5.3. Logical Operators

Recall that an operator is one or more characters that carries out an action on its operand(s). In Data and Variables we learned about three types of operators:

  • Arithmetic operators, such as +, -, *, /, and %.
  • The string operator +.
  • Compound assignment operators, such as += and -=.

Arithmetic and string operators take number and string operands, respectively, returning values of the same type. Compound assignment operators work similarly with numbers or strings while also reassigning the value of the first, variable operand.

5.3.1. Boolean Operators

In addition to these operators, we learned about comparison operators like ===, <, and others. These operators are part of a larger class known as boolean operators, so-called because they return a boolean value (true or false).

Three additional boolean operators allow us to create more complex expressions. These are described below.

5.3.1.1. Logical AND

A compound boolean expression is a boolean expression built out of smaller boolean expressions. JavaScript allows us to create a compound boolean expression using the logical AND operator, &&.

The operator takes two operands, and the resulting expression is true if both operands are true individually. If either operand is false, the overall expression is false.

Example

In English, the && operator mirrors the use of the word "and" (hence the name "logical AND"). A sentence like "Roses are red and violets are blue," is true as a whole precisely because roses are actually red, and violets are actually blue.

On the other hand, the sentence "Roses are red and violets are green," is false as a whole. While roses are indeed red, violets are not green.

Let's see how this works in code.

Example

1
2
3
console.log(7 > 5 && 5 > 3);
console.log(7 > 5 && 2 > 3);
console.log(2 > 3 && 'dog' === 'cat');

Console Output

true
false
false

In line 1, 7 > 5 && 5 > 3 evaluates to true because both 7 > 5 and 5 > 3 are true individually.

The expression 7 > 5 && 2 > 3 evaluates to false because one of the two expressions, 2 > 3, is false.

Like line 2, line 3 returns false because both sub-expressions are false. Notice that we can mix and match data types however we like, as long as both sides of the && expression are themselves boolean expressions.

5.3.1.2. Logical OR

JavaScript's logical OR operator, ||, also creates compound boolean expressions. This operator takes two operands, and the resulting expression is true if either of the operands are true individually. If both operands are false, the overall expression is false.

Example

As with logical AND, logical OR mirrors our experience of English language truth values. The sentence "Pigs can fly or dogs can run," is true as a whole. Joining the two clauses by "or" requires that only one of them is true in order for the full sentence to be true.

When both of the clauses joined by "or" are false, the statement as a whole is false. For example, "Pigs can fly or dogs can speak Spanish," is a false statement.

Let's look at some examples in JavaScript.

1
2
3
console.log(7 > 5 || 5 > 3);
console.log(7 > 5 || 2 > 3);
console.log(2 > 3 || 'dog' === 'cat');

Console Output

true
true
false

Lines 1 and 2 both return true because at least one of the comparison expressions joined by || is true. Line 3 returns false because both sub-expressions are false.

Warning

The single symbols & and | are themselves valid JavaScript operators, so accidentally leaving off one symbols when typing && or || will not result in an error message.

The operators & and | are bitwise operators, which are beyond the scope of this course.

Most programmers rarely use & and |, and it is not important for you to understand them at this point. However, you should never use them in place of && and ||.

5.3.1.3. Logical NOT

The logical NOT operator, !, takes only a single operand and reverses its boolean value.

Example

1
2
console.log(! true);
console.log(! false);

Console Output

false
true

The operator ! (sometimes called "bang") has the same semantic role as the word "not" in English.

Example

1
2
console.log( !(5 > 7) );
console.log( !('dog' === 'cat') );

Console Output

true
true

5.3.2. Operator Precedence

We now have a number of operators in our toolkit. It is important to understand how these operators relate to each other with respect to operator precedence. Operator precedence is the set of rules that dictate in which order the operators are applied.

JavaScript will always apply the logical NOT operator, !, first. Next, it applies the arithmetic operators, followed by the comparison operators. The logical AND and OR are applied last.

This means that the expression x * 5 >= 10 && y - 6 <= 20 will be evaluated so as to first perform the arithmetic and then check the relationships. The && evaluation will be done last. The order of evaluation is the same as if we were to use parentheses to group, as follows:

((x * 5) >= 10) && ((y - 6) <= 20)

While parentheses are not always necessary due to default operator precedence, they make expressions much more readable. As a best practice, we encourage you to use them, especially for more complicated expressions.

The following table lists operators in order of precedence, from highest (applied first) to lowest (applied last). A complete table for the entire language can be found in the MDN JavaScript Documentation.

Operator Precedence
Precedence Category Operators
(highest) Logical NOT !
  Exponentiation **
  Multiplication and division *, /, %
  Addition and subtraction +, -
  Comparison <=, >=, >, <
  Equality ===, !==, ==, !=
  Logical AND &&
(lowest) Logical OR ||

5.3.3. Truth Tables

Truth tables help us understand how logical operators work by calculating all of the possible return values of a boolean expression. Let's look at the truth table for &&, which assumes we have two boolean expressions, A and B, joined by &&.

Example

Truth Table for &&
A B A && B
true true true
true false false
false true false
false false false

Consider the first row of the truth table. This row states that if A is true and B is true, then A && B is true. This is a fact, regardless of what boolean expressions A and B might actually be. The two middle rows demonstrate that if either A or B is false, then A && B is false. (If this idea is hard to grasp, try substituting actual expressions for A and B.)

5.3.4. Check Your Understanding

Question

Complete the table below.

Truth Table for ||
A B A || B
true true  
true false  
false true  
false false  

Question

Which of the following properly expresses the order of operations (using parentheses) in the following expression?

5*3 > 10 && 4 + 6 === 11
  1. ((5*3) > 10) && ((4+6) === 11)
  2. (5*(3 > 10)) && (4 + (6 === 11))
  3. ((((5*3) > 10) && 4)+6) === 11
  4. ((5*3) > (10 && (4+6))) === 11

Question

What is returned by the following boolean expression?

4 < 3 || 2 < 3
  1. true
  2. false
  3. "true"
  4. "false"