Existential checks are when we have to detect whether or not a variable has a value - that is, checking to see if a variable exists. If the value is
undefined or otherwise falsy, then it fails the check. This usually takes the form of an if-statement.
Let’s say you’ve created a SaaS application that lets customers keep track of their vendors. Things are going well and you’re adding many customers. One day, a curious customer starts messing around with the IDs in URLs and stumbles across a vendor they shouldn’t have access to. This is Very Bad™, especially if your customers are relying on your application to store sensitive information!