How to test if a string is JSON or not

Question

I have a simple AJAX call  and the server will return either a JSON string with useful data or an error message string produced by the PHP function mysql error     How can I test whether this data is a JSON string or the error message     It would be nice to use a function called isJSON just like you can use the function instanceof to test if something is an Array   This is what I want   if  isJSON data          do some data stuff  else        report the error     alert data

User · Accepted Answer

Use JSON parse  function isJson str        try           JSON parse str         catch  e            return false            return true

User · Answer

You can try the following one because it also validates number  null  string but the above-marked answer is not working correctly it s just a fix of the above function  function isJson str      try         const obj   JSON parse str         if  obj  amp  amp  typeof obj      object             return true                catch  err          return false           return false

User · Answer

I like best answer but if it is an empty string it returns true  So here s a fix   function isJSON MyTestStr       try           var MyJSON   JSON stringify MyTestStr           var json   JSON parse MyJSON           if typeof MyTestStr      string               if MyTestStr length    0                  return false            catch e           return false            return true

User · Answer

There are probably tests you can do  for instance if you know that the JSON returned is always going to be surrounded by   and   then you could test for those characters  or some other hacky method  Or you could use the json org JS library to try and parse it and test if it succeeds   I would however suggest a different approach  Your PHP script currently returns JSON if the call is successful  but something else if it is not  Why not always return JSON   E g   Successful call      status    success    data      lt your data here gt        Erroneous call      status    error    error    Database not found      This would make writing your client side JS much easier - all you have to do is check the  status  member and the act accordingly

User · Answer

var parsedData   try       parsedData   JSON parse data    catch  e           is not a valid JSON string     However  I will suggest to you that your http call   service should return always a data in the same format  So if you have an error  than you should have a JSON object that wrap this error     error       code    123   message     Foo not supported         And maybe use as well as HTTP status a 5xx code

User · Answer

I used this one  kind of mix of different answers  but anyway     x000D   x000D  const isJSON   str   gt    x000D    if  typeof str      string    x000D      try   x000D        JSON parse str  x000D        return true x000D        catch e   x000D        x000D      x000D    return false x000D    x000D   x000D   x000D   x000D   null  undefined  false  true           x000D        asdf                   abc    2       abc      2      x000D     map el   gt    x000D        console log    gt   el  lt   -   isJSON el     x000D     x000D   x000D  console log  -----------------   x000D   x000D   x000D

User · Answer

I use just 2 lines to perform that   var isValidJSON   true  try   JSON parse jsonString    catch   isValidJSON   false     That s all   But keep in mind there are 2 traps  1  JSON parse null  returns null 2  Any number or string can be parsed with JSON parse   method    nbsp  nbsp   JSON parse  5   returns 5    nbsp  nbsp   JSON parse 5  returns 5   Let s some play on code      TEST 1 var data       a   1        Avoiding  null  trap  Null is confirmed as JSON  var isValidJSON   data   true   false try   JSON parse data    catch e    isValidJSON   false    console log  data isValidJSON     isValidJSON   console log  data isJSONArray     isValidJSON  amp  amp  JSON parse data  length   true   false    Console outputs  data isValidJSON   true data isJSONArray   false      TEST 2 var data2        b   2      var isValidJSON   data   true   false try   JSON parse data2    catch e    isValidJSON   false    console log  data2 isValidJSON     isValidJSON   console log  data2 isJSONArray     isValidJSON  amp  amp  JSON parse data2  length   true   false    Console outputs  data2 isValidJSON   true data2 isJSONArray   true      TEST 3 var data3       2      var isValidJSON   data   true   false try   JSON parse data3    catch e    isValidJSON   false    console log  data3 isValidJSON     isValidJSON   console log  data3 isJSONArray     isValidJSON  amp  amp  JSON parse data3  length   true   false    Console outputs  data3 isValidJSON   false data3 isJSONArray   false      TEST 4 var data4    2   var isValidJSON   data   true   false try   JSON parse data4    catch e    isValidJSON   false    console log  data4 isValidJSON     isValidJSON   console log  data4 isJSONArray     isValidJSON  amp  amp  JSON parse data4  length   true   false     Console outputs  data4 isValidJSON   true data4 isJSONArray   false      TEST 5 var data5       var isValidJSON   data   true   false try   JSON parse data5    catch e    isValidJSON   false    console log  data5 isValidJSON     isValidJSON   console log  data5 isJSONArray     isValidJSON  amp  amp  JSON parse data5  length   true   false     Console outputs  data5 isValidJSON   false data5 isJSONArray   false     TEST 6 var data6     undefined  var isValidJSON   data   true   false try   JSON parse data6    catch e    isValidJSON   false    console log  data6 isValidJSON     isValidJSON   console log  data6 isJSONArray     isValidJSON  amp  amp  JSON parse data6  length   true   false    Console outputs  data6 isValidJSON   false data6 isJSONArray   false

User · Answer

Well    It depends the way you are receiving your data  I think the server is responding with a JSON formated  string  using json encode   in PHP e g    If you re using JQuery post and set response data to be a JSON format and it is a malformed JSON  this will produce an error     ajax     type   POST     url   test2 php     data   data     success  function  response              Supposing x is a JSON property            alert response x           dataType   json       Invalid JSON   error  function     alert  error             But  if you re using the type response as text  you need use   parseJSON  According jquery site    Passing in a malformed JSON string may result in an exception being thrown   Thus your code will be     ajax     type   POST     url   test2 php     data   data     success  function  response            try               parsedData   JSON parse response             catch  e                   is not a valid JSON string                   dataType   text

