console.log(typeof myVar === "undefined")

Here's a good page that contains the "===" comparison operator: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Comparison_Operators

Knowing that a variable is undefined or null could be an important part of a function. For example:

You create a function that takes inputs and assigns them to variables (Name, Age, and Location), however Location is optional. For the sake of this example, imagine a form on a website. There are 2 text inputs, and 1 dropdown list for the Location.

The Rules: Both inputs (Name, Age) have to be filled out, and there's the option to opt-out of Location. Name has to be a string, Age has to be a number, and Location has to be selected or opted out of (null), not undefined.

To check for these rules in javascript, you'd probably use the typeof operator. Say Name and Age where filled out correctly, but someone didn't select their Location, and they didn't opt-out (set to null) via a checkbox, they simply left it empty. Via if (typeof === "undefined"), you could know that, and not allow them to continue, instead displaying to a user the appropriate error message.

Does that make sense to you? I hope so :P