Firebug在其命令行中执行代码时将打印一个附加数字

Question

I was executing JavaScript within Firebug's console. While running the below loop I have noticed it's printing one additional number:

var i = 0; 

while(i<10) {
  console.log(i);
  i++;
}

It's providing the following output:

var i = 0;   while(i<10) {   console.log(i);   i++; } 
0
1
2
3
4
5
6
7
8
9
9

Is this a bug or something else?

Answer 1

The last 9 , which is causing disturbance, is the return value of console.log() . Try alert(i) instead of console.log(i) .

Answer 2

The Console panel outputs the return values of scripts executed through the Command Line or the Command Editor . Unfortunately Firebug does not visually differentiate the return value from the output created through console.log() . So the second 9 at the end of the output is the return value.
Within the Firefox built-in DevTools this is more obvious:

Note that the output of the return value cannot be suppressed.