js_all_error_flashcards

Question 1

What is an Error in JavaScript?

Answer 1

An error indicates an unexpected or exceptional event during code execution.
It disrupts the normal program flow.
Examples: Syntax errors, type errors, invalid operations, network issues.

Question 2

Name the three most common built-in Error Types.

Answer 2

SyntaxError: Violation of JavaScript’s grammar rules.
TypeError: Incorrect data type used in an operation.
ReferenceError: Using an undeclared variable.

Question 3

What are some other built-in Error types in JavaScript?

Answer 3

RangeError: Number outside the allowed range.
URIError: Problem with encodeURI() or decodeURI().
EvalError: Error in the eval() function (largely deprecated).
InternalError: Internal JavaScript engine error (rare).

Question 4

Why use try…catch blocks?

Answer 4

To catch errors and prevent crashes.
To handle errors gracefully (e.g., provide fallback logic).
To isolate potentially problematic code.
To improve application reliability.

Question 5

Describe the try block.

Answer 5

Contains code that might throw an error.
If an error occurs, execution jumps to the catch block.

Question 6

Describe the catch block.

Answer 6

Contains code that handles an error thrown in the try block.
Receives the error object as an argument.

Question 7

What is the purpose of the finally block in a try…catch statement?

Answer 7

Contains code that always executes, regardless of whether an error occurred or not.
Used for cleanup tasks (e.g., closing files, releasing resources).

Question 8

Describe the Error Object.

Answer 8

Created when an error occurs.
Properties:
name: Error type (e.g., “TypeError”, “ReferenceError”).
message: Human-readable error description.
stack: Call stack showing function sequence (not always available).

Question 9

Explain what a ‘call stack’ is in the context of error handling.

Answer 9

A record of functions called leading up to an error.
Used to trace and debug the source of the error.

Question 10

What does the term ‘throw an error’ mean in JavaScript?

Answer 10

Intentionally create and raise an error using the throw keyword.
JavaScript engine searches for a try…catch block to handle it.

Question 11

Besides error handling, what’s another use of throwing errors?

Answer 11

To report validation errors.
To stop execution when encountering unresolvable issues.

Question 12

What is an ‘unhandled rejection’?

Answer 12

Occurs when a Promise is rejected and no .catch() handler is attached.
Browsers and Node.js report unhandled rejections.

Question 13

What is the primary purpose of the try…catch statement in JavaScript?

Answer 13

To handle errors gracefully, prevent application crashes, and provide safe code execution.

Question 14

What code goes inside the try block?

Answer 14

Code that you suspect might throw an error.

Question 15

What happens if an error occurs inside the try block?

Answer 15

Execution stops immediately and control jumps to the catch block.

Question 16

What code goes inside the catch block?

Answer 16

Code that handles the error, logs it, or provides a fallback.

Question 17

What is the purpose of the error parameter in the catch block?

Answer 17

It holds the error object with details like name, message, and stack.

Question 18

What are the typical properties of an error object?

Answer 18

name: Type of error
message: Description of error
stack: Call stack trace (optional)

Question 19

What happens after the catch block finishes executing?

Answer 19

Unless interrupted (return/throw), execution continues after the try…catch.

Question 20

What is the purpose of the finally block, and when is it executed?

Answer 20

Always executes, regardless of errors.
Used for cleanup (closing files, etc.).

Question 21

Give an example of a common use case for the finally block.

Answer 21

Closing a file, releasing a DB connection, or cleaning up resources.

Question 22

Why is try…catch useful when dealing with user input?

Answer 22

User input is unpredictable and may cause errors.

Question 23

Why is try…catch important when making API calls?

Answer 23

API calls may fail due to network/server issues.
try…catch lets you handle failures.

Question 24

What is ‘fallback logic’ and how is it used in try…catch?

Answer 24

Alternative actions when primary code fails.
Implemented in the catch block.

Question 25

What is an 'Unhandled Rejection' and how does it relate to try...catch?

Answer 25

Error in a Promise without a .catch(). Use try...catch with async/await or .catch() for Promises.

Question 26

Explain try...catch vs .catch for Promise handling.

Answer 26

try...catch handles sync and async with await. .catch handles rejections in Promises.

Question 27

Can you nest try...catch blocks? Why?

Answer 27

Yes. Use for localized and broader error handling.

Question 28

What is the purpose of re-throwing an error?

Answer 28

Allows higher-level error handling after partial handling or logging.

Question 29

What is the purpose of throw in JavaScript?

Answer 29

Raises an error, interrupts execution, and sends control to a try...catch block.

Question 30

Should you throw a string or an Error object? Why?

Answer 30

Use an Error object. Includes name, message, and stack trace. String lacks structure.

Question 31

What is the purpose of the message property when throwing an Error?

Answer 31

Provides a human-readable description of the error. Useful for debugging.

Question 32

Give examples of scenarios where throwing custom errors is useful.

Answer 32

Form validation, function misuse, API failure, missing files, invalid data.

Question 33

What are best practices for custom error messages?

Answer 33

Be specific, informative, and user-friendly. Avoid vague messages. Include relevant data.

Question 34

How can you create a custom error class in JavaScript?

Answer 34

Use `class CustomError extends Error` with a constructor that calls super(message).

Question 35

Why is creating a custom error class useful?

Answer 35

Enables instanceof checks, targeted error handling, better organization.

Question 36

How would you catch a specific custom error?

Answer 36

Use instanceof in the catch block to filter error types.

Question 37

What’s the most common pitfall when working with loops?

Answer 37

Infinite loops. Prevent by checking conditions, using debug tools, and iteration limits.

Question 38

Why is using for...in with arrays a bad idea?

Answer 38

Iterates property keys (not values), may include inherited properties. Use for or for...of.

Question 39

What’s the problem with having an empty catch block?

Answer 39

Suppresses errors, making debugging harder. Always handle or log errors.

Question 40

Why should you use meaningful error messages?

Answer 40

Helps identify and fix issues quickly. Generic messages are not helpful.

Question 41

What are strategies to avoid deeply nested loops?

Answer 41

Break logic into functions. Use map/filter/reduce. Avoid unnecessary nesting.

Question 42

What is 'premature optimization' in loops?

Answer 42

Optimizing before knowing if there's a performance issue. Focus on clarity first.

Question 43

What is a 'loop invariant' and why is it useful?

Answer 43

Condition true before/during/after each iteration. Helps ensure loop correctness.

Question 44

forEach vs for...of: What’s a key control flow difference?

Answer 44

You can’t use break/continue in forEach. Use for...of if you need them.

Question 45

What is an 'off-by-one' error and how to avoid it?

Answer 45

Loop runs 1 time too few/many. Check < vs <= conditions. Test edge cases.