typeof null

Allen Wirfs-Brock allen at wirfs-brock.com
Thu May 10 07:51:32 PDT 2012

On May 10, 2012, at 1:05 AM, Brandon Benvie wrote:

> On this tangent, I personally almost always mean "isArrayish" in this very common situation. That is, indexed with a length such that almost all Array.prototype methods would work on it out of the box. The number of arrayish interfaces provided by host environments has continued to grow. It's the epitome of generic, easily extended, usable, and efficient interfaces. Sometimes duck typing goes from "walks like a duck" to "is a vertebrate and is a bird and can fly and is found in sub-tropical areas and walks like a duck and quacks like a duck and can mate with ducks (offspring optional)".
> The following is the shortest check I can think of to accomplish this goal accurately. It isn't guaranteed to work with sparse collections but that are rare aside from built-in arrays themselves.
> function isIndexed(o){
>   return Boolean(o) && hasOwn(o, 'length') && hasOwn(o, o.length - 1);
> }

Note that the "generic" Array.prototype methods get away without have to do a "isArrayish" classification test. In general, the Array.prototype methods are specified such that all objects appear "arrayish" to them.  They do this by accessing the "length" property and doing a ToUint32 conversions on the result.  ToUint32 produces 0 for values that can't be converted into integers such as the undefined that was returned if the "length" didn't exist.  So an object without a length property is  considered to have a length of 0. The algorithms all iterate over indices in the  interval 0 to length-1, so no "array elements" get processed for such objects.


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