Check if an array is empty or exists

Question

When the page is loading for the first time  I need to check if there is an image in image array and load the last image   Otherwise  I disable the preview buttons  alert the user to push new image button and create an empty array to put the images   The problem is that image array in the else fires all time  If an array exists - it just overrides it  but alert doesn t work   if image array length  gt  0          images   append   lt img src    image array image array length-1     class  images  id  1    gt     else          prev image   attr  disabled    true            next image   attr  disabled    true        alert  Please get new image        var image array           UPDATE Before loading html  I have something like this    lt  php if count  images     0     gt   lt script type  text javascript  gt       lt  php echo  image array     json encode  images    gt   lt  script gt   lt  php endif    gt

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This is what I use   The first condition covers truthy  which has both null and undefined   Second condition checks for an empty array   if arrayName  amp  amp  arrayName length  gt  0         do something      or thanks to tsemer s comment I added a second version  if arrayName  amp  amp  arrayName length    Then I made a test for the second condition  using Scratchpad in Firefox    x000D   x000D  var array1  x000D  var array2       x000D  var array3     one    two    three    x000D  var array4   null  x000D   x000D  console log array1   x000D  console log array2   x000D  console log array3   x000D  console log array4   x000D   x000D  if  array1  amp  amp  array1 length    x000D    console log  array1  has a value     x000D    x000D   x000D  if  array2  amp  amp  array2 length    x000D    console log  array2  has a value     x000D    x000D   x000D  if  array3  amp  amp  array3 length    x000D    console log  array3  has a value     x000D    x000D   x000D  if  array4  amp  amp  array4 length    x000D    console log  array4  has a value     x000D    x000D   x000D   x000D    which also proves that if array2  amp  amp  array2 length  and if array2  amp  amp  array2 length  gt  0  are exactly doing the same

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The following is my solution wrapped in a function that also throws   errors to manage a couple of problems with object scope and all types   of possible data types passed to the function    Here s my fiddle used to examine this problem  source   var jill    0   var jack     Uncaught ReferenceError  jack is not defined     if  typeof jack      undefined     jack     null      if  jack      if  jack in window      if  window hasOwnP roperty  jack       if  jack in window    function isemptyArray  arraynamed         cam also check argument length   if  arguments length     0         throw  No argument supplied            console log arguments length   number of arguments found      if  typeof arraynamed      undefined   amp  amp  arraynamed     null            console log  found arraynamed has a value          if   arraynamed instanceof Array      true             console log  I m an array            if  arraynamed length     0                  console log   I m empty                return true            else             return false             end length check         else             bad type         throw  Argument is not an array             end type check     else         bad argument     throw  Argument is invalid  check initialization         end argument check    try     console log isemptyArray jill      catch  e        console log   error caught   e

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A simple way that doesn t result in exceptions if not exist and convert to boolean     array   Example   if    arr         array exists

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in ts   isArray obj  any         return Array isArray obj        in html   photos    undefined      isArray photos   amp  amp  photos length   0

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I think this simple code would work   if   image array lt  0      Specify function needed

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To check if an array is either empty or not  A modern way  ES5    if  Array isArray array   amp  amp  array length           array exists and is not empty     An old-school way   typeof array     undefined       amp  amp  array    null      amp  amp  array length    null      amp  amp  array length  gt  0   A compact way   if  typeof array     undefined   amp  amp  array    null  amp  amp  array length    null  amp  amp  array length  gt  0           array exists and is not empty     A CoffeeScript way   if array  length  gt  0     Why   Case Undefined Undefined variable is a variable that you haven t assigned anything to it yet   let array   new Array            array       array  typeof array     undefined        gt  true   Case Null Generally speaking  null is state of lacking a value  For example a variable is null when you missed or failed to retrieve some data   array   searchData        can t find anything array    null               gt  true   Case Not an Array Javascript has a dynamic type system  This means we can t guarantee what type of object a variable holds  There is a chance that we re not talking to an instance of Array   supposedToBeArray    new SomeObject    typeof supposedToBeArray length             gt   undefined   array   new Array    typeof array length                         gt   number    Case Empty Array Now since we tested all other possibilities  we re talking to an instance of Array  In order to make sure it s not empty  we ask about number of elements it s holding  and making sure it has more than zero elements   firstArray       firstArray length  gt  0        gt  false  secondArray    1 2 3   secondArray length  gt  0       gt  true

