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.

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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.

num is undefined.