Further discussion of this topic on es-discuss has convinced its original proposer, MarkM, that there is no coherent general convention for isArrayLike that can be rescued and reconciled with existing practice. Accordingly, I have stopped editing this page. If someone does believe that attempting a coherent proposal would still be productive, they should please edit this page accordingly. —MarkM 1/9/2010 10:48am PDT

isArrayLike(obj :any) :boolean

This es-discuss thread documents a variety of conventions currently in use to determine if an object should be array-like. The following proposed predicate has the virtues that it

  • admits native arrays, arguments objects, and (perhaps inappropriately) String wrapper objects,
  • admits those host objects that are typically considered array-like by other legacy array-like tests (is this true?),
  • does not admit any other legacy ES3R non-host objects (to prevent false positives),
  • allows ES5 programmers to easily define new array-like non-array objects,
  • takes bounded-by-constant time (i.e., no iteration) under the assumption that inheritance chains have bounded depth,
  • is a reasonably compatible compromise with the existing notions of array-like in legacy libraries as represented by the thread cited above.
function isArrayLike(obj) {
  return !!(obj &&
            typeof obj === 'object' && 
            'length' in obj && 
            !({}.propertyIsEnumerable.call(obj, 'length'));


  • It may be better for the above predicate to reject String wrapper objects, such as by adding a
({}).toString.call(obj) !== '[object String]'


  • Although the above code cannot cause side effects on ES3R or ES5 when obj is a non-host object, it can cause side effects when obj is a proxy. This is necessary to allow proxies to emulate array-like objects adequately to pass the above predicate.
  • Unfortunately, the propertyIsEnumerable test above, despite the name, actually tests whether a property is both enumerable and own. This means the above predicate will reject objects that inherit a non-enumerable length property. I can think of no reliable and portable ES3R test which tests only property enumerability, except by running a for-in loop and testing the output. Doing so would kill the bounded-by-constant time virtue.
conventions/isarraylike.txt · Last modified: 2011/09/17 22:46 by markm
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