JavaScript JSON — Interview Questions & Answers

Ques: What is JSON in JavaScript?

Ans: JSON (JavaScript Object Notation) is a lightweight data interchange format used to store and exchange data between a client and a server.

It is language-independent but based on JavaScript object syntax.

Example:

{
  "name": "Alice",
  "age": 25,
  "city": "Delhi"
}

Ques: What does JSON stand for?

Ans: JSON = JavaScript Object Notation

Ques: Why is JSON used?

• To exchange data between client and server.
• It’s easy to read and write.
• Supported by all major programming languages.
• Ideal for AJAX and APIs (e.g., REST APIs).

Ques: What is the syntax of JSON?

Ans: JSON is written as key-value pairs enclosed in {}.

Rules:

• Keys must be in double quotes " ".
• Data is separated by commas.
• Supports objects {} and arrays [].

Example:

{
  "employee": {
    "name": "John",
    "age": 30,
    "skills": ["HTML", "CSS", "JS"]
  }
}

Ques: What are the data types supported in JSON?

• String
• Number
• Boolean
• Null
• Object
• Array

Example:

{
  "name": "Ravi",
  "age": 22,
  "isStudent": true,
  "courses": ["Math", "Science"]
}

Ques: What are invalid JSON data types?

• ndefined
• function()
• Date objects (must be strings)
• Comments

These must be converted before JSON parsing.

Ques: What is the difference between JSON and JavaScript objects?

Feature	JSON	JavaScript Object
Syntax	Keys in double quotes	Keys can be without quotes
Type	Data format	Data structure
Methods	parse(), stringify()	Normal JS methods
Example	{"name": "John"}	{name: "John"}

Ques: How do you convert JSON to a JavaScript object?

Ans: Use the JSON.parse() method.

Example:

let jsonData = '{"name":"John","age":25}';
let obj = JSON.parse(jsonData);
console.log(obj.name); // Output: John

Ques: How do you convert a JavaScript object to JSON?

Ans: Use the JSON.stringify() method.

Example:

let person = { name: "John", age: 25 };
let jsonText = JSON.stringify(person);
console.log(jsonText); // Output: {"name":"John","age":25}

Ques: What is JSON.parse() used for?

Ans: JSON.parse() converts a JSON string into a JavaScript object.

Example:

let obj = JSON.parse('{"x":10, "y":20}');
console.log(obj.x + obj.y); // 30

Ques: What is JSON.stringify() used for?

Ans: JSON.stringify() converts a JavaScript object or array into a JSON string.

Example:

let student = { name: "Alice", marks: 90 };
let jsonString = JSON.stringify(student);
console.log(jsonString);

Ques: What happens if JSON syntax is incorrect during parsing?

Ans: It throws a SyntaxError exception.

Always wrap JSON.parse() in a try...catch block.

Example:

try {
  JSON.parse("{name:'John'}"); // invalid JSON
} catch (e) {
  console.error("Invalid JSON:", e);
}

Ques: How do you handle circular references when stringifying?

Ans: JSON does not support circular structures.

Use libraries like flatted or manually remove circular references.

Ques: Can JSON contain functions or undefined values?

Ans: No, JSON supports only data — not methods, functions, or undefined.

Ques: How do you format (pretty print) a JSON string?

Ans: Pass extra parameters to JSON.stringify():

let obj = { name: "Bob", age: 20 };
console.log(JSON.stringify(obj, null, 2));  // Here 2 adds indentation

Ques: How do you loop through JSON data in JavaScript?

Ans: Convert it to an object first, then loop:

let json = '{"name":"Sam","age":23,"city":"Delhi"}';
let obj = JSON.parse(json);
for (let key in obj) {
  console.log(key + ": " + obj[key]);
}

Ques: How is JSON used with AJAX?

Ans: AJAX requests often send/receive JSON data.

Example:

fetch("data.json")
  .then(res => res.json())
  .then(data => console.log(data));

Ques: How do you send JSON data to a server (POST)?

fetch("server.php", {
  method: "POST",
  headers: { "Content-Type": "application/json" },
  body: JSON.stringify({ name: "John", age: 30 })
})
.then(res => res.json())
.then(data => console.log(data));

Ques: How do you read JSON data from a server (GET)?

fetch("data.json")
  .then(response => response.json())
  .then(data => console.log(data));

Ques: How is JSON handled in PHP?

Ans: PHP provides built-in methods:

$json = '{"name":"John","age":25}';
$obj = json_decode($json);
echo $obj->name; // John

Convert PHP arrays to JSON:

echo json_encode(["name" => "John", "age" => 25]);

Ques: How do you display JSON data in HTML?

<div id="result"></div>
<script>
fetch("user.json")
  .then(res => res.json())
  .then(data => {
    document.getElementById("result").innerHTML = data.name;
  });
</script>

Ques: What’s the difference between JSON.parse() and eval()?

Method	Description
JSON.parse()	Safe and fast; only parses valid JSON
eval()	Executes any JS code (unsafe and insecure)

Ques: What are nested JSON objects?

Ans: When JSON objects contain other objects or arrays as values.

Example:

{
  "user": {
    "name": "Tom",
    "skills": ["HTML", "CSS", "JS"]
  }
}

Access in JS:

data.user.skills[1]; // CSS

Ques: How can you convert an array to JSON?

let arr = ["apple", "banana", "mango"];
let json = JSON.stringify(arr);
console.log(json); // ["apple","banana","mango"]

Ques: How do you convert JSON back to an array?

let json = '["red","green","blue"]';
let arr = JSON.parse(json);
console.log(arr[1]); // green

Ques: How do you validate if a string is valid JSON?

function isValidJSON(str) {
  try {
    JSON.parse(str);
    return true;
  } catch {
    return false;
  }
}

Ques: How do you merge multiple JSON objects?

let obj1 = { a: 1 }, obj2 = { b: 2 };
let merged = { ...obj1, ...obj2 };
console.log(merged); // { a: 1, b: 2 }

Ques: What is the MIME type of JSON?

Ans: application/json

Used in HTTP headers when sending JSON data:

headers: { "Content-Type": "application/json" }

Ques: What are common JSON file extensions?

Ans: .json → Used for configuration files, datasets, APIs.

Examples:

• package.json (Node.js)
• db.json (mock data)

Ques: What are common JSON use cases?

• RESTful APIs
• Configuration files
• Data storage
• Web application data exchange
• NoSQL databases (e.g., MongoDB)