JS Interview Question: What’s the difference between a variable that is: null, undefined or undeclared?

What’s the difference between a variable that is: null, undefined or undeclared? How would you go about checking for any of these states?

From what I understand, in Javascript, undefined and null are somewhat related on what value a variable contains. The case for undeclared differs. It tackles on how a variable is defined and how javascript treats these variables. So I am going to discuss undefined and null first since both are on Data Type category.

undefined is a variable that has been declared but no value exists and is a type of itself ‘undefined’.
null is a value of a variable and is a type of object.

We use ‘console.log();’ and ‘type of’ to check if a variable is undefined or null.

ref: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures

undeclared variables is a variable that has been declared without ‘var’ keyword.
testVar = ‘hello world’;
as opposed to
var testVar = ‘hello world’;
When former code is executed, undeclared variables are created as global variable and they are configurable (ex. can be deleted).

ref: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/var

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