Top JSON Interview Questions and Answers (2025)

Question: Can JSON support comments?

Answer:

No, JSON (JavaScript Object Notation) does not support comments.

Unlike some other data formats (such as JavaScript or XML), JSON is strictly designed to be a data format for transmitting and storing information. It is intended to be simple and lightweight, and as part of this, comments are not allowed in JSON syntax.

Why JSON Does Not Support Comments:

1. Simplicity: JSON was designed to be minimalistic and easy for both humans and machines to read and write.
2. Interoperability: Since JSON is widely used in APIs, configuration files, and data exchange between systems, allowing comments could lead to parsing errors or unintended behavior in different platforms or libraries that might not support them.
3. Parsing Efficiency: Allowing comments would make the parsing process more complex. Without comments, parsers can efficiently process the JSON data without needing to account for additional parsing rules.

Example of Invalid JSON with Comments:

{
  "name": "John",  // This is a comment
  "age": 30,
  "city": "New York" /* This is another comment */
}

• The above JSON is invalid because JSON parsers do not support comments (// for single-line comments or /* */ for multi-line comments).

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Workarounds for Adding Comments in JSON:

Although JSON itself does not support comments, there are some workarounds you can use to include metadata or explanatory information:

1. Using a “comment” key: You can include a property with a key like "comment" or "notes", but it will be treated as actual data.

   {
     "name": "John",
     "age": 30,
     "notes": "This is a comment"
   }

2. Using non-data keys: Some systems allow the inclusion of non-essential data that can act as “comments.” For example, using a special key such as _comment:

   {
     "name": "John",
     "_comment": "This is a comment explaining the name",
     "age": 30
   }

   However, this will still be part of the data and can be processed or ignored by consumers of the JSON.

3. External Documentation: If you need to document the structure or data format of your JSON, you can maintain separate documentation that explains the contents and purpose of each key and value.

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Summary:

• JSON does not support comments.
• If you need to add explanations, consider using a "comment" key or keeping separate documentation.
• Using comments in JSON is likely to break parsers or lead to errors in systems that expect pure JSON data.

Question: How do you validate a JSON string?

Answer:

Validating a JSON string typically involves checking whether the string is well-formed and conforms to the JSON syntax rules. In JavaScript, the most common way to validate a JSON string is by using the built-in JSON.parse() method, which tries to parse the string and throws an error if the string is invalid.

Here’s how you can validate a JSON string in JavaScript:

1. Using JSON.parse() Method

The simplest way to validate a JSON string in JavaScript is to use JSON.parse(). If the string is not valid JSON, it will throw an error, which you can catch and handle.

Example:

function isValidJSON(jsonString) {
    try {
        // Try parsing the JSON string
        JSON.parse(jsonString);
        return true;  // If parsing succeeds, it is valid JSON
    } catch (e) {
        return false;  // If parsing fails, it is invalid JSON
    }
}

// Example usage:
const jsonString = '{"name": "John", "age": 30}';
console.log(isValidJSON(jsonString));  // Output: true

const invalidJsonString = '{"name": "John", "age": 30';  // Missing closing brace
console.log(isValidJSON(invalidJsonString));  // Output: false

Steps:

1. JSON.parse() attempts to convert the input string into a JavaScript object.
2. If the input string is a valid JSON, it returns the corresponding object, and you can conclude that the JSON is valid.
3. If the string is malformed, JSON.parse() throws an error (typically a SyntaxError), and you can handle it by returning false.

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2. Online Tools and Libraries

If you’re working in environments where you cannot use JSON.parse() (such as non-JavaScript environments), there are other ways to validate JSON:

• Online Tools: Websites like JSONLint or JSON Formatter & Validator allow you to paste JSON strings and check whether they are valid.

• Libraries: In other languages or frameworks, you can use libraries to validate JSON:

  • Python: Use the json module’s json.loads() function.
  • Node.js: The JSON.parse() method works in Node.js, just like in the browser.
  • Go: Use the encoding/json package to validate JSON strings.

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3. Common JSON Validation Checks

In addition to using JSON.parse(), you can manually validate the following common mistakes in JSON strings:

1. Mismatched Braces and Brackets:

   • Ensure that curly braces {} for objects and square brackets [] for arrays are properly balanced.

2. Missing or Extra Commas:

   • Ensure there is no trailing comma after the last element in objects or arrays.

3. Improper Quotation Marks:

   • JSON keys and string values must be enclosed in double quotes ("), not single quotes (').

4. Correct Data Types:

   • Ensure values are of the correct types: string values in double quotes, numbers without quotes, booleans as true or false, etc.

5. Escape Sequences in Strings:

   • JSON strings must properly escape special characters like double quotes (\"), backslashes (\\), etc.

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Example of Invalid JSON Issues:

1. Trailing Comma:

   {
     "name": "John",
     "age": 30,  // Invalid: Trailing comma
   }

2. Incorrect Quotation Marks:

   {
     'name': 'John'  // Invalid: Single quotes used instead of double quotes
   }

3. Unbalanced Braces:

   {
     "name": "John"  // Invalid: Missing closing brace

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Summary:

• The most common way to validate a JSON string in JavaScript is using the JSON.parse() method inside a try-catch block.
• If the string is well-formed, JSON.parse() will successfully parse it into a JavaScript object. If it’s malformed, it will throw an error, allowing you to identify it as invalid.
• For other environments or additional tools, you can use online validators or specific libraries to check if the JSON is valid.

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