How to check if a string is a valid JSON string in JavaScript without using Try Catch

Question

Something like   var jsonString       Id   1   Name    Coke         should be true IsJsonString jsonString      should be false IsJsonString  foo    IsJsonString   lt div gt foo lt  div gt      The solution should not contain try catch  Some of us turn on  break on all errors  and they don t like the debugger breaking on those invalid JSON strings

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Maybe it will useful       function parseJson code        try           return JSON parse code         catch  e            return code          function parseJsonJQ code        try           return   parseJSON code         catch  e            return code           var str        a   1   b   2   c   3   d   4   e   5    alert typeof parseJson str    alert typeof parseJsonJQ str    var str b     c   alert typeof parseJson str b    alert typeof parseJsonJQ str b      output   IE7  string object string string  CHROME  object object string string

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This answer to reduce the cost of trycatch statement    I used JQuery to parse JSON strings and I used trycatch statement to handle exceptions  but throwing exceptions for un-parsable strings slowed down my code  so I used simple Regex to check the string if it is a possible JSON string or not without going feather by checking it s syntax  then I used the regular way by parsing the string using JQuery    if  typeof jsonData     string         if               s     w            test jsonData             return jsonData           try       jsonData     parseJSON jsonData     catch  e            I wrapped the previous code in a recursive function to parse nested JSON responses

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I thought I d add my approach  in the context of a practical example  I use a similar check when dealing with values going in and coming out of Memjs  so even though the value saved may be string  array or object  Memjs expects a string  The function first checks if a key value pair already exists  if it does then a precheck is done to determine if value needs to be parsed before being returned    function checkMem memStr        let first   memStr slice 0  1      if  first            first          return JSON parse memStr      else return memStr      Otherwise  the callback function is invoked to create the value  then a check is done on the result to see if the value needs to be stringified before going into Memjs  then the result from the callback is returned    async function getVal         let result   await o cb o params      setMem result      return result      function setMem result          if  typeof result      string             let value   JSON stringify result          setValue key  value                else setValue key  result             The complete code is below  Of course this approach assumes that the arrays objects going in and coming out are properly formatted  i e  something like  quot   key   testkey   quot  would never happen  because all the proper validations are done before the key value pairs ever reach this function   And also that you are only inputting strings into memjs and not integers or other non object arrays-types  async function getMem o      let resp   let key   JSON stringify o key    let memStr   await getValue key    if   memStr  resp   await getVal     else resp   checkMem memStr    return resp    function checkMem memStr        let first   memStr slice 0  1      if  first            first          return JSON parse memStr      else return memStr        async function getVal         let result   await o cb o params      setMem result      return result      function setMem result          if  typeof result      string             let value   JSON stringify result          setValue key  value                else setValue key  result

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The function IsJsonString str   that is using JSON parse str   doesn t work in my case  I tried to validate json output from GraphiQL it always return false  Lucky me  isJSON works better   var test   false      body   on  DOMSubtreeModified     resultWrap   function          if   test               var resultWrap             graphiql  resultWrap   text   split      pop            if isJSON resultWrap   test    test           console log resultWrap            console log resultWrap isJSON                  Sample output   THREE WebGLRenderer 79 draw js 170  xxxxxxxxxx  draw js 170 false draw js 170  xxxxxxxxxx    draw js 170 false draw js 170  xxxxxxxxxx    draw js 170 false draw js 170  xxxxxxxxxx    draw js 170 false draw js 170    draw js 170 false draw js 170      PI   3 141592653589793   draw js 170 false draw js 170      PI   3 141592653589793     time   1570751209006   draw js 170 false draw js 170      PI   3 141592653589793     time   1570751209006     tick   156   draw js 170 false draw js 170      PI   3 141592653589793     time   1570751209006     tick   156     tickr   1 56   draw js 170 false draw js 170      PI   3 141592653589793     time   1570751209006     tick   156     tickr   1 56     fps   41 666666666666664   draw js 170 false draw js 170      PI   3 141592653589793     time   1570751209006     tick   156     tickr   1 56     fps   41 666666666666664     width   396 984   draw js 170 false draw js 170      PI   3 141592653589793     time   1570751209006     tick   156     tickr   1 56     fps   41 666666666666664     width   396 984     height   327  draw js 170 false draw js 170      PI   3 141592653589793     time   1570751209006     tick   156     tickr   1 56     fps   41 666666666666664     width   396 984     height   327   draw js 170 false draw js 170      PI   3 141592653589793     time   1570751209006     tick   156     tickr   1 56     fps   41 666666666666664     width   396 984     height   327  draw js 170 true

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function get json txt     var data     try        data   eval     txt            catch e    data   false                    return data      If there are errors  return false    If there are no errors  return json data

