Undefined: A Detailed Guide

What is Undefined?

Undefined is a property in JavaScript that indicates that a variable has not been assigned a value. It is different from null, which is a value that indicates that a variable has been explicitly assigned the value of null.

How to Check if a Variable is Undefined

You can use the typeof operator to check if a variable is undefined. The following code will return “undefined” if the variable is undefined:


typeof myVariable; // "undefined"


When is Undefined Used?

Undefined is most commonly used to check if a variable has been assigned a value before using it. For example, the following code will check if the variable “myVariable” has been assigned a value, and if not, it will assign it the value of 0:


var myVariable = undefined;

if (typeof myVariable === "undefined") {
myVariable = 0;
}


Undefined vs. Null

As mentioned above, undefined is different from null. Null is a value that indicates that a variable has been explicitly assigned the value of null. Undefined, on the other hand, indicates that a variable has not been assigned a value. The following table summarizes the key differences between undefined and null:

Conclusion

Undefined is a property in JavaScript that indicates that a variable has not been assigned a value. It is different from null, which is a value that indicates that a variable has been explicitly assigned the value of null. Undefined is most commonly used to check if a variable has been assigned a value before using it.