Control flow is the order in which statements are executed in a program. The default control flow is for statements to be read and executed in order from left-to-right, top-to-bottom in a program file.
Control structures such as conditionals (
if statements and the like) alter control flow by only executing blocks of code if certain conditions are met. These structures essentially allow a program to make decisions about which code is executed as the program runs.
if statement accepts an expression with a set of parentheses:
- If the expression evaluates to a truthy value, then the code within its code body executes.
- If the expression evaluates to a falsy value, its code body will not execute.
else block can be added to an
if block or series of
else if blocks. The
else block will be executed only if the
if condition fails.
The logical NOT operator
! can be used to do one of the following:
- Invert a Boolean value.
- Invert the truthiness of non-Boolean values.
Truthy and Falsy
false when evaluated as Booleans.
- Values that evaluate to
trueare known as truthy
- Values that evaluate to
falseare known as falsy
Falsy values include
0, empty strings,
NaN. All other values are truthy.
Comparison operators are used to comparing two values and return
false depending on the validity of the comparison:
!==strict not equal
>=greater than or equal
<=less than or equal
Else If Clause
After an initial
else if blocks can each check an additional condition. An optional
else block can be added after the
else if block(s) to run by default if none of the conditionals evaluated to truthy.
The logical AND operator
&& checks two values and returns a boolean. If both values are truthy, then it returns
true. If one, or both, of the values is falsy, then it returns
The logical OR operator
|| checks two values and returns a boolean. If one or both values are truthy, it returns
true. If both values are falsy, it returns
switch statements provide a means of checking an expression against multiple
case clauses. If a case matches, the code inside that clause is executed.
case clause should finish with a
break keyword. If no case matches but a
default clause is included, the code inside
default will be executed.
break is omitted from the block of a
switch statement will continue to check against
case values until a break is encountered or the flow is broken.
The ternary operator allows for a compact syntax in the case of binary (choosing between two choices) decisions. It accepts a condition followed by a
? operator, and then two expressions separated by a
:. If the condition evaluates to truthy, the first expression is executed, otherwise, the second expression is executed.