What does ‘::’ (double colon) do in JavaScript?

Question

The documentation of some JavaScript APIs shows the following snippets as an example of how to invoke some function:

<button type="button" onClick="foo.DoIt(72930)">Click</button>

<button type="button" onClick="foo.DoIt(42342::37438)">Click</button>

:: is obviously used here to allow either one or two arguments to be passed to the function.

What does :: do in JavaScript?

And how does the function know if one or two values were passed? How does it read them?

On closer look, the examples show other weird stuff like

<button type="button" onClick="foo.Bar(72//893)">Click</button>

<button type="button" onClick="foo.Qux(425;1,34::)">Click</button>

At least the // looks just wrong.

So I guess it's not some fancy new syntax that I'm not aware of, but maybe the examples are just missing quotes around a single string argument.

AFAIK, that's a typo. The double-colon in that use makes no sense. — Robert K, Oct 05 '09 at 14:19
can you tell us more about the javascript API you are using.. — Xinus, Oct 05 '09 at 14:24
Sorry, the API and it's documentation are not publicly available, so I can't provide a link. — dtb, Oct 05 '09 at 14:33
see also [JavaScript double colon (bind operator)](http://stackoverflow.com/questions/31220078/javascript-double-colon-bind-operator) — Aprillion, Aug 27 '16 at 12:49

score 282 · Answer 1 · edited Mar 31 '18 at 16:00

282

It was certainly not the case at the time of your question, but right now :: is a valid ES7 operator. It's actually a shortcut for bind.

::foo.bar

is equivalent to

foo.bar.bind(foo)

See an explanation here for examples:

edited Mar 31 '18 at 16:00

Zearin

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answered Jul 02 '15 at 09:32

Brann

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If I'm reading that correctly, then it isn't valid in the context given in the question. The LHS and RHS of the `::` in the question are Numbers. – Quentin Jul 27 '15 at 07:49
58

This is the answer I was looking for when I Googled for what :: means though! Thanks – poshaughnessy Jul 30 '15 at 13:46
2

This answer has nothing to do with the question, because (1) ES7, which is published in June 2016, doesn't contain `::` operator; (2) `::` operator is only a stage 0 proposal currently, which is highly probable of being changed or removed; (3) In the context of the question, it is not a valid `::` operator syntax, and even the correct syntax will not work in any current major browsers. – rhgb Nov 14 '16 at 08:44
6

The question title doesn't match the question body, although, this answer suits the title question, in my case what I was looking for. – andrepaulo Jan 13 '17 at 13:56

score 15 · Accepted Answer · answered Oct 05 '09 at 14:18

15

Nothing. It is a syntax error.

>>> alert(42342::37438)
SyntaxError: missing ) after argument list

answered Oct 05 '09 at 14:18

Quentin

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Yep, unless for some ungodly reason said developer is actually parsing the onclick attribute value with `document.getElementsByTagName('button')[0].getAttribute('onclick')` and extracting that information, that is an invalid ECMAScript expression. – meder omuraliev Oct 05 '09 at 14:30
1

@Brann — It doesn't appear to be valid in the context given in the question. – Quentin Jul 03 '16 at 15:04
@Pawel — No it isn't. See my previous comment. See the context of the question. – Quentin Jul 14 '17 at 14:10

Joel Coehoorn · Answer 3 · 2017-05-18T19:28:30.063

7

:: has nothing to do with the number of parameters. You can do that already in JavaScript with a normal comma:

function SomeFunction(param1, param2) {
   //...
}

SomeFunction('oneParam'); // Perfectly legal

Also, based on Tzury Bar Yochay's answer, are you sure you're not looking at something like the following?

$('this::is all one::parameter'); // jQuery selector

edited May 18 '17 at 19:28

answered Oct 05 '09 at 14:21

Joel Coehoorn

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I guess you're right and it's missing the quotes. I will submit a bug report to the author :) – dtb Oct 05 '09 at 14:26

score 2 · Answer 4 · answered Oct 05 '09 at 14:19

2

In which example did you see that? So far, JavaScript does not have a double colon operator!

The double colon replaced the single-colon selectors for pseudo-elements in CSS3 to make an explicit distinction between pseudo-classes and pseudo-elements. But that is CSS3, not JavaScript! Not At ALL!

answered Oct 05 '09 at 14:19

Tzury Bar Yochay

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Given this, it sounds like perhaps he's looking at a jquery selector and omitted the quotes around the parameter. – Joel Coehoorn Oct 05 '09 at 14:23
probably. in any case, this question drew far more attention than I thought it would. – Tzury Bar Yochay Oct 05 '09 at 14:48

score 1 · Answer 5 · answered Jan 28 '13 at 18:20

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It could be using ECMAScript for XML (ECMA-357 standard) which would imply the double quotes are a XPath operator.

See ECMAScript for XML

answered Jan 28 '13 at 18:20

Jeff Lewis

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score 1 · Answer 6 · answered Oct 05 '09 at 14:26

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It must be a typo for

<button type="button" onClick="foo.DoIt('72930')">Click</button>

<button type="button" onClick="foo.DoIt('42342::37438')">Click</button>

answered Oct 05 '09 at 14:26

Xinus

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score 0 · Answer 7 · edited May 18 '17 at 17:46

0

Perhaps it's a typo, and the whole thing is expected to be in quotes.

edited May 18 '17 at 17:46

Peter Mortensen

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answered Oct 05 '09 at 14:19

Chris Kooken

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score 0 · Answer 8 · answered Oct 05 '09 at 14:21

0

I am guessing that the parameter list for foo.DoIt() is generated by code, and one the values was empty.

answered Oct 05 '09 at 14:21

D'Arcy Rittich

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What does ‘::’ (double colon) do in JavaScript?

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