User · Answer

All json strings start with     or     and end with the corresponding     or      so just check for that    Here s how Angular js does it   var JSON START                    var JSON ENDS                                  function isJsonLike str        var jsonStart   str match JSON START       return jsonStart  amp  amp  JSON ENDS jsonStart 0   test str       https   github com angular angular js blob v1 6 x src ng http js

User · Answer

Warning  For methods relying on JSON parse - Arrays and quote surrounded strings will pass too  ie  console log JSON parse   3     JSON parse    uD800       To avoid all non-object JSON primitives  boolean  null  array  number  string   I suggest using the following      Validate a possible object ie  o      a   2      const isJSONObject    o    gt       o  amp  amp   typeof o      object    amp  amp   Array isArray o   amp  amp           gt    try   return Boolean JSON stringify o      catch   return false            Validate a possible JSON object represented as string ie  s       a   3       function isJSONObjectString s        try           const o   JSON parse s           return   o  amp  amp   typeof o      object    amp  amp   Array isArray o        catch           return false           Code Explanation     o - Not falsy  excludes null  which registers as typeof  object    typeof o      object   - Excludes boolean  number  and string  Array isArray o  - Exclude arrays  which register as typeof  object   try     JSON stringify   JSON parse - Asks JavaScript engine to determine if valid JSON   Why not use the hasJsonStructure   answer   Relying on toString   is not a good idea  This is because different JavaScript Engines may return a different string representation  In general  methods which rely on this may fail in different environments or may be subject to fail later should the engine ever change the string result  Why is catching an exception not a hack   It was brought up that catching an exception to determine something s validity is never the right way to go  This is generally good advice  but not always  In this case  exception catching is likely is the best route because it relies on the JavaScript engine s implementation of validating JSON data   Relying on the JS engine offers the following advantages    More thorough and continually up-to-date as JSON spec changes Likely to run faster  as it s lower level code    When given the opportunity to lean on the JavaScript engine  I d suggest doing it  Particularly so in this case  Although it may feel hacky to catch an exception  you re really just handling two possible return states from an external method

User · Answer

You could try decoding it and catching the exception  native or json2 js    try     newObj   JSON parse myJsonString     catch  e      console log  Not JSON        However  I would suggest making the response always be valid JSON  If you get an error back from your MySQL query  simply send back JSON with the error     error   The MySQL error string      And then   if  myParsedJSON error      console log  An error occurred      myParsedJSON error

User · Answer

If the server is responding with JSON then it would have an application json content-type  if it is responding with a plain text message then it should have a text plain content-type  Make sure the server is responding with the correct content-type and test that

User · Answer

This code is JSON parse 1234  or JSON parse 0  or JSON parse false  or JSON parse null  all will return true   function isJson str        try           JSON parse str         catch  e            return false            return true      So I rewrote code in this way   function isJson item        item   typeof item      string            JSON stringify item            item       try           item   JSON parse item         catch  e            return false             if  typeof item      object   amp  amp  item     null            return true             return false      Testing result   isJson test result

User · Answer

Here is a code with some minor modification in Bourne s answer  As JSON parse number  works fine without any exception so added isNaN   function isJson str        try           JSON parse str         catch  e            return false            return isNaN str

User · Answer

In addition to previous answers  in case of you need to validate a JSON format like       you can use the following code   const validateJSON    str    gt      try       const json   JSON parse str       if  Object prototype toString call json  slice 8 -1       Object           return false            catch  e        return false        return true      Examples of usage   validateJSON       true validateJSON       false validateJSON     false validateJSON  2134   false validateJSON     Id   1   Name    Coke      true

User · Answer

when using jQuery   ajax   the response will have the responseJSON property if the response was JSON  this could be checked like this   if  xhr hasOwnProperty  responseJSON

User · Answer

Let s recap this  for 2019        Argument  Values such as true  false  null are valid JSON        FACT  These primitive values are JSON-parsable but they are not well-formed JSON structures  JSON specification indicates JSON is built on on two structures  A collection of name value pair  object  or an ordered list of values  array        Argument  Exception handling shouldn t be used to do something expected     This is a comment that has 25  upvotes     FACT  No  It s definitely legal to use try catch  especially in a case like this  Otherwise  you d need to do lots of string analysis stuff such as tokenizing   regex operations  which would have terrible performance   hasJsonStructure    This is useful if your goal is to check if some data text has proper JSON interchange format   function hasJsonStructure str        if  typeof str      string   return false      try           const result   JSON parse str           const type   Object prototype toString call result           return type       object Object                   type       object Array          catch  err            return false            Usage    hasJsonStructure  true                        false hasJsonStructure    x  true                   true hasJsonStructure   1  false  null             true   safeJsonParse    And this is useful if you want to be careful when parsing some data to a JavaScript value   function safeJsonParse str        try           return  null  JSON parse str          catch  err            return  err             Usage    const  err  result    safeJsonParse   Invalid JSON     if  err        console log  Failed to parse JSON      err message     else       console log result

[javascript] How to test if a string is JSON or not?

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