How to check if JavaScript object is JSON

Question

I have a nested JSON object that I need to loop through  and the value of each key could be a String  JSON array or another JSON object  Depending on the type of object  I need to carry out different operations  Is there any way I can check the type of the object to see if it is a String  JSON object or JSON array   I tried using typeof and instanceof but both didn t seem to work  as typeof will return an object for both JSON object and array  and instanceof gives an error when I do obj instanceof JSON   To be more specific  after parsing the JSON into a JS object  is there any way I can check if it is a normal string  or an object with keys and values  from a JSON object   or an array  from a JSON array    For example   JSON  var data      hi                  hello                    hi1   hi2                                hey   words                   Sample JavaScript  var jsonObj   JSON parse data   var path     hi   hello     function check jsonObj  path        var parent   jsonObj      for  var i   0  i  lt  path length-1  i              var key   path i           if  parent    undefined                parent   parent key                       if  parent    undefined            var endLength   path length - 1          var child   parent path endLength              if child is a string  add some text           if child is an object  edit the key value           if child is an array  add a new element           if child does not exist  add a new key value           How do I carry out the object checking as shown above

User · Answer

I know this is a very old question with good answers. However, it seems that it's still possible to add my 2¢ to it.

Assuming that you're trying to test not a JSON object itself but a String that is formatted as a JSON (which seems to be the case in your var data), you could use the following function that returns a boolean (is or is not a 'JSON'):

function isJsonString( jsonString ) {

  // This function below ('printError') can be used to print details about the error, if any.
  // Please, refer to the original article (see the end of this post)
  // for more details. I suppressed details to keep the code clean.
  //
  let printError = function(error, explicit) {
  console.log(`[${explicit ? 'EXPLICIT' : 'INEXPLICIT'}] ${error.name}: ${error.message}`);
  }


  try {
      JSON.parse( jsonString );
      return true; // It's a valid JSON format
  } catch (e) {
      return false; // It's not a valid JSON format
  }

}

Here are some examples of using the function above:

console.log('\n1 -----------------');
let j = "abc";
console.log( j, isJsonString(j) );

console.log('\n2 -----------------');
j = `{"abc": "def"}`;
console.log( j, isJsonString(j) );

console.log('\n3 -----------------');
j = '{"abc": "def}';
console.log( j, isJsonString(j) );

console.log('\n4 -----------------');
j = '{}';
console.log( j, isJsonString(j) );

console.log('\n5 -----------------');
j = '[{}]';
console.log( j, isJsonString(j) );

console.log('\n6 -----------------');
j = '[{},]';
console.log( j, isJsonString(j) );

console.log('\n7 -----------------');
j = '[{"a":1, "b":   2}, {"c":3}]';
console.log( j, isJsonString(j) );

When you run the code above, you will get the following results:

1 -----------------
abc false

2 -----------------
{"abc": "def"} true

3 -----------------
{"abc": "def} false

4 -----------------
{} true

5 -----------------
[{}] true

6 -----------------
[{},] false

7 -----------------
[{"a":1, "b":   2}, {"c":3}] true

Please, try the snippet below and let us know if this works for you. :)

IMPORTANT: the function presented in this post was adapted from https://airbrake.io/blog/javascript-error-handling/syntaxerror-json-parse-bad-parsing where you can find more and interesting details about the JSON.parse() function.

