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JavaScript – Logical Expressions

The logical operators &&, ||, and !
perform Boolean algebra and are often used in conjunction with the
relational operators to combine two relational expressions into one
more complex expression. These operators are described in the
subsections that follow. In order to fully understand them, you may
want to review the concept of “truthy” and “falsy” values introduced
in Boolean Values.

Logical AND (&&)

The && operator can
be understood at three different levels. At the simplest level, when
used with boolean operands, && performs the Boolean AND
operation on the two values: it returns true if and only if both its first operand
and its second operand are true. If one or both of these operands is
false, it returns false.

&& is often used as
a conjunction to join two relational expressions:

x == 0 && y == 0   // true if, and only if x and y are both 0

Relational expressions always evaluate to true or false, so when used like this, the
&& operator itself
returns true or false. Relational operators have higher
precedence than && (and
||), so expressions like these
can safely be written without parentheses.

But && does not
require that its operands be boolean values. Recall that all
JavaScript values are either “truthy” or “falsy.” (See Boolean Values for details. The falsy values are false, null, undefined, 0, -0,
NaN, and "". All other values, including all
objects, are truthy.) The second level at which && can be understood is as a
Boolean AND operator for truthy and falsy values. If both operands
are truthy, the operator returns a truthy value. Otherwise, one or
both operands must be falsy, and the operator returns a falsy value.
In JavaScript, any expression or statement that expects a boolean
value will work with a truthy or falsy value, so the fact that
&& does not always return
true or false does not cause practical
problems.

Notice that the description above says that the operator
returns “a truthy value” or “a falsy value,” but does not specify
what that value is. For that, we need to describe && at the third and final level.
This operator starts by evaluating its first operand, the expression
on its left. If the value on the left is falsy, the value of the
entire expression must also be falsy, so && simply returns the value on the
left and does not even evaluate the expression on the right.

On the other hand, if the value on the left is truthy, then
the overall value of the expression depends on the value on the
right-hand side. If the value on the right is truthy, then the
overall value must be truthy, and if the value on the right is
falsy, then the overall value must be falsy. So when the value on
the left is truthy, the && operator evaluates and returns
the value on the right:

var o = { x : 1 };
var p = null;
o && o.x     // => 1: o is truthy, so return value of o.x
p && p.x     // => null: p is falsy, so return it and don't evaluate p.x

It is important to understand that && may or may not evaluate its
right-side operand. In the code above, the variable p is set to null, and the expression p.x would, if evaluated, cause a TypeError. But the
code uses && in an
idiomatic way so that p.x is
evaluated only if p is truthy—not
null or undefined.

The behavior of &&
is sometimes called “short circuiting,” and you may sometimes see
code that purposely exploits this behavior to conditionally execute
code. For example, the following two lines of JavaScript code have
equivalent effects:

if (a == b) stop();   // Invoke stop() only if a == b
(a == b) && stop();   // This does the same thing

In general, you must be careful whenever you write an
expression with side effects (assignments, increments, decrements,
or function invocations) on the right-hand side of &&. Whether those side effects
occur depends on the value of the left-hand side.

Despite the somewhat complex way that this operator actually
works, it is most commonly used as a simple Boolean algebra operator
that works on truthy and falsy values.

Logical OR (||)

The || operator performs
the Boolean OR operation on its two operands. If one or both
operands is truthy, it returns a truthy value. If both operands are
falsy, it returns a falsy value.

Although the || operator is
most often used simply as a Boolean OR operator, it, like the
&& operator, has more
complex behavior. It starts by evaluating its first operand, the
expression on its left. If the value of this first operand is
truthy, it returns that truthy value. Otherwise, it evaluates its
second operand, the expression on its right, and returns the value
of that expression.

As with the &&
operator, you should avoid right-side operands that include side
effects, unless you purposely want to use the fact that the
right-side expression may not be evaluated.

An idiomatic usage of this operator is to select the first
truthy value in a set of alternatives:

// If max_width is defined, use that.  Otherwise look for a value in
// the preferences object.  If that is not defined use a hard-coded constant.
var max = max_width || preferences.max_width || 500;

This idiom is often used in function bodies to supply default
values for parameters:

// Copy the properties of o to p, and return p
function copy(o, p) {
   p = p || {};  // If no object passed for p, use a newly created object.
   // function body goes here
}

Logical NOT (!)

The ! operator is a unary
operator; it is placed before a single operand. Its purpose is to
invert the boolean value of its operand. For example, if x is truthy !x evaluates to false. If x is falsy, then !x is true.

Unlike the && and
|| operators, the ! operator converts its operand to a
boolean value (using the rules described in Chapter 3) before inverting the converted value. This
means that ! always returns
true or false, and that you can convert any value
x to its equivalent boolean value
by applying this operator twice: !!x (see Explicit Conversions).

As a unary operator, ! has
high precedence and binds tightly. If you want to invert the value
of an expression like p &&
q
, you need to use parentheses: !(p && q). It is worth noting two
theorems of Boolean algebra here that we can express using
JavaScript syntax:

// These two equalities hold for any values of p and q
!(p && q) === !p || !q
!(p || q) === !p && !q

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