Lesson test: Values, names and types

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Values, names and types

1.[2p]
Why does `0.1 + 0.2 === 0.3` evaluate to `false`?

One tenth has no exact binary representation, so the stored values are slightly off and their rounded sum is a different double from the nearest double to 0.3

Correct

JavaScript numbers are not accurate enough for two decimal places

The answer is: One tenth has no exact binary representation, so the stored values are slightly off and their rounded sum is a different double from the nearest double to 0.3

=== compares types rather than values, and the two sides have different types

The answer is: One tenth has no exact binary representation, so the stored values are slightly off and their rounded sum is a different double from the nearest double to 0.3

Addition of small numbers is performed in a different order than written

The answer is: One tenth has no exact binary representation, so the stored values are slightly off and their rounded sum is a different double from the nearest double to 0.3

2.[2p]
What string does `typeof null` return? Write it without quotes.

CorrectNot quite: object

3.[2p]
`"5" - 3` evaluates to the number 2, while `"5" + 3` evaluates to the string `"53"`. Why do the two coerce in opposite directions?

+ has a string meaning, concatenation, so the number becomes a string; - has no string meaning, so the string becomes a number

Correct

- is evaluated before + in all cases

The answer is: `+` has a string meaning, concatenation, so the number becomes a string; `-` has no string meaning, so the string becomes a number

Subtraction always returns a number and addition always returns a string

The answer is: `+` has a string meaning, concatenation, so the number becomes a string; `-` has no string meaning, so the string becomes a number

The left operand decides the type for - and the right operand decides it for +

The answer is: `+` has a string meaning, concatenation, so the number becomes a string; `-` has no string meaning, so the string becomes a number

4.[2p]
What does `1 + 2 + "3"` evaluate to? Write the result without quotes.

CorrectNot quite: 33

5.[1p]
`NaN === NaN` evaluates to `false`, which is why `Number.isNaN(x)` exists.

True

Correct

False

The answer is: True

6.[2p]
What does `2 ** 10 % 100` evaluate to?

CorrectNot quite: 24

7.[2p]
Which of these are true of `const`?

Select all that apply

A name declared with const cannot be rebound to another value

Correct

Reaching for const first and switching to let only when reassignment is needed is the working habit

Correct

const guarantees the value it holds can never change in any way

The answer is: A name declared with `const` cannot be rebound to another value, Reaching for `const` first and switching to `let` only when reassignment is needed is the working habit

const names must be written in capitals

The answer is: A name declared with `const` cannot be rebound to another value, Reaching for `const` first and switching to `let` only when reassignment is needed is the working habit

8.[2p]
Put these steps in the order the machine performs them when running `count = count + 1`.

Add 1 to that value
Look up the value currently held by count
Bind the name count to that result

Show the answer

b, c, a

9.[3p]
Match each expression to the value it produces.

7 / 2
"2" + "3"
Number("abc")
1 / 0
"hello".length

5
3.5
NaN
Infinity
the string "23"

Show the answer

7 / 2: 3.5 "2" + "3": the string "23" Number("abc"): NaN 1 / 0: Infinity "hello".length: 5

1.[2p] Why does 0.1 + 0.2 === 0.3 evaluate to false?

2.[2p] What string does typeof null return? Write it without quotes.

3.[2p] "5" - 3 evaluates to the number 2, while "5" + 3 evaluates to the string "53". Why do the two coerce in opposite directions?

4.[2p] What does 1 + 2 + "3" evaluate to? Write the result without quotes.

5.[1p] NaN === NaN evaluates to false, which is why Number.isNaN(x) exists.

6.[2p] What does 2 ** 10 % 100 evaluate to?

7.[2p] Which of these are true of const?

8.[2p] Put these steps in the order the machine performs them when running count = count + 1.

9.[3p] Match each expression to the value it produces.

1.[2p]
Why does `0.1 + 0.2 === 0.3` evaluate to `false`?

2.[2p]
What string does `typeof null` return? Write it without quotes.

3.[2p]
`"5" - 3` evaluates to the number 2, while `"5" + 3` evaluates to the string `"53"`. Why do the two coerce in opposite directions?

4.[2p]
What does `1 + 2 + "3"` evaluate to? Write the result without quotes.

5.[1p]
`NaN === NaN` evaluates to `false`, which is why `Number.isNaN(x)` exists.

6.[2p]
What does `2 ** 10 % 100` evaluate to?

7.[2p]
Which of these are true of `const`?

8.[2p]
Put these steps in the order the machine performs them when running `count = count + 1`.

9.[3p]
Match each expression to the value it produces.