the log lines and send some
logs to standard error. This option add the property
`LOG_TO_STANDARD_ERROR` which will send any failing status codes to
standard error.

README.md

@@ -156,6 +156,16 @@ You can disable new lines in the log output by setting the environment variable
 docker run -e LOG_WITHOUT_NEWLINE=true -p 8080:8080 -p 8443:8443 --rm -t mendhak/http-https-echo:31
 ```
 
+## Log to standard error
+
+By default the log output is always sent to standard out. If you want to vary the log output format you can set `LOG_TO_STANDARD_ERROR` which combined with `x-set-response-status-code` will log anything with a status code between 400 and 599, to standard error.
+
+```bash
+docker run -e LOG_TO_STANDARD_ERROR=true -p 8080:8080 -p 8443:8443 --rm -t mendhak/http-https-echo:31
+
+curl -v -H "x-set-response-status-code: 401" http://localhost:8080/
+```
+
 ## Send an empty response
 
 You can disable the JSON output in the response by setting the environment variable `ECHO_BACK_TO_CLIENT`. For example,

index.js

@@ -131,11 +131,17 @@ app.all('*', (req, res) => {
     if (process.env.LOG_IGNORE_PATH != req.path) {
 
       let spacer = 4;
-      if(process.env.LOG_WITHOUT_NEWLINE){
+      if(process.env.LOG_WITHOUT_NEWLINE) {
         spacer = null;
       }
 
-      console.log(JSON.stringify(echo, null, spacer));
+      //Allow logging to standard error if the status code is a failure
+      let loggerFunction = console.log;
+      if (process.env.LOG_TO_STANDARD_ERROR === "true" && setResponseStatusCode >= 400 && setResponseStatusCode < 600) {
+        loggerFunction = console.error;
+      }
+
+      loggerFunction(JSON.stringify(echo, null, spacer));
     }
   });