Lesson test: Objects and nested data

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Objects and nested data

1.[2p]
`const key = "port";` What does `config.key` give when `config` is `{ host: "localhost", port: 8080 }`?

undefined, because dot notation looks for a key literally spelled key

Correct

8080, the same as config[key]

The answer is: `undefined`, because dot notation looks for a key literally spelled `key`

"port", the value held by the variable

The answer is: `undefined`, because dot notation looks for a key literally spelled `key`

A TypeError, because key is not a property

The answer is: `undefined`, because dot notation looks for a key literally spelled `key`

2.[3p]
Why does `order.payment.method` throw while `order.customer.email` merely gives `undefined`?

order.payment is undefined and undefined has no properties, whereas order.customer is an object that simply lacks that key

Correct

Reading two dots deep is always an error

The answer is: `order.payment` is `undefined` and `undefined` has no properties, whereas `order.customer` is an object that simply lacks that key

method is a reserved word

The answer is: `order.payment` is `undefined` and `undefined` has no properties, whereas `order.customer` is an object that simply lacks that key

email has a default value and method does not

The answer is: `order.payment` is `undefined` and `undefined` has no properties, whereas `order.customer` is an object that simply lacks that key

3.[2p]
What operator makes `order.payment.method` give `undefined` instead of throwing? Write the two characters.

CorrectNot quite: ?.

4.[3p]
Which of these survive a `JSON.stringify` and `JSON.parse` round trip unchanged?

Select all that apply

A number

Correct

An array of strings

Correct

A nested object

Correct

A key whose value is undefined

The answer is: A number, An array of strings, A nested object

A Date

The answer is: A number, An array of strings, A nested object

5.[3p]
In `counts[key] = (counts[key] ?? 0) + amount`, why is `??` used rather than `||`?

|| would also replace a legitimate running total of 0, since 0 is falsy

Correct

|| cannot be used on object properties

The answer is: `||` would also replace a legitimate running total of 0, since 0 is falsy

?? is faster

The answer is: `||` would also replace a legitimate running total of 0, since 0 is falsy

|| returns a boolean rather than a value

The answer is: `||` would also replace a legitimate running total of 0, since 0 is falsy

6.[2p]
A tree has a root of size 0 with children of size 3 and 0, and the second of those has children of size 5 and 7. What does a recursive `totalSize` return for the root?

CorrectNot quite: 15

7.[3p]
Match each part of a recursive function to what it does.

Base case
Recursive case
Combining step
Stack overflow

answers directly, with no further call
what happens when the descent never reaches the base case
calls itself on smaller pieces of the same shape
turns the results of those calls into this call's answer

Show the answer

Base case: answers directly, with no further call Recursive case: calls itself on smaller pieces of the same shape Combining step: turns the results of those calls into this call's answer Stack overflow: what happens when the descent never reaches the base case

8.[2p]
Object keys are always strings, so `counts[1]` and `counts["1"]` reach the same entry.

True

Correct

False

The answer is: True

9.[2p]
When is a `Map` the better choice than a plain object?

When the keys are data rather than field names you wrote yourself

Correct

When the values are numbers

The answer is: When the keys are data rather than field names you wrote yourself

When there are fewer than ten entries

The answer is: When the keys are data rather than field names you wrote yourself

When the data has to be turned into JSON

The answer is: When the keys are data rather than field names you wrote yourself

1.[2p] const key = "port"; What does config.key give when config is { host: "localhost", port: 8080 }?

2.[3p] Why does order.payment.method throw while order.customer.email merely gives undefined?

3.[2p] What operator makes order.payment.method give undefined instead of throwing? Write the two characters.

4.[3p] Which of these survive a JSON.stringify and JSON.parse round trip unchanged?

5.[3p] In counts[key] = (counts[key] ?? 0) + amount, why is ?? used rather than ||?

6.[2p] A tree has a root of size 0 with children of size 3 and 0, and the second of those has children of size 5 and 7. What does a recursive totalSize return for the root?

7.[3p] Match each part of a recursive function to what it does.

8.[2p] Object keys are always strings, so counts[1] and counts["1"] reach the same entry.

9.[2p] When is a Map the better choice than a plain object?

1.[2p]
`const key = "port";` What does `config.key` give when `config` is `{ host: "localhost", port: 8080 }`?

2.[3p]
Why does `order.payment.method` throw while `order.customer.email` merely gives `undefined`?

3.[2p]
What operator makes `order.payment.method` give `undefined` instead of throwing? Write the two characters.

4.[3p]
Which of these survive a `JSON.stringify` and `JSON.parse` round trip unchanged?

5.[3p]
In `counts[key] = (counts[key] ?? 0) + amount`, why is `??` used rather than `||`?

6.[2p]
A tree has a root of size 0 with children of size 3 and 0, and the second of those has children of size 5 and 7. What does a recursive `totalSize` return for the root?

7.[3p]
Match each part of a recursive function to what it does.

8.[2p]
Object keys are always strings, so `counts[1]` and `counts["1"]` reach the same entry.

9.[2p]
When is a `Map` the better choice than a plain object?