What is Undefined?

In programming, the value undefined is a special value that indicates that a variable has not been assigned a value or that a function does not return a value.

When is undefined used?

undefined is used in several different scenarios:

• When a variable is declared but not assigned a value.
• When a function is called without any arguments and the function does not have default arguments.
• When a function is called and the function does not return a value.
• When an object property is accessed and the property does not exist.

How to check for undefined

There are several ways to check for undefined in JavaScript:

• The === operator can be used to check if a variable is strictly equal to undefined.
• The typeof operator can be used to check if a variable is of type undefined.

Example

// Check if a variable is undefined
if (typeof myVariable === "undefined") {
  // The variable is undefined
}

// Check if a function does not return a value
function myFunction() {}

if (typeof myFunction() === "undefined") {
  // The function does not return a value
}

Conclusion

undefined is a special value in JavaScript that indicates that a variable has not been assigned a value or that a function does not return a value. It is important to understand how undefined is used in order to write robust and error-free code.