What does !== undefined mean in JavaScript ?
Last Updated :
12 Feb, 2024
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);
}
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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.
Javascript
let num;
if (num != undefined) {
console.log( "num has a value assigned." );
} else {
console.log( "num is undefined." );
}
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