Operator // in JavaScript : 5.0 // 2.0 = 5?

Question

Is there a // operator in JavaScript?

Because in Python we have:

5 // 2.0 # => 2.0
5 / 2.0  # => 2.5

So I tried in JavaScript:

5.0//2.0

and I got 5! What's going on there?

I read that there is no such a thing as a // operator in JavaScript. In this case, why didn't I get an exception or, better, an error from the lexer?

I used this line:

document.write(eval("5.0//2.0"));

In Firefox 3.6.23.

As you might have seen by looking at the syntax highlighting, `// ..` are the markers of a comment in JavaScript. Also, if you need tips for your English, visit http://english.stackexchange.com/ — Rob W, Nov 02 '11 at 17:48
This is the first time I've seen a question that is answered by its syntax highlighting. — SLaks, Nov 02 '11 at 17:49
See [this question](http://stackoverflow.com/questions/4228356/integer-division-in-javascript) for integer division in JavaScript — Alex Turpin, Nov 02 '11 at 17:54
I think Yksin was ashamed of coming back to SO after he saw what his mistake was. @Yksin: come back, choose an answer, we all make silly mistakes — Juan Mendes, Nov 02 '11 at 18:03

score 7 · Answer 1 · edited Mar 16 '12 at 13:42

7

// is a comment in javascript.

Try:

   5 / 2; //yields 2.5
   Math.floor(5/2); //yields 2

Also do not use eval.

Just do document.write(5/2);

edited Mar 16 '12 at 13:42

Antony Hatchkins

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answered Nov 02 '11 at 17:48

Naftali

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+1, only answer that mentions what `//` does and how to do integer division. – Alex Turpin Nov 02 '11 at 17:55
what are extra parentheses for in the last line? – Antony Hatchkins Mar 12 '12 at 16:27
@AntonyHatchkins whatever are you talking about? – Naftali Mar 12 '12 at 17:18
@AntonyHatchkins that is perfectly fine ^_^ – Naftali Mar 16 '12 at 13:00
I've fixed it. Syntax of javascript is too clumsy in my opinion (eg `+a` to convert string to number) to use excess symbols in simple answers. – Antony Hatchkins Mar 16 '12 at 13:48
@AntonyHatchkins ahh that was accidental. Thanks for the fix ^_^ – Naftali Mar 16 '12 at 15:25

score 2 · Answer 2 · answered Nov 02 '11 at 17:47

2

In JavaScript, // is not an operator, it denotes a comment.

answered Nov 02 '11 at 17:47

John Hartsock

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score 1 · Answer 3 · answered Nov 02 '11 at 17:47

1

// is used for commenting in JavaScript.

answered Nov 02 '11 at 17:47

Fatih Acet

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score 0 · Answer 4 · answered Jun 16 '13 at 06:50

// starts a comment. To do integer division, first perform a regular division using / and then round it. This can be done with &-1, ^0, |0 or ~~, in order from fast to slow as measured on my laptop. There is a measurable difference between the first three but it's small. The last one is really slow in comparison.

Putting it all together, 5/2&-1 will yield 2. It rounds towards zero.

Operator // in JavaScript : 5.0 // 2.0 = 5?

4 Answers4