How to Check if the Variable is Undefined
Do the following to check if the value's type is "undefined":
To check if a variable is undefined, you can use comparison operators — the equality operator == or strict equality operator === .
If you declare a variable but not assign a value, it will return undefined automatically. Thus, if you try to display the value of such variable, the word "undefined" will be displayed.
undefined vs “undefined”
There are two ways of determining whether a variable is not defined either by value or by type. If you check by value, you will get that variable is assigned a value or not. In the case of undefined, the assigned variable don’t have any value but the variable exists.
Checking the type is done with the typeof operator. In the case of variable === “undefined”, the type of variable is undefined.