JavaScript(JS) JS check if a variable is undefined or null

In JavaScript, you can check if a variable is undefined or null using a combination of the typeof and === operators. Here's an example:

let myVar;

if (typeof myVar === 'undefined' || myVar === null) {
  console.log('myVar is either undefined or null');
} else {
  console.log('myVar is defined and not null');
}

In the above example, we declare a variable myVar but do not assign it a value. We then use the typeof operator to check whether myVar is undefined and the === operator to check whether it is null. We use the || (OR) operator to combine the two conditions.

If myVar is undefined or null, the code inside the if block will execute. If it is defined and not null, the code inside the else block will execute.

Note that in JavaScript, undefined is a primitive value that is automatically assigned to variables that are declared but not assigned a value. On the other hand, null is a special value that represents the intentional absence of any object value.