To check if a variable exists in JavaScript, you can use the “typeof” operator. The “typeof” operator returns a string representing the variable’s type or “undefined” if the variable is not defined.
Example
if (typeof mainVar !== 'undefined') {
console.log('mainVar exists');
} else {
console.log('mainVar does not exist');
}
Output
mainVar does not exist
The above code checks if mainVar is defined by comparing the result of typeof mainVar to the string “undefined”.
If the result is not “undefined”, the variable exists; otherwise, it does not exist.
Remember that the typeof operator checks for the existence of a variable, not whether its value is null.
If a variable is defined but has the value null, typeof will return “object”, and the check will indicate that the variable exists.

Niva Shah is a Software Engineer with over eight years of experience. She has developed a strong foundation in computer science principles and a passion for problem-solving.