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How about  ECMA 5 1     if Array isArray image array   amp  amp  image array length        array exists and is not empty

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How about this   checking for length of undefined array may throw exception   if image array     array exists     if image array length         array has length greater than zero

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if  typeof image array      undefined   amp  amp  image array length  gt  0           the array is defined and has at least one element     Your problem may be happening due to a mix of implicit global variables and variable hoisting  Make sure you use var whenever declaring a variable    lt  php echo  var image array     json encode  images    gt     add var      here   And then make sure you never accidently redeclare that variable later   else               image array          no var here

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When you create your image array  it s empty  therefore your image array length is 0  As stated in the comment below  i edit my answer based on this  question s answer     var image array        inside the else brackets doesn t change anything to the image array defined before in the code

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You should do this      if   image array             image array defined but not assigned automatically coerces to false       else if    0 in image array              empty array          doSomething

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Probably your image array is not array but some OBJECT with length property  like string  - try if image array instanceof Array  amp  amp  image array length    x000D   x000D  function test image array      if image array instanceof Array  amp  amp  image array length         console log image array  - it is not empty array        else       console log image array  - it is empty array or not array at all           test  length 5    test  undefined    test      test   abc     x000D   x000D   x000D

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I come across this issue quite a lot in Javascript  For me the best way to do it is to put a very broad check before checking for length  I saw some other solutions in this Q amp A  but I wanted to be able to check for either null or undefined or any other false value   if  array    array length    0       console log  Array is either empty or does not exist       This will first check for undefined  null  or other false values  If any of those are true  it will complete the boolean as this is an OR  Then the more risky check of array length  which could error us if array is undefined  can be checked  This will never be reached if array is undefined or null  so the ordering of conditions is very important

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If you do not have a variable declared as array you can create a check   if x  amp  amp  x constructor  Array  amp  amp  x length      console log  is array and filed     else      var x          console log  x   empty array        This checks if variable x exists and if it is  checks if it is a filled array  else it creates an empty array  or you can do other stuff    If you are certain there is an array variable created there is a simple check   var x        if  x length       console log  empty      else       console log  full        You can check my fiddle here with shows most possible ways to check array

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You can use jQuery s isEmptyObject   to check whether the array contains elements or not   var testArray  1 2 3 4 5    var testArray1     console log jQuery isEmptyObject testArray      false console log jQuery isEmptyObject testArray1      true    Source  https   api jquery com jQuery isEmptyObject

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The best is to check like      let someArray  string             let hasAny1  boolean     someArray  amp  amp    someArray length      let hasAny2  boolean     someArray  amp  amp  someArray length  gt  0    or like this     console log  quot And now on empty       quot   hasAny1  hasAny2    See full samples list

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optional chaining  As optional chaining proposal reached stage 4 and is getting wider support  there is a very elegant way to do this  if image array  length         image array is defined and has at least one element

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If you want to test whether the image array variable had been defined you can do it like this  if typeof image array      undefined            it is not defined yet   else if  image array length  gt  0           you have a greater than zero length array

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JavaScript    typeof myArray       undefined   amp  amp  Array isArray myArray   amp  amp  myArray length  gt  0     Lodash  amp  Underscore      isArray myArray   amp  amp  myArray length  gt  0

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For me sure some of the high rated answers  work  when I put them into jsfiddle  but when I have a dynamically generated amount of array list a lot of this code in the answers just doesn t work for ME   This is what IS working for me   var from        if typeof from 0      undefined                Notice   NO quotes around undefined and I m not bothering with the length

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You should use      if  image array     undefined  amp  amp  image array length  gt  0

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Using undescore or lodash     isArray image array   amp  amp     isEmpty image array

[javascript] Check if an array is empty or exists

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