What are undefined variables in JavaScript?

Undefined variables are variables that have not been assigned any value after their declaration. JavaScript automatically assigns the value undefined to such variables. This value can also be explicitly assigned.

Examples:

// Declaration without assignment
let myVariable;
console.log(myVariable); // Outputs undefined

// Explicit assignment of undefined
let anotherVariable = undefined;
console.log(anotherVariable); // Outputs undefined

// Accessing a non-existent property of an object
const myObject = {};
console.log(myObject.nonExistentProperty); // Outputs undefined

undefined is a primitive value that differs from null. null is usually used to explicitly indicate the absence of a value or emptiness, whereas undefined means no assigned initial value.

Comparison of undefined and null:

Property	undefined	null
Type with typeof	"undefined"	"object" (historical mistake)
Assignment	Automatically upon declaration without a value; can be explicitly assigned	Must be explicitly assigned
Purpose	Absence of an assigned initial value	Explicit absence of a value, "emptiness"

Checking for undefined can be done using the strict equality operator (===) or the loose (==), although strict is more reliable and avoids type coercion. You can also use typeof.

let value;

if (value === undefined) {
  console.log("Value is undefined");
}

if (typeof value === "undefined") {
  console.log("Type is undefined");
}