In JavaScript, when you declare a variable and do not assign any value to it, then JavaScript assigns undefined to that variable.
There are two ways to determine if a variable is undefined in JavaScript:
1) The typeof operator returns "undefined" when a variable has an undefined value. You can use the value returned by the typeof operator in an if statement to check whether a variable is undefined or not.
let x;
console.log(typeof x);
if(typeof x === "undefined"){
console.log("Variable x is not initialized.");
}else{
console.log(`The value of x is ${x}.`);
}
Output
"undefined" Variable x is not initialized.
2) You can directly use a strict equality operator (===) to check if a variable is undefined or not.
let x;
if(x === undefined){
console.log("Variable x has an undefined value.");
}else{
console.log(`The value of x is ${x}.`);
}
Output
Variable x has an undefined value.
It is important to note that you should not use the equality operator (==) because it returns true if null is tested against an undefined value.
console.log(null == undefined); //true
null is used to show that the variable does not have any object value. It is one of the primitive data types in JavaScript. In simple terms, null also represents an absence of value.
The simplest way to check if a variable is null is to use a strict equality operator (===) inside the if statement.
let x = null;
if(x === null){
console.log("Variable x is null.");
}else{
console.log("Variable x is not null.");
}
Output
Variable x is null.
It is recommended that you should not use the typeof operator for checking if a variable is null because the typeof operator returns "object" for the variable that has a null value. The typeof operator also returns "object" for reference type variables.
let x = null;
let pdt = {
name: "Amazon Alexa"
};
console.log(typeof x); //object
console.log(typeof pdt); //object
In this example, you can see that the typeof operator has returned "object" on the pdt variable.
