Double dot syntax in JavaScript - What is this expression doing and how is it valid syntax?

Question

    14.toString();
    // Result -> SyntaxError: Unexpected token ILLEGAL

    14..toString();
    // Result -> "14"

What is placing an extra dot after the number doing, and how is this valid syntax?

This Question has answer in http://stackoverflow.com/a/4211093/1787416 — Anto king, Jul 22 '14 at 15:22
Related: [Why 42.toString() fails in JS?](http://stackoverflow.com/questions/24398347/why-42-tostring-fails-in-js) — Jonathan Lonowski, Jul 22 '14 at 15:22
@AntoKing That question has *nothing* to do with this one... — Niet the Dark Absol, Jul 22 '14 at 15:23
@NiettheDarkAbsol Who unmarked this as a duplicate? This question is a direct duplicate of the one I posted NOT the one AntoKing posted... — PW Kad, Jul 22 '14 at 15:26
@NiettheDarkAbsol You are high enough rep to see who did it though. This is a clear duplicate — PW Kad, Jul 22 '14 at 15:28
@PWKad [Quentin did](http://i.imgur.com/AZ8PkJ9.png), presumably to let it be re-closed as a dupe of the right thing. — Niet the Dark Absol, Jul 22 '14 at 15:30
I looked at the questions it had been marked as a duplicate of, one of them looked related, but not actually a duplicate. — Quentin, Jul 22 '14 at 15:48

score 4 · Answer 1 · answered Jul 22 '14 at 15:22

4

14. is a Number. .toString() calls a method on that Number.

Thus 14..toString() is the same as 14.0.toString().

You couldn't have 14.toString() because the . is still the floating point and not the property accessing symbol.

answered Jul 22 '14 at 15:22

Quentin

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nitpick: `14.` is a `number`, not a `Number`; `.toString` autoboxes it – blgt Jul 22 '14 at 15:42

score 3 · Answer 2 · edited May 23 '17 at 10:30

3

It is important to remember that the parser is greedy.

It sees the 1, so it starts reading a number. 4 is valid in a number, . is valid in a number, t is not, so it stops.

So it has the number 14. (which is just 14). Now what to do with it? Uh... there's a t there, that's not valid, ERROR!

In the second case, . is valid in a number, . would be valid but we already have a dot so stop there.

We have 14. again, but this time when looking what to do it sees ., so it converts the 14. to a Number object, then calls toString() on it, result "14"

See also: Why does "a + + b" work, but "a++b" doesn't?

edited May 23 '17 at 10:30

Community

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answered Jul 22 '14 at 15:22

Niet the Dark Absol

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Of course, you should never be seeing this code "in real life". Just write `"14"` if it's a string that you want. – Niet the Dark Absol Jul 22 '14 at 15:25
Very nice explanation. – T.J. Crowder Jul 22 '14 at 15:25
I'm thinking pretty much the only valid use case would be calling `toLocaleString`: `1400000000..toLocaleString()` in my locale is `"1,400,000,000"` but for some other locales it would be `"1.400.000.000"`. – T.J. Crowder Jul 22 '14 at 15:39

Double dot syntax in JavaScript - What is this expression doing and how is it valid syntax?

2 Answers2