Count of Defined Array Elements

Question

Given following array   var arr    undefined  undefined  2  5  undefined  undefined     I d like to get the count of elements which are defined  i e   those which are not undefined    Other than looping through the array  is there a good way to do this

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If the undefined s are implicit then you can do   var len   0  for  var i in arr    len        undefined s are implicit if you don t set them explicitly    both are a 0  and a 3  are explicit undefined var arr    undefined  1  2  undefined    arr 6    3    now arr 4  and arr 5  are implicit undefined  delete arr 1    now arr 1  is implicit undefined  arr 2    undefined   now arr 2  is explicit undefined

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An array length is not the number of elements in a array  it is the highest index   1  length property will report correct element count only if there are valid elements in consecutive indices    var a       a 23     foo   a length      24   Saying that  there is no way to exclude undefined elements from count without using any form of a loop

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Unfortunately  No  You will you have to go through a loop and count them   EDIT    var arrLength   arr filter Number   alert arrLength

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Loop and count in all browsers   var cnt   0  for  var i   0  i  lt  arr length  i          if  arr i      undefined              cnt            In modern browsers   var cnt   0  arr foreach function val        if  val     undefined      cnt

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Remove the values then check  remove null check here if you want   const x   A filter item   gt  item     undefined    item     null  length   With Lodash  const x     size   filter A  item   gt     isNil item

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In recent browser  you can use filter  var size   arr filter function value    return value     undefined    length   console log size     Another method  if the browser supports indexOf for arrays   var size   arr slice 0  sort   indexOf undefined     If for absurd you have one-digit-only elements in the array  you could use that dirty trick   console log arr join     length     There are several methods you can use  but at the end we have to see if it s really worthy doing these instead of a loop

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No  the only way to know how many elements are not undefined is to loop through and count them  That doesn t mean you have to write the loop  though  just that something  somewhere has to do it   See  3 below for why I added that caveat    How you loop through and count them is up to you  There are lots of ways    A standard for loop from 0 to arr length - 1  inclusive   A for  in loop provided you take correct safeguards  Any of several of the new array features from ECMAScript5  provided you re using a JavaScript engine that supports them  or you ve included an ES5 shim  as they re all shim-able   like some  filter  or reduce  passing in an appropriate function  This is handy not only because you don t have to explicitly write the loop  but because using these features gives the JavaScript engine the opportunity to optimize the loop it does internally in various ways   Whether it actually does will vary on the engine        but it all amounts to looping  either explicitly or  in the case of the new array features  implicitly

[javascript] Count of "Defined" Array Elements

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