Safely turning a JSON string into an object

Given a string of JSON data, how can I safely turn that string into a JavaScript object?

Obviously I can do this unsafely with something like:

var obj = eval("(" + json + ')');

but that leaves me vulnerable to the JSON string containing other code, which it seems very dangerous to simply eval.

JSON.parse(jsonString) is a pure JavaScript approach so long as you can guarantee a reasonably modern browser.

The jQuery method is now deprecated. Use this method instead:

let jsonObject = JSON.parse(jsonString);

---

Original answer using deprecated jQuery functionality:

If you're using jQuery just use:

jQuery.parseJSON( jsonString );

It's exactly what you're looking for (see the jQuery documentation).

This answer is for IE < 7, for modern browsers check Jonathan's answer above.

This answer is outdated and Jonathan's answer above (JSON.parse(jsonString)) is now the best answer.

JSON.org has JSON parsers for many languages including four different ones for JavaScript. I believe most people would consider json2.js their goto implementation.

Use the simple code example in "JSON.parse()":

var jsontext = '{"firstname":"Jesper","surname":"Aaberg","phone":["555-0100","555-0120"]}';
var contact = JSON.parse(jsontext);

and reversing it:

var str = JSON.stringify(arr);

This seems to be the issue:

An input that is received via Ajax websocket etc, and it will be in String format, but you need to know if it is JSON.parsable. The touble is, if you always run it through JSON.parse, the program MAY continue "successfully" but you'll still see an error thrown in the console with the dreaded "Error: unexpected token 'x'".

var data;

try {
  data = JSON.parse(jqxhr.responseText);
} catch (_error) {}

data || (data = {
  message: 'Server error, please retry'
});

I'm not sure about other ways to do it but here's how you do it in Prototype (JSON tutorial).

new Ajax.Request('/some_url', {
  method:'get',
  requestHeaders: {Accept: 'application/json'},
  onSuccess: function(transport){
    var json = transport.responseText.evalJSON(true);
  }
});

Calling evalJSON() with true as the argument sanitizes the incoming string.

If you're using jQuery, you can also use:

$.getJSON(url, function(data) { });

Then you can do things like

data.key1.something
data.key1.something_else

etc.

Just for fun, here is a way using a function:

 jsonObject = (new Function('return ' + jsonFormatData))()

$.ajax({
  url: url,
  dataType: 'json',
  data: data,
  success: callback
});

The callback is passed the returned data, which will be a JavaScript object or array as defined by the JSON structure and parsed using the $.parseJSON() method.

Using JSON.parse is probably the best way.

Here's an example

var jsonRes = '{ "students" : [' +
          '{ "firstName":"Michel" , "lastName":"John" ,"age":18},' +
          '{ "firstName":"Richard" , "lastName":"Joe","age":20 },' +
          '{ "firstName":"James" , "lastName":"Henry","age":15 } ]}';
var studentObject = JSON.parse(jsonRes);

Try using the method with this Data object. ex:Data='{result:true,count:1}'

try {
  eval('var obj=' + Data);
  console.log(obj.count);
}
catch(e) {
  console.log(e.message);
}

This method really helps in Nodejs when you are working with serial port programming

The easiest way using parse() method:

var response = '{"result":true,"count":1}';
var JsonObject= JSON.parse(response);

Then you can get the values of the JSON elements, for example:

var myResponseResult = JsonObject.result;
var myResponseCount = JsonObject.count;

Using jQuery as described in the jQuery.parseJSON() documentation:

JSON.parse(jsonString);

I found a "better" way:

In CoffeeScript:

try data = JSON.parse(jqxhr.responseText)
data ||= { message: 'Server error, please retry' }

In Javascript:

var data;

try {
  data = JSON.parse(jqxhr.responseText);
} catch (_error) {}

data || (data = {
  message: 'Server error, please retry'
});

JSON parsing is always a pain. If the input is not as expected it throws an error and crashes what you are doing.

You can use the following tiny function to safely parse your input. It always turns an object even if the input is not valid or is already an object which is better for most cases:

JSON.safeParse = function (input, def) {
  // Convert null to empty object
  if (!input) {
    return def || {};
  } else if (Object.prototype.toString.call(input) === '[object Object]') {
    return input;
  }
  try {
    return JSON.parse(input);
  } catch (e) {
    return def || {};
  }
};

Parse the JSON string with JSON.parse(), and the data becomes a JavaScript object:

JSON.parse(jsonString)

Here, JSON represents to process JSON dataset.

