Error response format in Rest API - Time & Space Complexity

Time Complexity: Error response format

O(n)

Understanding Time Complexity

When a REST API sends an error response, it often builds a structured message. We want to understand how the time to create this error message changes as the error details grow.

How does the work to prepare the error response grow with the size of the error information?

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

function createErrorResponse(errors) {
  const response = { status: "error", errors: [] };
  for (const err of errors) {
    response.errors.push({ code: err.code, message: err.message });
  }
  return response;
}

This code builds an error response object by looping over a list of error details and adding each to the response.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: Looping through the list of errors to add each error detail to the response.
• How many times: Once for each error in the input list.

How Execution Grows With Input

As the number of errors increases, the code adds more items to the response one by one.

Input Size (n)	Approx. Operations
10	About 10 additions to the errors array
100	About 100 additions to the errors array
1000	About 1000 additions to the errors array

Pattern observation: The work grows directly with the number of errors; doubling errors doubles the work.

Final Time Complexity

Time Complexity: O(n)

This means the time to create the error response grows in a straight line with the number of errors.

Common Mistake

[X] Wrong: "Adding more errors won't affect the time much because it's just one response."

[OK] Correct: Each error detail is added one by one, so more errors mean more work and more time.

Interview Connect

Understanding how error responses scale helps you design APIs that handle many errors efficiently and predict how your code behaves as input grows.

Self-Check

What if we changed the error list to a nested structure with sub-errors? How would the time complexity change?

Practice

1. What is the main purpose of an error response format in a REST API?

easy

A. To speed up the API response time

B. To clearly communicate what went wrong during a request

C. To encrypt the data sent to the client

D. To store user information securely

Solution

1. Step 1: Understand the role of error responses

   Error responses tell the client what problem happened when processing their request.

2. Step 2: Identify the main goal

   The main goal is to communicate the error clearly so the client can handle it properly.

3. Final Answer:

   To clearly communicate what went wrong during a request -> Option B

4. Quick Check:

   Error response purpose = clear communication [OK]

Hint: Error responses explain problems clearly to clients [OK]

Common Mistakes:

• Thinking error responses speed up API
• Confusing error format with encryption
• Assuming error responses store data

2. Which of the following is a correct JSON structure for a simple error response in a REST API?

easy

A. {"error": {"code": 404, "message": "Not Found"}}

B. {"status": 200, "message": "Success"}

C. {"data": {"id": 1, "name": "Item"}}

D. {"error_code": "404", "error_msg": "Not Found"}

Solution

1. Step 1: Identify standard error response keys

   Common keys include "error", "code", and "message" to describe the problem.

2. Step 2: Check each option

   {"error": {"code": 404, "message": "Not Found"}} uses "error" with "code" and "message" correctly in JSON format.

3. Final Answer:

   {"error": {"code": 404, "message": "Not Found"}} -> Option A

4. Quick Check:

   Standard error keys = code and message [OK]

Hint: Look for 'error', 'code', and 'message' keys in JSON [OK]

Common Mistakes:

• Confusing success response with error
• Using wrong key names like error_code
• Missing nested error object

3. Given this error response JSON:

{"error": {"code": 401, "message": "Unauthorized"}}

What HTTP status code should the API return?

medium

A. 401

B. 500

C. 404

D. 200

Solution

1. Step 1: Match error code to HTTP status

   The error code 401 means "Unauthorized", which is an HTTP status code for authentication failure.

2. Step 2: Confirm correct HTTP status

   The API should return HTTP status 401 to match the error code in the response body.

3. Final Answer:

   401 -> Option A

4. Quick Check:

   Error code 401 = HTTP status 401 [OK]

Hint: Match error code with HTTP status code [OK]

Common Mistakes:

• Returning 200 for error responses
• Confusing 401 with 404
• Using 500 for client errors

4. You receive this error response from an API:

{"error": {"code": "403", "message": "Forbidden"}}

But your client expects the code as an integer. What is the best fix?

medium

A. Change the message to "Access Denied"

B. Remove the code field entirely

C. Change the code value to an integer 403 in the response

D. Wrap the code in quotes to make it a string

Solution

1. Step 1: Identify data type mismatch

   The code is a string "403" but the client expects an integer 403.

2. Step 2: Fix the data type in the response

   Change the code value from a string to an integer to match client expectations.

3. Final Answer:

   Change the code value to an integer 403 in the response -> Option C

4. Quick Check:

   Match data types for code field [OK]

Hint: Ensure error code data type matches client expectation [OK]

Common Mistakes:

• Removing important error fields
• Changing message instead of code type
• Leaving code as string when integer needed

5. You want to design an error response format that includes an error code, a message, and optional details for debugging. Which JSON structure below best fits this need?

hard

A. {"error": "Bad Request", "code": 400}

B. {"status": "error", "code": 400, "msg": "Bad Request"}

C. {"error_code": 400, "error_message": "Bad Request"}

D. {"error": {"code": 400, "message": "Bad Request", "details": "Missing 'id' parameter"}}

Solution

1. Step 1: Check required fields in each option

   We need code, message, and optional details inside a clear error object.

2. Step 2: Evaluate each JSON structure

   {"error": {"code": 400, "message": "Bad Request", "details": "Missing 'id' parameter"}} includes all three: code, message, and details nested inside "error".

3. Final Answer:

   {"error": {"code": 400, "message": "Bad Request", "details": "Missing 'id' parameter"}} -> Option D

4. Quick Check:

   Include code, message, and details in error object [OK]

Hint: Include optional details inside error object for clarity [OK]

Common Mistakes:

• Omitting details field
• Using inconsistent key names
• Not nesting error info properly