5.1. Booleans¶

One of the core features of any programming language is the ability to conditionally execute a segment of code. This means that a program will run a segment of code only if a given condition is met.

Example

Consider a banking application that can remind you when a bill is due. The application will notify you that a bill is due soon, but only if the bill has not already been paid.

The condition for the above example is: Send a notification of an upcoming bill only if the statement "the bill is unpaid" is true. In order to state something like this in JavaScript, we need to understand how programming languages represent true and false.

5.1.1. Boolean Values¶

The JavaScript data type for storing true and false values is boolean, named after the British mathematician George Boole.

Fun Fact

George Boole created Boolean Algebra, which is the basis of all modern computer arithmetic.

There are only two boolean values---true and false. JavaScript is case-sensitive, so True and False are not valid boolean values.

Example

console.log(true);
console.log(typeof true);
console.log(typeof false);

Console Output

true
boolean
boolean

The values true and false are not strings. If you use quotes to surround booleans ("true" and "false"), those values become strings.

Example

console.log(typeof true);
console.log(typeof "true");

Console Output

boolean
string

5.1.2. Boolean Conversion¶

As with the number and string data types, the boolean type also has a conversion function, Boolean. It works similarly to the Number and String functions, attempting to convert a non-boolean value to a boolean.

Try It!

Explore how Boolean converts various non-boolean values.

console.log(Boolean("true"));
console.log(Boolean("TRUE"));
console.log(Boolean(0));
console.log(Boolean(1));
console.log(Boolean(''));
console.log(Boolean('LaunchCode'));

repl.it

5.1.3. Boolean Expressions¶

A boolean expression is an expression that evaluates to either true or false. The equality operator, ==, compares two values and returns true or false depending on whether the values are equal.

Example

console.log(5 == 5);
console.log(5 == 6);

Console Output

true
false

In the first statement, the two operands are equal, so the expression evaluates to true. In the second statement, 5 is not equal to 6, so we get false.

We can also use == to see that true and "true" are not equal.

Example

console.log(true == "true");

Console Output

false

5.1.3.1. Comparison Operators¶

The == operator is one of six common comparison operators.

Comparison Operators¶
Operator	Description	Examples Returning `true`	Examples Returning `false`
Equal (`==`)	Returns `true` if the two operands are equal, and `false` otherwise.	`7 == 7` `"dog" == "dog"`	`7 == 5` `"dog" == "cat"`
Not equal(`!=`)	Returns `true` if the two operands are not equal, and `false` otherwise.	`7 != 5` `"dog" != "cat"`	`7 != 7` `"dog" != "dog"`
Greater than (`>`)	Returns `true` if the left-hand operand is greater than the right-hand operand, and `false` otherwise.	`7 > 5` `'b' > 'a'`	`5 > 7` `'a' > 'b'`
Less than (`<`)	Returns `true` if the left-hand operand is less than the right-hand operand, and `false` otherwise.	`5 < 7` `'a' < 'b'`	`7 < 5` `'b' < 'a'`
Greater than or equal (`>=`)	Returns `true` if the left-hand operand is greater than or equal to the right-hand operand, and `false` otherwise.	`7 >= 5` `7 >= 7` `'b' >= 'a'` `'b' >= 'b'`	`5 >= 7` `'a' >= 'b'`
Less than or equal (`<=`)	Returns `true` if the left-hand operand is less than or equal to the right-hand operand, and `false` otherwise.	`5 <= 7` `5 <= 5` `'a' <= 'b'` `'a' <= 'a'`	`7 <= 5` `'b' <= 'a'`

Although these operations are probably familiar, the JavaScript symbols are different from the mathematical symbols. A common error is to use a single equal sign (=) instead of a double equal sign (==). Remember that = is an assignment operator and == is a comparison operator. Also note that =< and => are not recognized operators.

An equality test is symmetric, meaning that we can swap the places of the operands and the result is the same. For a variable a, if a == 7 is true then 7 == a is also true. However, an assignment statement is not symmetric: a = 7 is legal while 7 = a is not.

Warning

If you explore the equality operator in more depth, you will find some surprises. For example, the following comparisons return true:

7 == "7"
0 == false
0 == ''

We will explore the nuances of == in the upcoming section Equality, and introduce two new operators, === and !==, that will align more closely with our intuitive notion of equality.

5.1.4. Check Your Understanding¶

Question

Under which conditions does Boolean convert a string to true?

Only when the string is "true".
Whenever the string contains any non-whitespace character.
Whenever the string is non-empty.
Never. It converts all strings to false.

Question

Which of the following is a Boolean expression? Select all that apply.

3 == 4
3 + 4
3 + 4 === 7
"false"