Understanding the Distinction Between null and undefined in JavaScript

In JavaScript, both null and undefined are primitive types with different meanings. It is crucial to understand the distinctions between them.

1. undefined:

   • When a variable is declared but not assigned any value, it is considered undefined.
   • For example:

     let age; // age is undefined
     

   • It is important to note that attempting to access a variable that has not been declared will result in a ReferenceError: <variable> is not defined error, which is different from undefined.

2. null:

   • When a variable has the value null, it means that it has been intentionally assigned to have no value.
   • For example:

     let age = null; // age is null
     

   • To check if a variable has been assigned the value null, you can use the comparison operator ===.
   • To check for both null and undefined, you can use the statement:

     if (!age) {
       // code here
     }
     

   • The typeof operator can also be used to determine the type of a variable. However, null is evaluated as an object, even though it is a primitive type:

     let age;
     typeof age; // 'undefined'
     
     let age = null;
     typeof age; // 'object'
     

Understanding the difference between null and undefined is essential for writing clean and reliable JavaScript code.