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isValidJsonString - check for valid json string  JSON data types - string  number  object  JSON object   array  boolean  null  https   www json org json-en html   falsy values in javascript -  false  0  -0  0n   quot   null  undefined  NaN -  https   developer mozilla org en-US docs Glossary Falsy   JSON parse  works well for number   boolean  null and valid json String won t raise any error   please refer example below  JSON parse 2     2 JSON parse null     null JSON parse true     true JSON parse    quot name quot   quot jhamman quot         name   quot jhamman quot   JSON parse   1 2 3        1  2  3    break when you parse undefined   object  array etc  it gave Uncaught SyntaxError  Unexpected end of JSON input    please refer example below JSON parse     JSON parse     JSON parse undefined  JSON parse  quot jack quot        function isValidJsonString jsonString            if   jsonString  amp  amp  typeof jsonString      quot string quot             return false             try         JSON parse jsonString          return true       catch error           return false

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Use a JSON parser like JSON parse   function IsJsonString str        try           JSON parse str         catch  e            return false            return true

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If you re dealing with a response from an AJAX  or XMLHttpRequest  call  what worked for me is to check the response content type and parse or not the content accordingly

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x000D   x000D  var jsonstring     ConnectionString   aaaaaa   Server   ssssss      x000D   x000D  if   x   gt  try JSON parse x  return true  catch e  return false    jsonstring    x000D   x000D  document write  valide json   x000D   x000D   else  x000D  document write  invalide json   x000D    x000D   x000D   x000D

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Very Simple one-liner code   But Hacky approach    if  expected json id     undefined         not a json   else        json        NOTE  This only works if you are expecting something is JSON string   like id   I am using it for an API and expecting the result either in   JSON or some error string

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In prototypeJS  we have method isJSON  You can try that  Even json might help     something  isJSON       - gt  false    something    isJSON       - gt  true    foo  42    isJSON       - gt  false      foo    42    isJSON

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if resp        try           resp     parseJSON resp           console log resp         catch e            alert e             hope this works for you too

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I think I know why you want to avoid that  But maybe try  amp  catch     try  amp  catch   o  This came into my mind   var json verify   function s   try   JSON parse s   return true    catch  e    return false        So you may also dirty clip to the JSON object  like   JSON verify   function s   try   JSON parse s   return true    catch  e    return false        As this as encapsuled as possible  it may not break on error

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From Prototype framework String isJSON definition here            String isJSON   - gt  Boolean            Check if the string is valid JSON by the use of regular expressions        This security method is called internally                   Examples                 something  isJSON                 - gt  false              something    isJSON                 - gt  true              foo  42    isJSON                 - gt  false                foo    42    isJSON                 - gt  true         function isJSON         var str   this      if  str blank    return false      str   str replace             bfnrt  u 0-9a-fA-F  4   g            str   str replace         n r    true false null -  d       d       eE    -   d    g            str   str replace               s      g           return            s      test str         so this is the version that can be used passing a string object  function isJSON str        if      s    test str    return false      str   str replace             bfnrt  u 0-9a-fA-F  4   g            str   str replace         n r    true false null -  d       d       eE    -   d    g            str   str replace               s      g           return            s      test str          x000D   x000D  function isJSON str    x000D      if      s    test str    return false  x000D      str   str replace             bfnrt  u 0-9a-fA-F  4   g        x000D      str   str replace         n r    true false null -  d       d       eE    -   d    g        x000D      str   str replace               s      g       x000D      return            s      test str   x000D      x000D   x000D  console log   this is a json    isJSON       key     1    key2 e       val         x000D   x000D  console log  this is not a json   isJSON       key     1    key2 e     pippo        x000D   x000D   x000D

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For people who like the  Net convention of  try  functions that return a boolean and handle a byref param containing the result  If you don t need the out parameter you can omit it and just use the return value   StringTests js    var obj1         var bool1      h   happy    tryParse obj1      false   var obj2         var bool2    2114509 GOODLUCKBUDDY 315852  tryParse obj2       false    var obj3         if    house number   1   road   Mauchly   city   Irvine   county   Orange County   state   California   postcode   92618   country   United States of America   country code   us    tryParse obj3       console log obj3     StringUtils js  String prototype tryParse   function jsonObject      jsonObject   jsonObject          try       if           test this               test this      begin   end with    or          return false     avoid error handling for strings that obviously aren t json     var json   JSON parse this       if typeof json      object          jsonObject merge json         return true            catch  e        return false          ObjectUtils js  Object defineProperty Object prototype   merge       value  function mergeObj       for  var propertyName in mergeObj          if  mergeObj hasOwnProperty propertyName             this propertyName    mergeObj propertyName                           return this         enumerable  false     this is actually the default

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Here my working code   function IsJsonString str      try       var json   JSON parse str       return  typeof json      object        catch  e        return false

