ES6 doesn't need opt-in

John J Barton johnjbarton at
Sat Dec 31 21:12:13 PST 2011

On Sat, Dec 31, 2011 at 7:53 PM, Axel Rauschmayer <axel at> wrote:

> 1. We want sane isObject and isNull predicates, ideally using typeof. Lack
> of them continues to bite people, as the web contains code that wrongly
> assumes typeof x == "object" => won't throw on null x.
> What are the use cases for typeof? Offhand, I see five and for most of
> them, other solutions seem to be better.

In my experience the major use of typeof is to implement dynamic function
overloading.  That is, a function that is flexible in the arguments it
takes, such that one logical name maps to multiple use cases.

> 1. Checking whether a variable has been declared.
>     Problematic: verbose and conflated with checking for a declared
> variable having the value `undefined`.
>     Better: a dedicated operator or predicate for performing this check.

Sorry, I don't think anyone checks if a is variable declared. You just look
at the source code.

> 2. Checking that a value is neither null nor undefined.
>     Problematic: can’t be done via only typeof currently.
>     Better: a predicate for performing this check. This use case will
> become less important with default parameter values.

if(!name) {}

> 3. Distinguishing between objects and primitives.
>     Problematic: Made more difficult by typeof null === "object" and
> typeof function () {} === "function".
>     Better: predicates such as isObject() and isPrimitive()

Mostly we are checking string, array, function or object.  Only Java devs
pass/return |null| so that case is not that big of a deal. Or another way
to look at it: if you use null, it's your own little signal, so enjoy.

Array is the famously difficult case.

> 4. Determining the type of a primitive value.
>     Better: typeof is OK here, but changing it so that typeof null ===
> "null" would help.
> 5. Determining the type of a value (primitive or otherwise).
>     Better: I would want a function, e.g. getTypeName() that works like
> (null-enabled) typeof for primitives and returns the value of the [[Class]]
> property for objects.

This would be great!

> Everything except #1 can be easily implemented as functions (and be
> brought to ES5 via shims). A function such the #5 getTypeName() could take
> care of use case #4, as well.
> Ideas for getTypeName():
> --
> Dr. Axel Rauschmayer
> axel at
> home:
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> blog:
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