If you've been programming for a while, these terms might be familiar to you and not in the good sense. Usually we find these words written in red text with a whole bunch of errors. Now this article won't be focused on those errors but more so on what these 3 keywords mean and how they differ from each other.
Let's start off with null. null is an actual value that can be given to a variable. It is similar to saying a variable is empty.
Secondly we have undefined. Undefined is an error that is thrown when the programmer is trying to assign or use a variable without having first declared it. Nowadays with ES6's new let and const, these undefined's are much easier to spot. Back before ES6, there would be a lot of silent errors that would occur with the var keyword and how it hoisted itself.
Lastly, we have a more uncommon error, undeclared. With the release of ES6, these errors have been seen a lot less as for this error occurs when you try to assign a value to a variable that has not been declared with var.
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