Why does this JavaScript code print “undefined” on the console?

问题

I have 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 it prints \"undefined\" after first iteration? Important: I see such behavior only when the code executed in JavaScript console. If it\'s the part of a page, it works fine.

回答1:

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 :-)

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

来源：https://stackoverflow.com/questions/10263190/why-does-this-javascript-code-print-undefined-on-the-console