Table 5-5. Truth table for AND (
&&
) with nonboolean operands
x
y
x && y
falsy
falsy
x
(falsy)
falsy
truthy
x
(falsy)
truthy falsy
y
(falsy)
truthy truthy
y
(truthy)
Table 5-6. Truth table for OR (
||
) with nonboolean operands
x
y
x || y
falsy
falsy
y
(falsy)
falsy
truthy
y
(truthy)
truthy falsy
x
(truthy)
truthy truthy
x
(truthy)
Note that if you convert the result to a boolean, it will be correct according to the
boolean definitions of AND and OR. This behavior of the logical operators allows
you to take certain convenient shortcuts. Here’s one that you will see a lot:
const
options
=
suppliedOptions
||
{
name
:
"Default"
}
Remember that objects (even if they are empty) always evaluate to truthy. So if
suppliedOptions
is an object,
options
will refer to
suppliedOptions
. If no options
were supplied—in which case,
suppliedOptions
will be
null
or
undefined
—
options
will get some default values.
In the case of NOT, there’s no reasonable way to return anything other than a
boolean, so the NOT operator (
!
) always returns a boolean for an operand of any
type. If the operand is truthy, it returns
false
, and if the operand is falsy, it returns
true
.
Conditional Operator
The conditional operator is JavaScript’s sole ternary operator, meaning it takes three
operands (all other operands take one or two). The conditional operator is the
expression equivalent of an
if...else
statement. Here’s an example of the condi‐
tional operator:
92 | Chapter 5: Expressions and Operators
