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