Why this piece of code prints `undefined` at the end?

Question

While I am running the following piece of code in Chrome's developer tools javascript console,

var solution = 1;
for(var i = 1; i <= 12; ++i)
    solution *= i;
console.log(solution.toString());

I get the following output:

479001600
undefined

When I remove the last line, that is console.log(solution.toString());, I get just 479001600. What happens?

That is because the code you run does not return anything.... there has to be a dupe on this. — epascarello, Jul 19 '17 at 16:53
The console records the last returned value of whatever you typed. `console.log` "returns" `undefined` so that's what you see. Without it, the last `solution *= i` is recorded, showing the result you wanted. — Niet the Dark Absol, Jul 19 '17 at 16:53
@NiettheDarkAbsol That's exactly the case. Thanks for your help. — Ali Shakiba, Jul 19 '17 at 16:55
When working directly in the console, there's no need to use `console` to see the variable's value — j08691, Jul 19 '17 at 16:56
Wow..... thought someone would have looked for a dupe instead of an easy answer... — epascarello, Jul 19 '17 at 17:00
@epascarello Thanks. How can one disable this action of console, that is disabling the printing of the last returned value? — Ali Shakiba, Jul 19 '17 at 17:02
This is "baked-in" to, not only Chrome, but most other developer tool's as well. The feature is not user configurable. Why do you want to do that anyway? — Scott Marcus, Jul 19 '17 at 17:05

score 5 · Accepted Answer · answered Jul 19 '17 at 16:53

The undefined is simply the console's way of telling you that the statement:

console.log(solution.toString());

doesn't, itself, return a value (it outputs a value, but doesn't return one). It's not something to worry about when your actual code executes because it's a specific behavior to the developer's console.

As another example, if you type: 5 + 6 into the console, it will report 11 on the next console line, because the console always wants to report any value returned from the statement it just executed.

score 1 · Answer 2 · answered Jul 19 '17 at 16:57

The console in google chrome will always return the value of the last executed statement.

Since your last statement in the console was

console.log()

one which did not return any value it showed 'undefined';

Try

var a=[];a.push(5);

It will show 1 on the next line as the push method returns the length of the newly formed array

Why this piece of code prints `undefined` at the end?

2 Answers2