_x000D_

function isJsonString( jsonString ) {_x000D_
_x000D_
  let printError = function(error, explicit) {_x000D_
  console.log(`[${explicit ? 'EXPLICIT' : 'INEXPLICIT'}] ${error.name}: ${error.message}`);_x000D_
  }_x000D_
_x000D_
_x000D_
  try {_x000D_
      JSON.parse( jsonString );_x000D_
      return true; // It's a valid JSON format_x000D_
  } catch (e) {_x000D_
      return false; // It's not a valid JSON format_x000D_
  }_x000D_
_x000D_
}_x000D_
_x000D_
_x000D_
console.log('\n1 -----------------');_x000D_
let j = "abc";_x000D_
console.log( j, isJsonString(j) );_x000D_
_x000D_
console.log('\n2 -----------------');_x000D_
j = `{"abc": "def"}`;_x000D_
console.log( j, isJsonString(j) );_x000D_
_x000D_
console.log('\n3 -----------------');_x000D_
j = '{"abc": "def}';_x000D_
console.log( j, isJsonString(j) );_x000D_
_x000D_
console.log('\n4 -----------------');_x000D_
j = '{}';_x000D_
console.log( j, isJsonString(j) );_x000D_
_x000D_
console.log('\n5 -----------------');_x000D_
j = '[{}]';_x000D_
console.log( j, isJsonString(j) );_x000D_
_x000D_
console.log('\n6 -----------------');_x000D_
j = '[{},]';_x000D_
console.log( j, isJsonString(j) );_x000D_
_x000D_
console.log('\n7 -----------------');_x000D_
j = '[{"a":1, "b":   2}, {"c":3}]';_x000D_
console.log( j, isJsonString(j) );

_x000D_

User · Answer

I wrote an npm module to solve this problem  It s available here   object-types  a module for finding what literal types underly objects  Install    npm install --save object-types     Usage  const objectTypes   require  object-types     objectTypes          gt   object   objectTypes          gt   array   objectTypes new Object true        gt   boolean    Take a look  it should solve your exact problem  Let me know if you have any questions  https   github com dawsonbotsford object-types

User · Answer

If you are trying to check the type of an object after you parse a JSON string  I suggest checking the constructor attribute   obj constructor    Array    obj constructor    String    obj constructor    Object   This will be a much faster check than typeof or instanceof   If a JSON library does not return objects constructed with these functions  I would be very suspiciouse of it

User · Answer

Why not check Number - a bit shorter and works in IE Chrome FF node js   x000D   x000D  function whatIsIt object    x000D      if  object     null    x000D          return  null   x000D        x000D      else if  object     undefined    x000D          return  undefined   x000D        x000D      if  object constructor name    x000D              return object constructor name  x000D        x000D      else      last chance 4 IE    nfunction Number     n     native code  n  n    node js   function String      native code     x000D          var name   object constructor toString   split       x000D          if  name  amp  amp  name length  gt  1    x000D              name   name 1   x000D              return name substr 0  name indexOf        x000D            x000D          else      unreachable now    x000D              return  don t know   x000D            x000D        x000D    x000D   x000D  var testSubjects     string    1 2 3    foo   bar    4   x000D     Test all options x000D  console log whatIsIt null    x000D  console log whatIsIt     x000D  for  var i 0  len   testSubjects length  i  lt  len  i      x000D      console log whatIsIt testSubjects i     x000D    x000D   x000D   x000D

User · Answer

You could make your own constructor for JSON parsing   var JSONObj   function obj      extend this  JSON parse obj      var test   new JSONObj    a    apple         a   apple     Then check instanceof to see if it needed parsing originally  test instanceof JSONObj

User · Answer

The answer by  PeterWilkinson didn t work for me because a constructor for a  typed  object is customized to the name of that object  I had to work with typeof  function isJson obj        var t   typeof obj      return   boolean    number    string    symbol    function   indexOf t     -1

User · Answer

I combine the typeof operator with a check of the constructor attribute  by Peter    var typeOf   function object        var firstShot   typeof object      if  firstShot      object             return firstShot             else if  object constructor        constructor            return  array             else if  object constructor        constructor            return  object             else if  object     null            return  null             else           return  don  t know                Test var testSubjects    true  false  1  2 3   string    4 5 6    foo   bar    null  undefined    console log   typeOf      input parameter   join   t    console log new Array 28  join  -     testSubjects map function testSubject       console log  typeOf testSubject   JSON stringify testSubject   join   t t           Result   typeOf      input parameter --------------------------- boolean     true boolean     false number      1 number      2 3 string       string  array        4 5 6  object        foo   bar   null        null undefined

