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
- Using Equality Operator with Negation
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); } |
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." ); } |
num is undefined.