Control Flow

Question 1

What is flow control?

Answer 1

When writing programs, you want your data to take the correct path. You want it to turn left or right, up, down, reverse, or proceed straight ahead when it’s supposed to.

We call this flow control.

Question 2

What is a conditional?

Answer 2

A conditional is a fork (or multiple forks) in the road.

They use the keywords ‘if’ and ‘else’

Question 3

What is a clause in this context?

Answer 3

A block, statement, or expression

Question 4

What are operands?

Answer 4

The expressions or values that an operator uses are its operands. In comparisons, the expressions to the left and right of the operator are the operands. For instance, the equality comparison x === y uses the === operator with two operands: x and y.

Question 5

What is this ‘===’ operator called?

Answer 5

The strict equality operator, also known as the identity operator, returns true when the operands have the same type and value, false otherwise.

Question 6

What is this ‘!==’ operator called?

Answer 6

The strict inequality operator returns false when the operands have the same type and value, true otherwise. Note that !== is the inverse of ===: when === returns true, !== returns false, and vice versa.

Question 7

What is this ‘==’ operator called?

Answer 7

The non-strict equality operator, also known as the loose equality operator, is similar to ===.

However, when the operands have different types, == attempts to coerce one of the operands to the other operand’s type before it compares them, and it may coerce both operands in some cases.

The result is true when the final values are the same, false otherwise. The coercion behavior can lead to unexpected results. For instance, when we compare the number 5 to the string ‘5’ using ==, we get true; with ===, we get false.

Question 8

What is this ‘!=’ operator called?

Answer 8

The non-strict inequality operator, also known as the loose inequality operator, is similar to !==. However, when the operands have different types, != attempts to coerce one of the operands to the other operand’s type before it compares them, and it may coerce both operands in some cases. The result is false when the final values are the same, true otherwise.

Question 9

What is this ‘<’ operator called?

Answer 9

The less than operator returns true when the value of the left operand has a value that is less than the value of the right operand, false otherwise.

Question 10

What is happening here?

Answer 10

When comparing strings, the comparison is character-by-character. JavaScript moves from left-to-right in the strings looking for the first character that is different from its counterpart in the other string. Once it finds a character that differs, it compares that character with its counterpart, and makes a decision based on that. If both strings are equal up to the length of the shorter string as in the next to last example, then the shorter string is considered less than the longer string.

The final example shows that if you use < with two different types, some sort of coercion will take place. In this case, “42” gets coerced to a number, so a numeric comparison takes place. Don’t try to remember this.

Question 11

What is this ‘>’ operator called?

Answer 11

The greater than operator returns true when the value of the left operand has a value that is greater than the value of the right operand, false otherwise.

Question 12

What is this ‘<=’ operator called?

Answer 12

The less than or equal to operator returns true when the value of the left operand has a value that is less than or equal to the value of the right operand, false otherwise. Note that =< is not a valid comparison operator.

Question 13

What is this ‘>=’ operator called?

Answer 13

The greater than or equal to operator returns true when the value of the left operand has a value that is greater than or equal to the value of the right operand, false otherwise. Note that => is not a valid comparison operator.

Question 14

What is this ‘!’ operator called?

Answer 14

The not operator returns true when its operand is false and returns false when the operand is true. That is, it negates its operand. Note that, unlike most operators, ! takes a single operand; the operand appears to the right of the operator.

Question 15

What is the ‘&&’ operator called?

Answer 15

The ‘and’ operator returns true when both operands are true and false when either operand is false.

Question 16

What is the ‘||’ operator called?

Answer 16

The or operator returns true when either operand is true and false when both operands are false.

Question 17

What is short circuit evaluation?

Answer 17

Consider these two expressions:

The first expression returns true when item is both red and portable. If either condition is false, then the overall result must be false. Thus, if the program determines that item is not red, it doesn’t have to check whether it is portable. JavaScript short-circuits the entire expression by terminating evaluation as soon as it determines that item isn’t red. It doesn’t need to call isPortable() since it already knows that the entire expression must be false.

Question 18

Why would it print ‘how can this be true’?

Answer 18

The first example logs “how can this be true?” while the second logs “it is not true.” This works since JavaScript coerces the value 5 to true, and the value 0 to false. To repeat, JavaScript can coerce any value to a boolean. Thus, you can use any expression in a conditional expression. We often say that the expression evaluates as or evaluates to true or false.

Question 19

What would be printed here?

Answer 19

The first example logs “how can this be true?” while the second logs “it is not true.” This works since JavaScript coerces the value 5 to true, and the value 0 to false. To repeat, JavaScript can coerce any value to a boolean. Thus, you can use any expression in a conditional expression. We often say that the expression evaluates as or evaluates to true or false.

Question 20

What are the JS versions of ‘false’?