User · Answer

Based on  Martin Wantke answer  but with some recommended improvements adjusts       NOTE  Check JavaScript type  By Questor function getJSType valToChk         function isUndefined valToChk    return valToChk     undefined        function isNull valToChk    return valToChk     null        function isArray valToChk    return valToChk constructor    Array        function isBoolean valToChk    return valToChk constructor    Boolean        function isFunction valToChk    return valToChk constructor    Function        function isNumber valToChk    return valToChk constructor    Number        function isString valToChk    return valToChk constructor    String        function isObject valToChk    return valToChk constructor    Object         if isUndefined valToChk     return  quot undefined quot         if isNull valToChk     return  quot null quot         if isArray valToChk     return  quot array quot         if isBoolean valToChk     return  quot boolean quot         if isFunction valToChk     return  quot function quot         if isNumber valToChk     return  quot number quot         if isString valToChk     return  quot string quot         if isObject valToChk     return  quot object quot         NOTE  I found this approach very didactic  so I submitted this answer

User · Answer

You can use Array isArray to check for arrays   Then typeof obj     string   and typeof obj     object    var s    a string   a       o       i   5  function getType p        if  Array isArray p   return  array       else if  typeof p     string   return  string       else if  p    null  amp  amp  typeof p     object   return  object       else return  other     console log   s  is     getType s    console log   a  is     getType a    console log   o  is     getType o    console log   i  is     getType i          s  is string a  is array  o  is object i  is other

User · Answer

An JSON object is an object  To check whether a type is an object type  evaluate the constructor property   function isObject obj        return obj     undefined  amp  amp  obj     null  amp  amp  obj constructor    Object      The same applies to all other types   function isArray obj        return obj     undefined  amp  amp  obj     null  amp  amp  obj constructor    Array     function isBoolean obj        return obj     undefined  amp  amp  obj     null  amp  amp  obj constructor    Boolean     function isFunction obj        return obj     undefined  amp  amp  obj     null  amp  amp  obj constructor    Function     function isNumber obj        return obj     undefined  amp  amp  obj     null  amp  amp  obj constructor    Number     function isString obj        return obj     undefined  amp  amp  obj     null  amp  amp  obj constructor    String     function isInstanced obj        if obj     undefined    obj     null    return false         if isArray obj     return false        if isBoolean obj     return false        if isFunction obj     return false        if isNumber obj     return false        if isObject obj     return false        if isString obj     return false         return true

User · Answer

Peter s answer with an additional check  Of course  not 100  guaranteed   var isJson   false  outPutValue      var objectConstructor      constructor  if jsonToCheck constructor     objectConstructor       outPutValue   JSON stringify jsonToCheck       try              JSON parse outPutValue               isJson   true       catch err               isJson   false           if isJson       alert  Is json      JSON stringify jsonToCheck           else      alert  Is other

User · Answer

I d check the constructor attribute   e g   var stringConstructor    test  constructor  var arrayConstructor      constructor  var objectConstructor        constructor   function whatIsIt object        if  object     null            return  null             if  object     undefined            return  undefined             if  object constructor     stringConstructor            return  String             if  object constructor     arrayConstructor            return  Array             if  object constructor     objectConstructor            return  Object                       return  don t know            var testSubjects     string    1 2 3    foo   bar    4    for  var i 0  len   testSubjects length  i  lt  len  i          alert whatIsIt testSubjects i         Edit  Added a null check and an undefined check

User · Answer

Try this  if   typeof is json     function    function is json   obj         var  has keys   0       for  var  pr in  obj                 if    obj hasOwnProperty   pr    amp  amp        d    test   pr                             has keys   1              break                        return    has keys  amp  amp   obj constructor    Object  amp  amp   obj constructor    Array     1   0       It works for the example below  var  a      name     me           surname     I           nickname                             first     wow                          second     super                          morelevel                                             3level1    1                                         3level2    2                                         3level3    3                                                                       var  b      name    surname    nickname      var  c    abcdefg     console log  is json   a      console log  is json   b      console log  is json   c

User · Answer

you can also try to parse the data and then check if you got object   var testIfJson   JSON parse data   if  typeOf testIfJson     object       Json   else     Not Json

User · Answer

try this dirty way          obj  includes

[javascript] How to check if JavaScript object is JSON

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