Evaluating Expression Syntax

Question 1

Differences between ‘Undefined’ and ‘Null’? When would a return value be ‘Undefined’ vs. ‘Null’?

Answer 1

• ‘Null’ is considered a value; can be assigned
• Typeof null is object; means ‘no object’
• ‘Null’ is return value for undefined objects or invalid expressions
• Undefined indicates that no value assigned
• Undefined is returned for primitives

Question 2

Undefined != Null

Answer 2

True. ‘Null’ is a value and has type of object. Undefined has no value.

Question 3

NaN != NaN

Answer 3

True. NaN can only be used in comparison expressions

Question 4

Bool value of ‘Null’

Answer 4

False

Question 5

Bool value of ‘Undefined’

Answer 5

False

Question 6

ES6 method to check NaN?

Answer 6

Number.isNAN()

Question 7

Default return value of a function if not specified?

Answer 7

Undefined

Question 8

Are semicolons required after statements?

Answer 8

No, but they’re recommended for clarity

Question 9

What comparison is safe to evaluate null?

Answer 9

Strict equality (returns 0 if type coersion is applied by ==)

Question 10

Type of NaN?

Answer 10

Number

Question 11

Type of Infinity?

Answer 11

Number (+ or - infinity)

Question 12

In dot notation, everything to the left of a dot is ________

Answer 12

an object

Question 13

Numeric value of ‘undefined’

Answer 13

NaN

Question 14

Numeric value of ‘null’

Answer 14

0

ex: value of a null variable used in a calculation, value if type coercion is applied

Question 15

What is the bool value of an object?

Answer 15

True

Question 16

Name 6 things considered ‘falsey’

Answer 16

false, 0, NaN, ‘ ‘, undefined, null

Question 17

Operator to concat strings

Answer 17

+

Question 18

Escape character

Answer 18

\ backslash

Question 19

What type of expression is…

{ foo: bar(3, 5)}

Answer 19

Object literal. Expression that looks like a statement

Question 20

What type of expression is…

function foo( ){ }

Answer 20

Named function expression

Question 21

let n = “25”

1. Convert to number with single operator
2. Convert to number with method

Answer 21

1. +n //not ideal because it’s unclear

2. number(n) //verbose but clear

Question 22

Preferred quote style for JS?

Answer 22

Single

Question 23

Preferred quote style for HTML?

Answer 23

Double

Question 24

Quote style for JSON?

Answer 24

Double (required)

Question 25

What is the difference between these function definitions? Do they have the same scope?

A) var square = function(x) { return x * x; };

B) function square(x) {
return x*x;
}

Answer 25

A) Expression with function keyword assigned to variable

B) Function declaration with keyword

• var ‘square’ has function scope; as a variable, it cannot be hoisted. ; function ‘square’ can be hoisted

Question 26

Can a function be used in an arbitrary expression?

Answer 26

Yes, a function is considered a ‘value’. It can be part of an expression (not called like a method).