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)?

Firefox's JavaScript is a superset of ECMAScript 3. It has features that are not standard syntax. Included among those are array comprehensions, let expressions, and destructuring assignment. — the system
– the system, Commented Feb 2, 2013 at 0:12

lostsource · Accepted Answer · 2013-02-02 00:12:40Z

18

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

edited Feb 2, 2013 at 0:12

answered Feb 2, 2013 at 0:08

lostsource

22k9 gold badges71 silver badges90 bronze badges

Sign up to request clarification or add additional context in comments.

3 Comments

Jason Sperske Over a year ago

Ahhh! I knew it. Thanks for the link :)

matt Over a year ago

Darn, my O'Reilly Javascript: The Definitive Guide is out of date and I didn't know it.

Elias Van Ootegem Over a year ago

You can turn it on in chrome, too, by enabling experimental features, just open a tab and type chrome://flags/ in the address bar... The implementation may differ from mozilla's, just like the ECMA6 new var aka let

Elias Van Ootegem · Accepted Answer · 2013-02-02 00:28:05Z

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;

john_science · Accepted Answer · 2013-02-02 00:07:00Z

4

Sort of. You can do something like:

function the_func(some_val) {
    // if some_val is not passed to the_func 
    if (typeof some_val == 'undefined') {
        some_val = 'default_some_val';
    }

    // now you can use some_val in the remaining parts of the method
}

answered Feb 2, 2013 at 0:07

john_science

6,5976 gold badges45 silver badges62 bronze badges

4 Comments

Popnoodles Over a year ago

maybe with two fewer lines though.

Jason Sperske Over a year ago

And this is how 100% of my code and every other argument checking code I have ever seen in JavaScript looks.

javaBean007 Over a year ago

Can do something like: some_val = (typeof some_val !== 'undefined') ? some_val : 'defaultVal';

john_science Over a year ago

@javaBean007 Correct, there are shorter ways to do this. But it is a beginner question, so I opted for the most descriptive solution that is the easiest to read.

Collectives™ on Stack Overflow

Do function arguments support a default value in JavaScript?

3 Answers 3

3 Comments

Comments

4 Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

3 Comments

Comments

4 Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related