syntax - What does ‘::’ (double colon) do in JavaScript? - Stack Overflow

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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.

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.

• javascript
• syntax

Share Improve this question Follow edited May 18, 2017 at 17:45 Peter Mortensen 31.6k2222 gold badges109109 silver badges133133 bronze badges asked Oct 5, 2009 at 14:14 dtbdtb 217k3737 gold badges411411 silver badges437437 bronze badges 4

• 1 AFAIK, that's a typo. The double-colon in that use makes no sense. – Robert K Commented Oct 5, 2009 at 14:19
• can you tell us more about the javascript API you are using.. – Xinus Commented Oct 5, 2009 at 14:24
• Sorry, the API and it's documentation are not publicly available, so I can't provide a link. – dtb Commented Oct 5, 2009 at 14:33
• see also JavaScript double colon (bind operator) – Aprillion Commented Aug 27, 2016 at 12:49

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8 Answers 8

Sorted by: Reset to default 284

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:

Nothing. It is a syntax error.

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

:: 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

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!

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>

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

See ECMAScript for XML

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

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

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