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' — Peter Mortensen
– Peter Mortensen, Commented Oct 20, 2020 at 17:22

Peter Mortensen · Accepted Answer · 2020-10-20 17:03:53Z

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, 2020 at 17:03

Peter Mortensen

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answered Apr 21, 2012 at 21:34

Pointy

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2 Comments

user166390 Over a year ago

...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...

Anton Moiseev Over a year ago

Yes, thank you, you are right. I was confused because it's not the case for all browsers.

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