JavaScript variables may be declared without setting a value for them.
They can also be set to undefined .
In this article, we’ll look at how to check if a JavaScript variable is initialized or whether it’s undefined .
The typeof Operator
One way to check if a JavaScript variable exists is to use the typeof operator.
For instance, we can write:
let x;
console.log(typeof x === 'undefined')
If x isn’t initialized, then it’s the same as setting it to undefined .
So typeof x returns 'undefined' .
Therefore, the console.log returns true .
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 .
Truthy value is any value other than those values.
For instance, we can write:
let x;
console.log(Boolean(x))
Then console log logs false since x is uninitialized, so it’s undefined .
And since Boolean converts any falsy value to false , that’s what we get.
Equivalently, we can use !! to do the same thing:
let x;
console.log(!!(x))
Conclusion
We can use the typeof operator or the Boolean function to check if a variable is initialized or not.
