To check if a variable exists in JavaScript, use the “typeof” operator. The “typeof” operator returns a string representing the variable’s type or “undefined” if the variable is not defined.
Example 1: Check whether a variable is defined or not
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.
Example 2: Check whether a variable is null or not
let data = null;
if (typeof data === null) {
console.log("Variable is null");
}
else {
console.log("Variable is defined and value is "
+ data);
}
Output
Variable is defined and value is null
Example 3: Falsy check
We can also check if a variable is falsy to see if it’s a falsy value, which includes undefined, null, ” (empty string), 0, NaN, or false.
let data;
console.log(Boolean(data))
Output
false
The console.log() function logs false since data is uninitialized, so it’s undefined.
And since Boolean converts any falsy value to false, that’s what we get.
You can also use the “!!” operator.
let data;
console.log(!!(data))
Output
false
That’s it!

Krunal Lathiya is a seasoned Computer Science expert with over eight years in the tech industry. He boasts deep knowledge in Data Science and Machine Learning. Versed in Python, JavaScript, PHP, R, and Golang. Skilled in frameworks like Angular and React and platforms such as Node.js. His expertise spans both front-end and back-end development. His proficiency in the Python language stands as a testament to his versatility and commitment to the craft.