JavaScript JSON & Data Handling - Practice Questions 2026

Mastering data manipulation is the backbone of modern web development. Whether you are building dynamic web applications, working with APIs, or managing complex state, your ability to handle JSON and JavaScript data structures determines the quality of your code. Welcome to the most comprehensive practice exams designed specifically to help you prepare for your JavaScript JSON & Data Handling challenges.

Why Serious Learners Choose These Practice Exams

Serious learners understand that watching tutorials is only half the battle. To truly master a topic, you must test your knowledge under pressure. This course is designed for developers who want to move beyond syntax and understand the logic of data flow. By choosing these practice exams, you are investing in a rigorous assessment tool that mirrors the complexity of professional development environments. We provide a massive, original question bank that forces you to think critically about how data is structured, parsed, and manipulated in JavaScript.

Course Structure

This course is organized into distinct modules to ensure a logical progression of difficulty:

• Basics / Foundations: This section focuses on the fundamental syntax of JSON and JavaScript objects. You will be tested on the differences between JSON strings and JavaScript objects, valid data types in JSON, and basic object access patterns.

• Core Concepts: Here, we dive deeper into essential methods like JSON. parse() and JSON. stringify(). You will learn to navigate common pitfalls, such as handling deep-nested objects and understanding how JavaScript treats arrays versus objects during serialization.

• Intermediate Concepts: This module covers data transformation techniques. You will practice using array methods like map(), filter(), and reduce() specifically in the context of processing JSON data retrieved from external sources.

• Advanced Concepts: Challenge yourself with complex topics like the reviver and replacer parameters in JSON methods, handling circular references, and managing asynchronous data fetching using Promises and Async/Await.

• Real-world Scenarios: These questions simulate actual tasks you will face on the job, such as cleaning "dirty" API data, restructuring objects for UI components, and optimizing data handling for performance.

• Mixed Revision / Final Test: A comprehensive cumulative exam that pulls from all previous sections to ensure you have retained the knowledge and are ready for any professional assessment.

Sample Questions

QUESTION 1

What will be the result of the following code?

const user = { id: 1, name: "Alice", age: undefined };

const jsonString = JSON. stringify(user);

console. log(jsonString);

• OPTION 1: {"id":1,"name":"Alice","age":undefined}

• OPTION 2: {"id":1,"name":"Alice","age":null}

• OPTION 3: {"id":1,"name":"Alice"}

• OPTION 4: {"id":1,"name":"Alice","age":""}

• OPTION 5: SyntaxError

CORRECT ANSWER: OPTION 3

CORRECT ANSWER EXPLANATION: In JavaScript, when using JSON. stringify(), any property whose value is undefined is omitted (skipped) from the resulting JSON string. This is because undefined is not a valid data type in the JSON specification.

WRONG ANSWERS EXPLANATION:

• OPTION 1: This is wrong because "undefined" is not a valid JSON value; JSON only supports null, strings, numbers, booleans, objects, and arrays.

• OPTION 2: This is wrong because JSON. stringify() does not automatically convert undefined to null for object properties (though it does so for array elements).

• OPTION 4: This is wrong because the method does not convert undefined to an empty string.

• OPTION 5: This is wrong because the code is syntactically correct; JSON. stringify() handles undefined values gracefully by ignoring them.

QUESTION 2

Which of the following is a valid JSON string?

• OPTION 1: { 'name': 'John' }

• OPTION 2: { "name": "John", }

• OPTION 3: { "age": 025 }

• OPTION 4: [ "Apple", "Orange", ]

• OPTION 5: { "isVerified": true }

CORRECT ANSWER: OPTION 5

CORRECT ANSWER EXPLANATION: A valid JSON object must use double quotes for both keys and string values, and it must contain valid data types like booleans (true/false).

WRONG ANSWERS EXPLANATION:

• OPTION 1: This is wrong because JSON strictly requires double quotes (") for keys and strings, not single quotes (').

• OPTION 2: This is wrong because JSON does not allow trailing commas after the last property in an object.

• OPTION 3: This is wrong because JSON numbers cannot have leading zeros (unless the number is 0 followed by a decimal point).

• OPTION 4: This is wrong because, like objects, arrays in JSON cannot have a trailing comma after the last element.

Course Features

• You can retake the exams as many times as you want to ensure mastery.

• This is a huge original question bank designed to challenge all skill levels.

• You get support from instructors if you have questions regarding specific logic or answers.

• Each question has a detailed explanation to turn mistakes into learning opportunities.

• Mobile-compatible with the Udemy app so you can practice on the go.

• 30-days money-back guarantee if you are not satisfied with the course quality.

We hope that by now you are convinced! And there are a lot more questions inside the course.