Imagine we received this text from a web server:

'{ "name":"John", "age":30, "city":"New York"}'

To parse into a JSON object:

var obj = JSON.parse('{ "name":"John", "age":30, "city":"New York"}'); 

Here obj is the respective JSON object which looks like:

{ "name":"John", "age":30, "city":"New York"}

To fetch a value use the . operator:

obj.name // John
obj.age //30

Convert a JavaScript object into a string with JSON.stringify().

JSON.parse() converts any JSON string passed into the function into a JSON object.

To understand it better, press F12 to open "Inspect Element" in your browser and go to the console to write the following commands:

var response = '{"result":true,"count":1}'; //sample json object(string form)
JSON.parse(response); //converts passed string to JSON Object.

Now run the command:

console.log(JSON.parse(response));

You'll get output as an Object {result: true, count: 1}.

In order to use that Object, you can assign it to the variable, maybe obj:

var obj = JSON.parse(response);

By using obj and the dot (.) operator you can access properties of the JSON object.

Try to run the command:

console.log(obj.result);

JSON.parse(jsonString);

json.parse will change into object.

Official documentation:

The JSON.parse() method parses a JSON string, constructing the JavaScript value or object described by the string. An optional reviver function can be provided to perform a transformation on the resulting object before it is returned.

Syntax:

JSON.parse(text[, reviver])

Parameters:

text : The string to parse as JSON. See the JSON object for a description of JSON syntax.

reviver (optional) : If a function, this prescribes how the value originally produced by parsing is transformed, before being returned.

Return value

The Object corresponding to the given JSON text.

Exceptions

Throws a SyntaxError exception if the string to parse is not valid JSON.

Converting the object to JSON, and then parsing it, works for me, like:

JSON.parse(JSON.stringify(object))

If we have a string like this:

"{\"status\":1,\"token\":\"65b4352b2dfc4957a09add0ce5714059\"}"

then we can simply use JSON.parse twice to convert this string to a JSON object:

var sampleString = "{\"status\":1,\"token\":\"65b4352b2dfc4957a09add0ce5714059\"}"
var jsonString= JSON.parse(sampleString)
var jsonObject= JSON.parse(jsonString)

And we can extract values from the JSON object using:

// instead of last JSON.parse:
var { status, token } = JSON.parse(jsonString);

The result will be:

status = 1 and token = 65b4352b2dfc4957a09add0ce5714059

You also can use reviver function to filter.

var data = JSON.parse(jsonString, function reviver(key, value) {
   //your code here to filter
});

For more information read JSON.parse.

Older question, I know, however nobody notice this solution by using new Function(), an anonymous function that returns the data.

---

Just an example:

 var oData = 'test1:"This is my object",test2:"This is my object"';
 
 if( typeof oData !== 'object' )
  try {
   oData = (new Function('return {'+oData+'};'))();
  }
  catch(e) { oData=false; }
  
 if( typeof oData !== 'object' )
  { alert( 'Error in code' ); }
 else {
        alert( oData.test1 );
        alert( oData.test2 );
      }

---

This is a little more safe because it executes inside a function and do not compile in your code directly. So if there is a function declaration inside it, it will not be bound to the default window object.

I use this to 'compile' configuration settings of DOM elements (for example the data attribute) simple and fast.

Summary:

Javascript (both browser and NodeJS) have a built in JSON object. On this Object are 2 convenient methods for dealing with JSON. They are the following:

1. JSON.parse() Takes JSON as argument, returns JS object
2. JSON.stringify() Takes JS object as argument returns JSON object

Other applications:

Besides for very conveniently dealing with JSON they have can be used for other means. The combination of both JSON methods allows us to make very easy make deep clones of arrays or objects. For example:

let arr1 = [1, 2, [3 ,4]];
let newArr = arr1.slice();

arr1[2][0] = 'changed'; 
console.log(newArr); // not a deep clone

let arr2 = [1, 2, [3 ,4]];
let newArrDeepclone = JSON.parse(JSON.stringify(arr2));

arr2[2][0] = 'changed'; 
console.log(newArrDeepclone); // A deep clone, values unchanged

Just to the cover parse for different input types

Parse the data with JSON.parse(), and the data becomes a JavaScript object.

var obj = JSON.parse('{ "name":"John", "age":30, "city":"New York"}');

When using the JSON.parse() on a JSON derived from an array, the method will return a JavaScript array, instead of a JavaScript object.

var myArr = JSON.parse(this.responseText);
console.log(myArr[0]);