Answer 20

-false

-The number 0. This includes all 3 variations of zero in JavaScript:
0: The ordinary zero value.
-0: A negative zero. That’s mathematical nonsense, but a real thing in JavaScript.
0n: The BigInt version of zero.

An empty string (‘’)

undefined

null

NaN

Question 21

What is truthy and falsy?

Answer 21

We often use the term falsy to refer to values that evaluate as false, while the values that evaluate as true are truthy. We use these terms when we need to distinguish between boolean true and false values. We can also discuss truthiness: whether something is a truthy or falsy value.

Question 22

What is the return value when using ‘&&’ and ‘||’ operators?

Answer 22

They use short circuit evaluation.

The return value will be the last evaluated operand.

Question 23

What is happening here?

Answer 23

In reality, !! isn’t a separate operator in JavaScript. Instead, it’s two consecutive ! operators. The expression !!a is equivalent to writing !(!a). The inner ! converts the value of a to false if it is truthy, or true if a is falsy. The outer ! then flips true to false or false to true. In the end, we end up with a boolean value instead of a truthiness value:

Question 24

What is this ‘??’ operator called?

Answer 24

The nullish coalescing operator evaluates to the right-hand operand if the left-hand operand is nullish (either null or undefined). Otherwise, it evaluates to the value of the left-hand operand.

Question 25

How is '||' different from '??' operator?

Answer 25

|| will return the right-hand operand if the left operand is a falsy value. ?? will return the right-hand operand if the left operand is nullish.

Question 26

When is '??' operator useful?

Answer 26

?? is most useful when dealing with code that could result in undefined or null, and you want to handle the situation gracefully:

Question 27

How does JS evaluate expressions with several operators and sub-expressions?

Answer 27

JavaScript has a set of precedence rules it uses to evaluate expressions that use multiple operators and sub-expressions. The following is a list of the comparison operations from the highest precedence (top) to lowest (bottom). <=, <, >, >= - Comparison ===, !==, ==, != - Equality && - Logical AND || - Logical OR

Question 28

In what order does this expression get evaluated?

Answer 28

For the moment, let's ignore the fact that both || and && are short-circuit operators. The program first evaluates the y && z sub-expression since && has higher precedence than ||. It then takes the result of that evaluation and evaluates x || result.

Question 29

Can we override the precedence order in any way?

Answer 29

Yes! Using parenthesis. We can use parentheses to override the precedence: sub-expressions in parentheses get evaluated before un-parenthesized expressions at the same depth in the main expression

Question 30

How does this expression get evaluated?

Answer 30

In this code, x || y gets evaluated first, and then result && z. That's a different result from the un-parenthesized expression. Parentheses help the computer and other programmers understand your intentions; you should strive to use parentheses in any expression that uses two or more different operators.

Question 31

What happens when there are multiple parenthesized subexpressions?

Answer 31

When multiple parenthesized subexpressions appear at the same depth, it evaluates them from left to right. Once it evaluates the parenthesized expressions, it evaluates the final expression value.

Question 32

What is the 'ternary operator'?

Answer 32

The ternary operator is a quick and easy way to write a short, concise, and simple if/else conditional. It uses a combination of the ? and : symbols and takes 3 operands (hence, the name "ternary"):

Question 33

What is the biggest advantage of ternary operators vs if/else statements?

Answer 33

The chief advantage that the ternary operator has over an if/else statement is that the entire structure is an expression. What that means is that we can treat the ternary expression as a value: we can assign it to a variable, pass it as an argument, and so on. Since if/else is a statement, we can't capture its result to a variable.

Question 34

When should I use a ternary expression?

Answer 34

Ternary expressions should usually be used to select between 2 values, not to choose between two actions. (An action would be something like logging a value to the console or setting a variable to a new value.) The ternary expression's result should almost always be assigned to a variable, passed to a function as an argument, or returned by a function. If you're not doing one of those things, an if/else statement is a better choice.

Question 35

Is this good practice?

Answer 35

No! The following snippets use ternaries that choose between actions, and should be considered inappropriate uses.

Question 36

How does the 'switch' statement work?

Answer 36

A switch statement is similar to an if statement, but it has a different interface. It compares a single value against multiple values for strict equality (as with the === operator), whereas if can test multiple expressions with any condition.

Question 37

Why is the break statement important in a 'switch' statement?

Answer 37

The break statement in each case is crucial. Without a break, execution "falls through" to the next case clause.

Question 38

What is a fall-through in a switch statement?

Answer 38

When we execute multiple case clauses for a single value

Question 39

When are fall-throughs appropriate?

Answer 39

For instance, suppose you want to execute the same action for two or more cases:

Question 40

Is this a fall through?

Answer 40

Technically, this is fall-through, but, since each case executes a single clause, it's safe to use and doesn't suggest a possible error.