What does !== undefined mean in JavaScript ?

In JavaScript, !== is a strict inequality operator, and undefined is a special value representing the absence of a value or the lack of an assigned value to a variable. The !== operator checks for both value and type equality, and !== undefined is used to verify if a variable is not equal to the undefined value.

Using strict inequality

The !== operator checks for both value and type equality, and !== undefined is used to verify if a variable is not equal to the undefined value.

Example: To demonstrate the use of the strict inequality operator by checking whether a number is undefined or not.

Javascript

let num = 5;

if (num !== undefined) {
  console.log(num);
} else {
  console.log(undefined);
}

Output

5

Using equality operator with negation

The non-strict equality operator != lead to unexpected behaviour in certain cases. Using the strict inequality operator !== is generally recommended for checking against undefined.

Example: To demonstrate using equality operator with negation.

Javascript

let num;

if (num != undefined) {
  console.log("num has a value assigned.");
} else {
  console.log("num is undefined.");
}

Output

num is undefined.