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?

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()?

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)