syntax - 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.

Answer 1

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: