javascript while loop giving different results in node and browser

Question

I am executing the following seemingly straightforward code

var number = 0;
while (number <= 12) {
    console.log(number);
    number = number + 2;  
}

and I am getting different results in the browser and in Node. When I run it in the Firebug(v 2.0.4 ) console on Firefox(v. 32.0.3) the result I get is

0 2 4 6 8 10 12 14

which is not`the result I expected.

In Node, the same code gives me the correct answer which is

0 2 4 6 8 10 12

Is there anything I'm missing regarding the behaviour in the browser???

Thanks in advance.

Could it be that you are seeing 14 0 2 4 6 8 10 12? The 14 seems to be kind of the "result" of the loop (I guess because of the assignment in the loop body). Just like 1 + 1 prints 2. This is in FF 32.0.3. I can't tell yoy why FF is doing that though. — Felix Kling
– Felix Kling, Commented Oct 7, 2014 at 19:37
Can't reproduce either, outside of what @FelixKling described. Are you sure you're seeing the 14 on the end? — jmar777
– jmar777, Commented Oct 7, 2014 at 19:38
After the loop the console is printing the value of the last evaluated expression, which is number = number + 2. — JJJ
– JJJ, Commented Oct 7, 2014 at 19:38
You can reproduce by copy-pasting the entire code to the JS console, at least in Chrome. — JJJ
– JJJ, Commented Oct 7, 2014 at 19:38

Jeremy J Starcher · Accepted Answer · 2014-10-07 19:50:48Z

2

If you slightly change your code:

CHROME

var number = 0;
while (number <= 12) {
    console.log("z" + number);
    number = number + 2;  
}

z0
z2
z4
z6
z8
z10
z12
14

You'll see that the 14 is not being printed by the loop. Rather, that is the end value of the expression when the loop finishes running and is printed by the console itself.

FIREFOX

while (number <= 12) {
    console.log("z" + number);
    number = number + 2;  
}

14
"z0"
"z2"
"z4"
"z6"
"z8"
"z10"
"z12"

In Firefox, it runs the entire loop, prints the result and then catches up with the console.

answered Oct 7, 2014 at 19:50

Jeremy J Starcher

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2 Comments

kevgathuku Over a year ago

I have tried it in Firebug and it gives the same results as Chrome. How can I avoid printing out the 14?

Jeremy J Starcher Over a year ago

Running inside the console? You can't. Run it inside of a page or something.

Sebastian Zartner · Accepted Answer · 2016-03-09 19:25:18Z

1

If you run a script in Firebug's console, then it will evaluate the code. So it's evaluating the value of the last number in the while loop (which is now 14) and prints that out. It's actually printing out the value of number 8 times but groups them in 1 print out.

You can see the same effect by just typing in "window" in the command line. It's evaluating it's value and prints it out in the console.

More info can be found in the description to Firebug's command line.

edited Mar 9, 2016 at 19:25

Sebastian Zartner

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answered Oct 8, 2014 at 13:12

kevinius

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