The Ultimate Guide to Understanding Undefined

What is Undefined?

In JavaScript, the undefined value is a primitive value that represents the absence of a value. It is one of the two falsy values in JavaScript, along with null.

Undefined is assigned to variables that have not been declared or initialized.

How to Check for Undefined

There are two ways to check if a variable is undefined:

1. Using the typeof operator
2. Using the strict equality operator (===)

Using the typeof Operator

The typeof operator returns the type of a variable. If the variable is undefined, typeof will return “undefined”.

let myVar;

console.log(typeof myVar); // Output: "undefined"

Using the Strict Equality Operator (===)

The strict equality operator (===) checks if two values are exactly the same, including their type. If the variable is undefined, === will return false for any other value.

let myVar;

console.log(myVar === undefined); // Output: true
console.log(myVar === null); // Output: false
console.log(myVar === 0); // Output: false

When is Undefined Used?

Undefined is most commonly used to check if a variable has been initialized. This can be useful in debugging or when working with third-party code.

For example, the following code checks if the myVar variable has been initialized before using it:

if (myVar !== undefined) {
  // Do something with myVar
}

Conclusion

Undefined is a primitive value in JavaScript that represents the absence of a value. It is one of the two falsy values in JavaScript, along with null. Undefined is assigned to variables that have not been declared or initialized.

You can check if a variable is undefined using the typeof operator or the strict equality operator (===). Undefined is most commonly used to check if a variable has been initialized.