Understand and Learn javascript JSON Object with examples

JSON is abbreviated as javascript object notation. Before going to understand javascript JSON object, First, we will go through JSON basics.

JSON introduction

  • JSON is a text-based file format that is used to save and transfer data in between software applications. Usually, the browser sends the request data to the server and receives the response data in JSON format.
  • JSON is simple to read and lightweight
  • JSON is language independent. All the programming languages have the support for understanding JSON notation
  • It only represents data in this format, No methods and functions are supported.
  • Comments are not allowed in data JSON Sample
    "name": "Tom",  
    "id": "1",  
    "age": 25  
    "name": "Ram",  
    "id": "2",  
    "age": 35  
```**JSON Syntax**   
The JSON object is similar to javascript syntax.  
JSON object holds key and value pairs separated by a colon. Each object and both key and value pair is separated by a comma.  
A JSON array represents using brackets which holds a list of similar elements  
Keys only accepts Strings, numbers but not arrays or object arrays  
Values can be number,Arrays,String,Objects,boolean,null  
_Simple JSON example_  

var emp={ "name":"Tom" }

_JSON Array Example_  

var emp=[{ "name":"John" }, { "name":"Ram" }]

 JSON data can be represented in the form of Strings as well as files. The file extension is json. And content type for these files is application/json.  

## JSON object in javascript


The JSON object is used to manipulate JSON format-based files and strings. we can do the convert to/from a string using this method. There are only two methods available in javascript.

### JSON.parse() method

Take the input as a string and returns the object.

**Syntax** is JSON.parse(stringObject\[, reviver\])


_stringObject_ is a string to parse as JSON

_reviver_ \- is called during transformation to manipulate key and values and return it. and the return value is the object 

The below are valid strings for parsing into a javascript object.  

var jsonString = '{"name": "John","id": "1","age": 25}'; var jsonObject = JSON.parse(jsonString); console.log(typeof jsonString);// returns string console.log(typeof jsonObject);// returns object console.log(jsonString);// returns {"name": "John","id": "1","age": 25} console.log(jsonObject);// returns Objectage: 25id: "1"name: "John"__proto__: Object console.log(jsonObject.name); // returns John // valid json parsing JSON.parse('{}'); // returns {} JSON.parse('"hello"'); // returns "hello" JSON.parse('null'); // returns null JSON.parse('false'); // returns false

var test = JSON.parse('{"name":"kiran"}', (key, value) =>{ return typeof value === 'string' ? value.toUpperCase() // return value in uppercase : value // return everything else unchanged } ); console.log('t', test); // returns json object like {"name":"KIRAN"}

**JSON parse SyntaxError Example**   
throws _Syntaxerror_ when unable to parse string format to JSON object.  
 **Uncaught SyntaxError: Unexpected token \] in JSON at position**   

JSON.parse('[4, 1, 9, ]');  
JSON.parse('{"key" : "value", }');  

Solution or fix to check JSON string format and fix it. You can also check online JSON format validation tools to validate it if required.

JSON Stringify method

stringify method is used to serialize JSON value as JSON string.
Syntax is JSON.stringify(jsonvalue[, replacer[, space]])
Arguments- jsonvalue is value converted to JSON string
replacer - is a function written to filter/values from full JSON string. keys/values can be omitted using this function. This is optional.
spacer - the third argument can be string or number
And the return value is JSON string
Throws a TypeError (“cyclic object value”) exception when a circular reference is found. Valid JSON stringify example

const arrayJson = ['kiran', 'john', 'frank'];  
const stringArrayJson = JSON.stringify(arrayJson);  
console.log(stringArrayJson); // return ["kiran","john","frank"]  

JSON stringify reviver and space example

 `const employeeString = {  
  id: 1,  
  name: 'Kiran',  
  age: 35  
function replacer(key, value) {  
  console.log(typeof value);  
  if (key === 'age') {  
    return undefined;  
  return value;  
const empJsonString = JSON.stringify(employeeString, replacer);  
console.log(empJsonString) // returns {"id":1,"name":"Kiran"}  
const empJsonString1 = JSON.stringify(employeeString, null,'--');  
console.log(empJsonString1) // returns {--"id": 1,--"name": "Kiran",--"age": 35}  
```**JSON stringify TypeError error**   
we are going to use stringify for the circular reference use case where parent and child objects are referenced. Uncaught TypeError: Converting circular structure to JSON  

const parentObject = {'childObject':null}; parentObject.childObject = parentObject; JSON.stringify(parentObject);

## Difference between json and jsonp



Plain JSON

JSON with padding, JSON data + function call

It works with transferring data from the client and server from the same domain.

get the data from a server from the different domain

Content type is application/json

The content type is application/javascript.

{ "name": "Tom" }

function({ "name": "Tom" })

Easy to use and consume it

Not simple to understand and use it

### How to read data from JSON files in javascript? 

Reading data from JSON files is similar to retrieving data from the Ajax API Call. we can achieve this using **XMLHttpRequest** or **ajax jquery call**  

1.  Create XMLHttpRequest object
2.  Handles onreadystatechange event- this will be called whenever status changed 
3.  readyState state is 4 and status is 200, request successfully connected and a successful response is returned 
4.  parse data using the JSON parse method
5.  onError event is used to handle the errors.
6.  open and send methods are used to connect to the resource and send the request to the server.


` getJSONFileData: function() {
// XMLHttpRequest read json example
const xmlRequest = new XMLHttpRequest();
let path = ‘file:///C:/myproject/tsconfig.json’;
xmlRequest.open(‘GET’, path, true);
xmlRequest.onreadystatechange = function() {
if (this.readyState == 4 && this.status == 200) {
const files = JSON.parse(xmlRequest.response);
} else {
console.log(‘Returned an error while reading json from file’);

xmlRequest.onerror = function() {  
  console.log('Error occured during XMLHttpRequest');  


// ajax read json example $.ajax({ url : 'file:///C:/myproject/tsconfig.json', dataType : 'json', method : 'GET', success : function(response){ data = JSON.parse(response); console.log(data); } });

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