Why does this JavaScript code print "undefined" on the console?

Question

I have the following JavaScript code:

var counter = 0;
function printCounter(){
   console.log("counter=" + ++counter);
   setTimeout(printCounter, 1000);
}
printCounter();

I expect that it should print this output:

counter=1
counter=2
counter=3
...

But instead it prints following:

counter=1
undefined  // <-- Notice this "undefined"
counter=2
counter=3
...

Why does it print "undefined" after the first iteration?

Important: I see such behavior only when the code executed in the JavaScript console. If it's the part of a page, it works fine.

The real canonical question is probably *[Chrome/Firefox console.log always appends a line saying 'undefined'](https://stackoverflow.com/questions/14633968/chrome-firefox-console-log-always-appends-a-line-saying-undefined)* — Peter Mortensen, Oct 20 '20 at 17:22

score 9 · Accepted Answer · edited Oct 20 '20 at 17:03

9

It's because the "printCounter()" function itself returns undefined. That's the console telling you the result of the expression.

Change "printCounter()" by adding return "Hello Anton!"; to the end :-)

It's a little confusing to say it "returns undefined"; really, it has no explicit return, but it's the same effect.

edited Oct 20 '20 at 17:03

Peter Mortensen

answered Apr 21 '12 at 21:34

Pointy

2

...and so the `undefined` is the result of *the REPL evaluation* but since the *asynchronous* timer is running it will continue to call `console.log`... – Apr 21 '12 at 21:36
Yes, thank you, you are right. I was confused because it's not the case for all browsers. – Anton Moiseev Apr 21 '12 at 21:40

1 Answers1