Understanding Undefined in JavaScript

What is Undefined?

In JavaScript, the value undefined represents the absence of a value. It is a primitive value, like null, boolean, number, and string.

A variable is assigned the value undefined if:

• It is declared but not assigned a value.
• It is assigned the value undefined.
• A function is called without any arguments and the parameter is not assigned a default value.
• An object property is accessed but does not exist.
• An array element is accessed but does not exist.

Checking for Undefined

There are several ways to check if a value is undefined:

• Use the === operator to compare the value to undefined.
• Use the typeof operator to check if the value is “undefined”.

Differences Between Undefined and Null

Undefined and null are often confused, but they are not the same:

• Undefined represents the absence of a value, while null represents a deliberate assignment of no value.
• Undefined is automatically assigned to variables, while null must be explicitly assigned.

Example

  let myVariable; // myVariable is undefined

  console.log(myVariable); // undefined

  myVariable = null; // myVariable is now null

  console.log(myVariable); // null
  

Conclusion

Understanding undefined is essential for writing robust JavaScript code. By knowing when and how to check for undefined, you can avoid errors and ensure that your code works as intended.