In Javascript, the better way to check if the variable is undefined or not using typeof operator.

In Javascript, the undefined is a property which indicates that a variable has not been assigned a value, or not declared at all.

Check undefined in JavaScript?

Example 1: Test if a variable is undefined:

var x;
if (typeof x === "undefined") {
    console.log("x is undefined");
} else {
    console.log("x is defined");
}

Output:

x is undefined

if (typeof x === "undefined") {
    console.log("x is undefined");
} else {
    console.log("x is defined");
}

Output:

x is undefined

Note that, we have not declared x variable in the above example.