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Just keeping it simple  x000D   x000D  function isValidJsonString tester          early existing     if    s   undefined  test tester        number object array string boolean  test typeof tester                       return false           go ahead do you parsing via try catch return true      x000D   x000D   x000D

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I infer from the opening comment that the use case is delineating whether a response is HTML or JSON  In which case  when you do receive JSON  you probably ought to be parsing it and handling invalid JSON at some point in your code anyway  Aside from anything  I imagine you would like to be informed by your browser should JSON be expected but invalid JSON received  as will users by proxy of some meaningful error message    Doing a full regex for JSON is unnecessary therefore  as it would be - in my experience - for most use-cases    You would probably be better off using something like the below   function  someString         test string is opened with curly brace or machine bracket   if  someString trim   search           1     gt  -1        try      it is  so now let s see if its valid JSON       var myJson   JSON parse someString            yep  we re working with valid JSON       catch  e             nope  we got what we thought was JSON  it isn t  let s handle it            else          nope  we re working with non-json  no need to parse it fully         that should save you having to exception handle valid non-JSON code and take care of duff json at the same time

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Oh you can definitely use try catch to check whether its or not a valid JSON  Tested on Firfox Quantom 60 0 1  use function inside a function to get the JSON tested and use that output to validate the string  hears an example       function myfunction text             function for validating json string         function testJSON text               try                  if  typeof text    string                        return false                   else                      JSON parse text                       return true                                                                          catch  error                   return false                               content of your real function            if testJSON text                console log  json             else              console log  not json                       use it as a normal function         myfunction    name   kasun   age  10

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vanillaJS function isJSON str        try           return  JSON parse str   amp  amp    str         catch  e            return false            Usage  isJSON     will be false  isJSON       will be true   To check if something is an Array or Object  parsed JSON       vanillaJS function isAO val        return val instanceof Array    val instanceof Object   true   false        ES2015 var isAO    val    gt  val instanceof Array    val instanceof Object   true   false    Usage  isAO     will be true  isAO       will be false

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You can use the javascript eval   function to verify if it s valid   e g   var jsonString       Id   1   Name    Coke      var json   try     json   eval jsonString     catch  exception        It s advisable to always catch an exception since eval   is a javascript executor      json   null     if  json        this is json     Alternatively  you can use JSON parse function from json org   try     json   JSON parse jsonString     catch  exception      json   null     if  json        this is json     Hope this helps   WARNING  eval   is dangerous if someone adds malicious JS code  since it will execute it  Make sure the JSON String is trustworthy  i e  you got it from a trusted source   Edit For my 1st solution  it s recommended to do this    try         json   eval       jsonString               catch  exception            It s advisable to always catch an exception since eval   is a javascript executor          json   null          To guarantee json-ness  If the jsonString isn t pure JSON  the eval will throw an exception

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I know i m 3 years late to this question  but I felt like chiming in   While Gumbo s solution works great  it doesn t handle a few cases where no exception is raised for JSON parse  something that isn t JSON    I also prefer to return the parsed JSON at the same time  so the calling code doesn t have to call JSON parse jsonString  a second time   This seems to work well for my needs   function tryParseJSON  jsonString       try           var o   JSON parse jsonString               Handle non-exception-throwing cases             Neither JSON parse false  or JSON parse 1234  throw errors  hence the type-checking             but    JSON parse null  returns null  and typeof null      object               so we must check for that  too  Thankfully  null is falsey  so this suffices          if  o  amp  amp  typeof o      object                 return o                      catch  e           return false

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I used a really simple method to check a string how it s a valid JSON or not   function testJSON text       if  typeof text    string            return false            try          JSON parse text           return true            catch  error           return false            Result with a valid JSON string   var input    foo   bar    foo   bar      testJSON input      returns true    Result with a simple string   var input  This is not a JSON string    testJSON input      returns false    Result with an object   var input     testJSON input      returns false    Result with null input   var input null  testJSON input      returns false    The last one returns false because the type of null variables is object   This works everytime

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A comment first  The question was about not using try catch  If you do not mind to use it  read the answer below  Here we just check a JSON string using a regexp  and it will work in most cases  not all cases   Have a look around the line 450 in https   github com douglascrockford JSON-js blob master json2 js  There is a regexp that check for a valid JSON  something like   if            s     test text replace          bfnrtu  g        replace         n r    true false null -  d       d       eE    -   d    g        replace               s      g               the json is ok   else       the json is not ok      EDIT  The new version of json2 js makes a more advanced parsing than above  but still based on a regexp replace   from the comment of  Mrchief

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Here is the typescript version too  JSONTryParse input  any        try             check if the string exists         if  input                var o   JSON parse input                  validate the result too             if  o  amp  amp  o constructor     Object                    return o                                    catch  e  any               return false

[javascript] How to check if a string is a valid JSON string in JavaScript without using Try/Catch

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