Lesson test: Errors and exceptions

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Errors and exceptions

1.[3p]
What does a stack trace tell you?

Where the program stopped and the chain of calls that led there, which is not necessarily where the mistake is

Correct

The exact line containing the bug

The answer is: Where the program stopped and the chain of calls that led there, which is not necessarily where the mistake is

Which variables held wrong values

The answer is: Where the program stopped and the chain of calls that led there, which is not necessarily where the mistake is

How long each function took to run

The answer is: Where the program stopped and the chain of calls that led there, which is not necessarily where the mistake is

2.[3p]
Match each error kind to what causes it.

ReferenceError
TypeError
RangeError
SyntaxError

text that is not a valid program, including malformed JSON
a name that does not exist, usually a typo
a value outside an allowed range, or a recursion with no reachable base case
a value that is not the kind the operation needs

Show the answer

ReferenceError: a name that does not exist, usually a typo TypeError: a value that is not the kind the operation needs RangeError: a value outside an allowed range, or a recursion with no reachable base case SyntaxError: text that is not a valid program, including malformed JSON

3.[3p]
Put the frames of a stack trace in the order they are printed, from top to bottom.

The function that called the one that failed
The function where execution stopped
The top level line that started everything

Show the answer

c, a, b

4.[3p]
Why is `catch (err) { }` with an empty body worse than not catching at all?

A crash names the failure, points at a line and stops before the damage spreads, and swallowing it destroys all three

Correct

It is slower than letting the error travel

The answer is: A crash names the failure, points at a line and stops before the damage spreads, and swallowing it destroys all three

An empty block is a syntax error in strict mode

The answer is: A crash names the failure, points at a line and stops before the damage spreads, and swallowing it destroys all three

It converts the error into a TypeError further up

The answer is: A crash names the failure, points at a line and stops before the damage spreads, and swallowing it destroys all three

5.[2p]
`finally` runs even when the `try` block exits through a `return`.

True

Correct

False

The answer is: True

6.[2p]
`throw "oops"` is legal. Why throw `new Error("oops")` instead?

An Error object carries a name, a message and a stack, and a bare string carries none of them

Correct

A string cannot be caught by catch

The answer is: An `Error` object carries a name, a message and a stack, and a bare string carries none of them

Strings are copied by reference when thrown

The answer is: An `Error` object carries a name, a message and a stack, and a bare string carries none of them

throw requires an object in strict mode

The answer is: An `Error` object carries a name, a message and a stack, and a bare string carries none of them

7.[3p]
Which of these are reasons to validate input at the edge of a program rather than in every function?

Select all that apply

The checks that remain are few enough to be worth reading

Correct

The failure is reported at the point the bad data arrived

Correct

Values inside the program can then be trusted

Correct

It removes the need for a stack trace

The answer is: The checks that remain are few enough to be worth reading, The failure is reported at the point the bad data arrived, Values inside the program can then be trusted

8.[2p]
Which grade of wrongness produces no message of any kind? Write one word.

CorrectNot quite: logic

9.[2p]
A syntax error is reported at line 60, but the file looks correct there. Where is the mistake most likely to be?

Above line 60, where an unclosed brace or quote left everything after it parseable

Correct

Below line 60, in code the parser had not reached

The answer is: Above line 60, where an unclosed brace or quote left everything after it parseable

On line 60, in a character that does not display

The answer is: Above line 60, where an unclosed brace or quote left everything after it parseable

Nowhere in the file, since syntax errors come from the runtime

The answer is: Above line 60, where an unclosed brace or quote left everything after it parseable

1.[3p] What does a stack trace tell you?

2.[3p] Match each error kind to what causes it.

3.[3p] Put the frames of a stack trace in the order they are printed, from top to bottom.

4.[3p] Why is catch (err) { } with an empty body worse than not catching at all?

5.[2p] finally runs even when the try block exits through a return.

6.[2p] throw "oops" is legal. Why throw new Error("oops") instead?

7.[3p] Which of these are reasons to validate input at the edge of a program rather than in every function?

8.[2p] Which grade of wrongness produces no message of any kind? Write one word.

9.[2p] A syntax error is reported at line 60, but the file looks correct there. Where is the mistake most likely to be?

1.[3p]
What does a stack trace tell you?

2.[3p]
Match each error kind to what causes it.

3.[3p]
Put the frames of a stack trace in the order they are printed, from top to bottom.

4.[3p]
Why is `catch (err) { }` with an empty body worse than not catching at all?

5.[2p]
`finally` runs even when the `try` block exits through a `return`.

6.[2p]
`throw "oops"` is legal. Why throw `new Error("oops")` instead?

7.[3p]
Which of these are reasons to validate input at the edge of a program rather than in every function?

8.[2p]
Which grade of wrongness produces no message of any kind? Write one word.

9.[2p]
A syntax error is reported at line 60, but the file looks correct there. Where is the mistake most likely to be?