What Values Does JavaScript Consider Falsy?

In addition to the boolean value false, javascript also considers the other five values as equivalent to FALSE and they are, undefined, NaN, null, 0, and ""(empty string).

The number 0 in any data type such as int(0), float(0.0), BigInt(0n), hexadecimal(0x00), negatives(-0) is considered as a falsy value.

Description

Falsy values are values that are considered as false by the online javascript compiler. These false values are often used to evaluate different conditions in the program. These falsy values return false with the Boolean() constructor.

A Complete List of JavaScript Falsy Values

The following are the javascript falsy values :

Code Example of Falsy Values

The Boolean object is used to represent the boolean data type and the parameter passed into the Boolean() constructor will be converted into an object of the boolean data type.

The following examples illustrate the different use cases of the javascript falsy values,

The false and true are keywords of the data type boolean that are mostly used as flags to check conditions in a program.

Output :

The numbers(0,-0), decimal(0.0), and hexadecimal number(0x0) are all falsy values and are equivalent to false.

Checking for Falsy Values on Variables

The variables can be checked for falsy values under if...else statements to control the flow of the program.

Let us consider the following example,

Example :

Let us see how the variables are loosely compared using the == operator. The following code illustrates how variables are compared using the == operator,

Code :

Output :

The following case illustrates a difference in comparison between the falsy values,

Code :

Output :

Explanation :

The result is not equal because the null variable is not equal to any values other than undefined according to the standards of javascript and hence proves the execution of the above example. This is also the same for the undefined keyword.

The == operator behaves differently in the presence of the ! operator. The following example illustrates this,

Code :

Output :

Explanation :

The output results in equal because the ! operator performs a casting to the boolean data type which is then evaluated using the expression.

General Examples

The following example illustrates the use of such falsy values,

Code :

Explanation :

In the above example, the javascript falsy value is used as a condition in the while() loop, and the falsy value is inverted using the ! operator when a certain condition is met to end the loop. Many use cases of the falsy variable are to handle unseen situations in which we may encounter an empty value for a variable or zero or an empty string.

The javascript falsy values are also used to check if an object has a certain property. The following example illustrates this,

Example :

The presence of the property unavailable_property in the Array object is checked using the falsy values.

Output :

We can also find the same results using the following code,

Code :

Output :

Explanation :

Since length is a property that belongs to the Array object, it returns true. The method discussed before the current example is more suitable to check the presence of a method in an object because the current example can't be used to check a method or data member that has the value of 0 or "".

A Note on Strict Equality

The strict equality operator or the === operator will not make any type casting to the values that are compared and will return false if the data types of the compared values are different.

The following example illustrates how the === operator works,

Code :

Output :

Explanation :

Since the data types of the values compared are different, the strict equality operator returns false, and therefore the else part of the code is executed.

Examples with Arrays

The array object will never be a falsy value, even empty arrays will return true when passed as a parameter to the Boolean() function. The following example shows this,

Output :

The array object acts differently when compared with the false value and the following code illustrates this,

Code :

Output :

Explanation :

The reason for this is due to the Abstract Equality Algorithm followed by the == operator. By the algorithm, the following steps occur,

• The value false is converted to 0.
• By the rule, if the value is to be compared in our object and number, then the object is converted to primitive form. The empty array([]) is converted to an empty string('').
• Then the empty string is again converted to 0.

Hence, now both sides are equal and this results in the above output.

Caveat

The typeof operator is used to find the data type of the variable and can be used to check for undefined variables, as when checking for the availability of a property or data member of an object as numbers and strings may behave differently due to falsy values.

The following example illustrates this,

Code :

Output :

In the above example, even though the data member name is available, the condition couldn't evaluate it, because of its falsy value. The above error can be rectified by using the typeof operator,

Code :

Explanation :

The above code only checks the availability of the name data member and doesn't consider the falsy values.

The Logical AND Operator(&&)

The AND(&&) operator is used to evaluate two conditions at the same time. Falsy values in javascript can be used along with the AND operator to perform several functions. Let us consider the following example,

Code :

Explanation :

The above example produces no output as both test1 and test2 are fasly values. On using the AND operator, the test2 is not evaluated as the test1 is false. If the test1 is true, then the test2 is evaluated and if both are true, the if block is executed.