Do function arguments support a default value in JavaScript?

Question

While looking at a question related to porting a PHP function to JavaScript. I saw what I assumed was incorrect JavaScript:

function my_isnum(str, negative=false, decimal=false)

Then I tried this in JSFiddle:

function my_isnum(str, negative=false, decimal=-2)
{
    console.log(arguments);
    console.log(str);
    console.log(negative);
    console.log(decimal);
}
my_isnum("5", "Hi");

And to my utter amazement this is what I see in the Firebug console:

["5", "Hi"]
5
Hi
-2

Now in Chrome this is what I see:

Uncaught SyntaxError: Unexpected token = 

What I don't understand is this an example of some early standard being supported by Firefox (the MDN on function doesn't seem to mention this)?

Answer 1

This seems to be in the ECMAScript 6 specification and is at the moment only supported by Firefox

https://developer.mozilla.org/en-US/docs/JavaScript/Reference/default_parameters

Answer 2

Lostsource's answer is correct, come ECMA6, default values are very likely to be supported, I think it will, but as it is still a working draft, you can't really be sure... For now, you can use the logical or:

function foo(bar, foobar)
{
    bar = bar || 'foobar';//once
    foobar = foobar || !!bar || true;//or multiple times

This works, sort of, like a ternary. The expressions are resolved, from left to right: as soon as JS encounters a truthy value, that's what will be assigned:

var someVar = false || undefined || null || 0 || 1;

Will assign 1 to someVar. If no values are passed to a function, all arguments are assigned undefined by default, so in that case:

myArgument = myArgument || 'default';
//is the same as:
myArgument = undefined || 'default';//evaluates to "default"

But when you pass false as an argument, or null, or an empty string, the default value will be assigned, so be careful.
In those cases an if/ternary is a better fit (as seen in theJollySin's answer). The ternary equivalent of which is:

some_val = typeof some_val === 'undefined' ? 'default' : some_val;