How to check if variable is false or undefined in JavaScript?

In this article, we explored two methods to check if a variable is false or undefined in JavaScript.

The first method uses the typeof operator, and the second method uses loose equality (==) with the undefined keyword.

Check undefined variable using the typeof Operator #

In JavaScript, you can check if a variable is false or undefined using the typeof operator. The typeof operator returns a string indicating the type of the variable.

If the variable is not declared or is declared but not assigned any value, it will return undefined. If the variable is explicitly set to false, it will return boolean.

1. We define a function checkFalseOrUndefined that takes a variable as an argument.
2. Inside the function, we use the typeof operator to check the type of the variable.
3. If the type is boolean, we check if the value is false or not and display appropriate messages.
4. If the type is undefined, we log that the variable is undefined.
5. If the type is anything else, we log that the variable is of a different type.

Using Loose Equality (==) for Undefined Check #

Another approach to check if a variable is false or undefined is by using loose equality with the undefined keyword. In JavaScript, the loose equality operator treats null and undefined as equal.

1. We define the same function checkFalseOrUndefined . Inside the function, we use the loose equality operator == to check if the variable is equal to undefined.
2. If the variable is equal to undefined, we log that the variable is either undefined or null.
3. If the variable is explicitly set to false, we log that the variable is explicitly false. Otherwise, we log that the variable is defined and not false.