Undefined Variable Checks in JavaScript

This morning, I stumbled upon an issue surrounding checking for the existence of a variable in JavaScript. I was under the impression that I could perform this check as follows:

const isPresent = foo !== undefined;
// ReferenceError: foo is not defined

Looking at the code above, you will notice that this did not work, and instead, a ReferenceError exception was thrown. This is due to the JavaScript specification stating that an exception of this kind should be thrown when trying to dereference a variable that has not yet been declared. Upon review, however, I was able to resolve this issue by being more specific in our check, like so:

const isPresent = window.foo !== undefined;

This implementation works as the initial variable that we are dereferencing (window) exists, and instead, the check is now performed on the window value (an object) instead. An object in JavaScript returns undefined when trying to access a non-existent property. This check, however, can seem a little limited in scope. Instead, the more generic (and recommended) way for checking if a variable has been defined or not is as follows:

const isPresent = typeof foo !== 'undefined';