diff --git a/README.md b/README.md
index 6e1b2a1..03e9a57 100644
--- a/README.md
+++ b/README.md
@@ -102,6 +102,20 @@ $ npm install -g lodash
 $ cat package.json | fx 'require("lodash").keys(this.dependencies)'
 ```
 
+### Formatting
+
+If you need something different then JSON (for example arguments for xargs) do not return anything from reducer.
+Undefined value printed into stderr by default.
+```
+echo '[]' | fx 'void 0'
+undefined
+```
+
+```
+echo '[1,2,3]' | fx 'this.forEach(x => console.log(x))' 2>/dev/null | xargs echo
+1 2 3
+```
+
 ### Other examples
 
 Convert object to array:
diff --git a/index.js b/index.js
index 3c7e39c..0610cc6 100755
--- a/index.js
+++ b/index.js
@@ -38,7 +38,7 @@ async function main() {
   const result = cli.input.reduce(reduce, json)
 
   if (typeof result === 'undefined') {
-    console.log(undefined)
+    process.stderr.write('undefined\n')
   } else {
     const text = JSON.stringify(result, null, 4)
     console.log(highlight(text, {theme}))