undefined being treated as a missing optional argument

Luke Hoban lukeh at microsoft.com
Thu Apr 12 19:31:13 PDT 2012

>> The wiki says that this should fill the array with 'undefined' (I believe the spec draft aims to say the same, but I can't tell where this is established).

> The same runtime semantics routines are used for both default parameter initialization and for initializing array destructurings in let/const declarations (and hence same semantics for places).  For formal parameters it starts in the Runtime Semantics of 13.1 and (in theory) for individual formal parameters of the form (id=expr) will eventually reach Keyed Binding Initialisation for the production singeNameBind : BindingIdentifier Initialiser in 12.2.4.  It's "in theory" because it looks like the current draft is missing a couple intermediate steps in 13.1.

Got it.  That's where I was looking, but didn't see the place where (id=expr) was explicitly handled and passed into Keyed Binding Initialisation.  Sounds like that is the step that is missing in current drafts.

> > 1) Existing uses of default value patterns in JavaSscript use undefined as the sentinel for not present, so a semantics for default values in ES6 that behaves differently will not be usable as a shorthand for any of these existing APIs without changing semantics.

> Undefined is not always such a sentential.  Some examples of built-ins that treat undefined different form a missing parameter: ...

The examples cited are arguably cases where the built-in behaviour is unintuitive to many JavaScript developers, because it doesn't match their expectation with user code functions and most other built-ins.  I don't view any of the cases listed as validation that we would *want* that behaviour by default, just that there are a non-zero number of cases where it exists today.

> I suggest that the appropriate way to think of about the current behavior, in the context of ES6, is that function f(a) {...} is equivalent to function f(a=undefined) {...}
> In other words, there is a default initialization expression, if one is not specified. So, f() and f(undefined) appear to produce the same result.  

This is a good way of explaining the proposed semantics, but...

> But I see why somebody calling a function defined as function(a={ }){...} explicitly as f(undefined) would expect to trigger the default  value initializer.

Right.  This is exactly the sort of thing I'm worried about, and seems like the practical common case for default values.  

>> 2) The fact that JavaScript (at least for user objects) currently doesn't differentiate between missing arguments and undefined arguments is a nice simplifying rule in the language that is easy to understand..

> It does differentiate, at least in regard to the arguments object.  

True.  But this is uncommon enough as to not be something most developers deal with.  Default values aim to be a much more commonly used tool.

> > 3) The example above, of wanting to have an API that allows explicitly passing in undefined to override a default value, seems outside of the common case (out of curiosity - are there any realistic example of this?).  If truly desired, it is easy to not use default values.  But the common case seems more likely to be to emulate what is done today - and avoid having any undefined values flow into the API call.

> Why is the example, outside of common sense.  It is a straightforward function to fill every element of an array with a common value. Undefined is certain something that can be stored in arrays so why wouldn't there be situations where where you would want to pass undefined.  Particularly if the fill function was written by an unreformed Java programmer who used a peculiar default fill value.

The last point was why I considered it outside of the common case.  It seems unusual to intentionally want to fill with null by default, but still allow overriding with an undefined fill.  Not impossible, but I would expect this to be rare enough that I don't mind making it the one case where default values can't be used. 

> I agree that there is some confusion among some JS programmer about the current missing argument default value rules.  However, I don't think what you are suggesting is going to reduce that confusion.  I think it will increase it. 

At the end of the day - I see value in enabling the patterns developers are using today to be refactorable into default values.  I worry that the current proposed semantics are too far away from what is used today to make that practical.

Of course, there is enough inconsistency in what is used currently anyway - so this may be a difficult goal to achieve fully.  But I suspect that treating undefined the same as not present at least keeps things close enough the common forms below could reasonably consider migrating to default values.

// All fairly common..
if(!param) { param = 3; }
if(param == null) { param = 3; }
if(typeof param == 'undefined') { param = 3; }
param = param || 3;
var param = arguments[1] || 3;

// Not sure I've ever seen this... which seems to be the proposed default value semantics
if(arguments.length < f.length) { param = 3; }


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