Representing null in JSON

Question

What is the preferred method for returning null values in JSON  Is there a different preference for primitives  For example  if my object on the server has an Integer called  quot myCount quot  with no value  the most correct JSON for that value would be      or        quot myCount quot   null    or        quot myCount quot   0    Same question for Strings - if I have a null string  quot myString quot  on the server  is the best JSON      or        quot myString quot   null    or        quot myString quot    quot  quot     or  lord help me         quot myString quot    quot null quot     I like the convention for collections to be represented in the JSON as an empty collection http   jtechies blogspot nl 2012 07 item-43-return-empty-arrays-or html An empty Array would be represented         quot myArray quot         EDIT Summary The  personal preference  argument seems realistic  but short sighted in that  as a community we will be consuming an ever greater number of disparate services sources  Conventions for JSON structure would help normalize consumption and reuse of said services  As far as establishing a standard  I would suggest adopting most of the Jackson conventions with a few exceptions   Objects are preferred over primitives  Empty collections are preferred over null  Objects with no value are represented as null  Primitives return their value   If you are returning a JSON object with mostly null values  you may have a candidate for refactoring into multiple services         quot value1 quot   null       quot value2 quot   null       quot text1 quot   null       quot text2 quot    quot hello quot        quot intValue quot   0    use primitive only if you are absolutely sure the answer is 0      quot myList quot            quot myEmptyList quot   null    NOT BEST PRACTICE - return    instead      quot boolean1 quot   null    use primitive only if you are absolutely sure the answer is true false      quot littleboolean quot   false    The above JSON was generated from the following Java class  package jackson      import java util ArrayList  import java util List      import com fasterxml jackson databind ObjectMapper       public class JacksonApp           public static class Data               public Integer value1              public Integer value2          public String text1          public String text2    quot hello quot           public int intValue          public List lt Object gt  myList   new ArrayList lt Object gt             public List lt Object gt  myEmptyList          public Boolean boolean1          public boolean littleboolean                  public static void main String   args  throws Exception          ObjectMapper mapper   new ObjectMapper           System out println mapper writeValueAsString new Data              Maven dependency   lt dependency gt       lt groupId gt com fasterxml jackson core lt  groupId gt       lt artifactId gt jackson-core lt  artifactId gt       lt version gt 2 3 0 lt  version gt   lt  dependency gt

User · Answer

I would use null to show that there is no value for that particular key   For example  use null to represent that  number of devices in your household connects to internet  is unknown   On the other hand  use    if that particular key is not applicable   For example  you should not show a count  even if null  to the question  number of cars that has active internet connection  is asked to someone who does not own any cars   I would avoid defaulting any value unless that default makes sense   While you may decide to use null to represent no value  certainly never use  null  to do so

User · Answer

null is not zero  It is not a value  per se  it is a value outside the domain of the variable indicating missing or unknown data   There is only one way to represent null in JSON  Per the specs  RFC 4627 and json org     2 1   Values  A JSON value MUST be an object  array  number  or string  or one of the following three literal names     false null true

User · Answer

This is a personal and situational choice  The important thing to remember is that the empty string and the number zero are conceptually distinct from null    In the case of a count you probably always want some valid number  unless the count is unknown or undefined   but in the case of strings  who knows  The empty string could mean something in your application  Or maybe it doesn t  That s up to you to decide

User · Answer

Let s evaluate the parsing of each  http   jsfiddle net brandonscript Y2dGv  var json1         var json2      quot myCount quot   null    var json3      quot myCount quot   0    var json4      quot myString quot    quot  quot     var json5      quot myString quot    quot null quot     var json6      quot myArray quot          console log JSON parse json1          console log JSON parse json2        myCount  null  console log JSON parse json3        myCount  0  console log JSON parse json4        myString   quot  quot   console log JSON parse json5        myString   quot null quot   console log JSON parse json6        myArray         The tl dr here  The fragment in the json2 variable is the way the JSON spec indicates null should be represented  But as always  it depends on what you re doing -- sometimes the  quot right quot  way to do it doesn t always work for your situation  Use your judgement and make an informed decision    JSON1    This returns an empty object  There is no data there  and it s only going to tell you that whatever key you re looking for  be it myCount or something else  is of type undefined   JSON2   quot myCount quot   null  In this case  myCount is actually defined  albeit its value is null  This is not the same as both  quot not undefined and not null quot   and if you were testing for one condition or the other  this might succeed whereas JSON1 would fail  This is the definitive way to represent null per the JSON spec   JSON3   quot myCount quot   0  In this case  myCount is 0  That s not the same as null  and it s not the same as false  If your conditional statement evaluates myCount  gt  0  then this might be worthwhile to have  Moreover  if you re running calculations based on the value here  0 could be useful  If you re trying to test for null however  this is actually not going to work at all   JSON4   quot myString quot    quot  quot   In this case  you re getting an empty string  Again  as with JSON2  it s defined  but it s empty  You could test for if  obj myString     quot  quot   but you could not test for null or undefined   JSON5   quot myString quot    quot null quot   This is probably going to get you in trouble  because you re setting the string value to null  in this case  obj myString     quot null quot  however it is not    null   JSON6   quot myArray quot       This will tell you that your array myArray exists  but it s empty  This is useful if you re trying to perform a count or evaluation on myArray  For instance  say you wanted to evaluate the number of photos a user posted - you could do myArray length and it would return 0  defined  but no photos posted

User · Answer

There is only one way to represent null  that is with null   console log null     null        true console log null     true        false console log null     false       false console log null      null       false console log null      null       false console log null                 false console log null                 false console log null     0           false   That is to say  if any of the clients that consume your JSON representation use the     operator  it could be a problem for them   no value  If you want to convey that you have an object whose attribute myCount has no value      myCount   null     no attribute   missing attribute  What if you to convey that you have an object with no attributes        Client code will try to access myCount and get undefined  it s not there   empty collection  What if you to convey that you have an object with an attribute myCount that is an empty list      myCount

User · Answer

I would pick  default  for data type of variable  null for strings objects  0 for numbers   but indeed check what code that will consume the object expects  Don t forget there there is sometimes distinction between null default vs   not present    Check out null object pattern - sometimes it is better to pass some special object instead of null  i e     array instead of null for arrays or    for strings

User · Answer

According to the JSON spec  the outermost container does not have to be a dictionary  or  object   as implied in most of the comments above  It can also be a list or a bare value  i e  string  number  boolean or null   If you want to represent a null value in JSON  the entire JSON string  excluding the quotes containing the JSON string  is simply null  No braces  no brackets  no quotes  You could specify a dictionary containing a key with a null value    key1  null    or a list with a null value   null    but these are not null values themselves - they are proper dictionaries and lists  Similarly  an empty dictionary      or an empty list      are perfectly fine  but aren t null either   In Python    gt  gt  gt  print json loads    key1  null     u key1   None   gt  gt  gt  print json loads   null     None   gt  gt  gt  print json loads           gt  gt  gt  print json loads           gt  gt  gt  print json loads  null   None

[json] Representing null in JSON

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