JavaScript provides some primitive as well as non-primitive data types. For example “null”, “boolean”, “undefined”, etc. belong to the primitive data type while “arrays”, and “objects” are non-primitive or complex data types. In JavaScript, any variable that is not assigned with any value has a default value of “undefined”.
This write-up will explain different use cases of the “undefined” type in JavaScript and it will be organized as follows:
So, let’s get started!
What is undefined in JavaScript?
The undefined type belongs to the primitive data types that can have only one value i.e. undefined. A variable that is declared/created but not assigned with a value anywhere in the program has a default value of “undefined”.
Syntax
Here is the basic syntax for the JavaScript undefined type:
1 | undefined |
How to use undefined in JavaScript?
Let’s consider some use cases of JavaScript undefined type:
Example1: variable declared but not defined
In this example, we will check the type of a variable that is declared but not defined anywhere in the program:
1 2 | var x; console.log(typeof(x)); |
In this example:
- We declared/created a variable “x” and didn’t assign it a value.
- Next, we utilized the “typeof” operator to find the type of “x”.
- Consequently, we get the below-given result:
The typeof operator returned undefined which verified that an unassigned variable has a default value of “undefined”.
Example2: type of an empty string
Let’s consider we have an empty string as shown in the below snippet:
1 | var x = ""; |
The task is to check the type and value of an empty string:
1 2 3 | var x = ""; console.log(x); console.log(typeof(x)); |
The above snippet utilized the console.log() method:
- To print the value of x.
- To print the data type of x.
- As a result, we get the below-given output:
The output verified that an empty string and an undefined variable are two different things. The undefined variable has a data type “undefined” while an empty string has a data type “string”.
Example3: check if a variable is defined or not
In this example program, we will check whether a specific variable is defined or not. If the program does not define the variable, display the message “variable not defined!”. Else show the message “variable defined”:
1 2 3 4 5 6 7 8 | var x; if (typeof x === "undefined") { message = "variable not defined!"; console.log(message); } else { message = "variable defined!"; console.log(message); } |
The above code served the below given functionalities:
- Declared a variable “x”.
- Next, compared the value of “x” with the “undefined” type.
- Printed “variable not defined” if the program does not define the variable.
- Else displayed the message “variable defined”:
This is how the undefined type works in JavaScript.
Conclusion
In JavaScript, the undefined is a primitive type and has only one value “undefined”. A variable that is declared/created but not assigned with a value anywhere in the program has a default value of “undefined”. This write-up explained what exactly an undefined type is? It’s syntax and use-cases with the help of examples.