Date objects are not allowed in JSON. For Dates do somthing like this

var text = '{ "name":"John", "birth":"1986-12-14", "city":"New York"}';
var obj = JSON.parse(text);
obj.birth = new Date(obj.birth);

Functions are not allowed in JSON. If you need to include a function, write it as a string.

var text = '{ "name":"John", "age":"function () {return 30;}", "city":"New York"}';
var obj = JSON.parse(text);
obj.age = eval("(" + obj.age + ")");

Performance

There are already good answer for this question, but I was curious about performance and today 2020.09.21 I conduct tests on MacOs HighSierra 10.13.6 on Chrome v85, Safari v13.1.2 and Firefox v80 for chosen solutions.

Results

• eval/Function (A,B,C) approach is fast on Chrome (but for big-deep object N=1000 they crash: "maximum stack call exceed)
• eval (A) is fast/medium fast on all browsers
• JSON.parse (D,E) are fastest on Safari and Firefox

Details

I perform 4 tests cases:

• for small shallow object HERE
• for small deep object HERE
• for big shallow object HERE
• for big deep object HERE

Object used in above tests came from HERE

let obj_ShallowSmall = {
  field0: false,
  field1: true,
  field2: 1,
  field3: 0,
  field4: null,
  field5: [],
  field6: {},
  field7: "text7",
  field8: "text8",
}

let obj_DeepSmall = {
  level0: {
   level1: {
    level2: {
     level3: {
      level4: {
       level5: {
        level6: {
         level7: {
          level8: {
           level9: [[[[[[[[[['abc']]]]]]]]]],
  }}}}}}}}},
};

let obj_ShallowBig = Array(1000).fill(0).reduce((a,c,i) => (a['field'+i]=getField(i),a) ,{});


let obj_DeepBig = genDeepObject(1000);



// ------------------
// Show objects
// ------------------

console.log('obj_ShallowSmall:',JSON.stringify(obj_ShallowSmall));
console.log('obj_DeepSmall:',JSON.stringify(obj_DeepSmall));
console.log('obj_ShallowBig:',JSON.stringify(obj_ShallowBig));
console.log('obj_DeepBig:',JSON.stringify(obj_DeepBig));




// ------------------
// HELPERS
// ------------------

function getField(k) {
  let i=k%10;
  if(i==0) return false;
  if(i==1) return true;
  if(i==2) return k;
  if(i==3) return 0;
  if(i==4) return null;
  if(i==5) return [];
  if(i==6) return {};  
  if(i>=7) return "text"+k;
}

function genDeepObject(N) {
  // generate: {level0:{level1:{...levelN: {end:[[[...N-times...['abc']...]]] }}}...}}}
  let obj={};
  let o=obj;
  let arr = [];
  let a=arr;

  for(let i=0; i<N; i++) {
    o['level'+i]={};
    o=o['level'+i];
    let aa=[];
    a.push(aa);
    a=aa;
  }

  a[0]='abc';
  o['end']=arr;
  return obj;
}

Below snippet presents chosen solutions

// src: https://stackoverflow.com/q/45015/860099
function A(json) {
  return eval("(" + json + ')');
}

// https://stackoverflow.com/a/26377600/860099
function B(json) {
  return (new Function('return ('+json+')'))()
}


// improved https://stackoverflow.com/a/26377600/860099
function C(json) {
  return Function('return ('+json+')')()
}

// src: https://stackoverflow.com/a/5686237/860099
function D(json) {
  return JSON.parse(json);
}

// src: https://stackoverflow.com/a/233630/860099
function E(json) {
  return $.parseJSON(json)
}



 
// --------------------
// TEST
// --------------------

let json = '{"a":"abc","b":"123","d":[1,2,3],"e":{"a":1,"b":2,"c":3}}';

[A,B,C,D,E].map(f=> {  
  console.log(
    f.name + ' ' + JSON.stringify(f(json))
  )})

<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
This shippet only presents functions used in performance tests - it not perform tests itself!

And here are example results for chrome

Try this.This one is written in typescript.

         export function safeJsonParse(str: string) {
               try {
                 return JSON.parse(str);
                   } catch (e) {
                 return str;
                 }
           }

/**
 * Safely turning a JSON string into an object
 *
 * @param {String} str - JSON String
 * @returns deserialized object, false if error
 */
export function jsonParse(str) {
  let data = null;
  try {
    data = JSON.parse(str);
  } catch (err) {
    return false;
  }
  return data;
}