null-undefined-undeclared.md

In JavaScript, null, undefined, and undeclared are three states that a variable can be in, each with its own distinct meaning:

1. undefined: A variable is undefined if it has been declared, but no value has been assigned to it (yet).

let myVar;
console.log(myVar); // undefined

2. null: null is a value that represents no value. It is often used to indicate that a variable should have no value. It's different from undefined in the sense that it has been explicitly assigned.

let myVar = null;
console.log(myVar); // null

3. undeclared: A variable is undeclared if it has not been declared anywhere in the code. Trying to access an undeclared variable results in a ReferenceError.

console.log(myVar); // ReferenceError: myVar is not defined

It's important to note that undefined and null are actually values in JavaScript, while undeclared means that the variable has not been created at all.

Also, JavaScript has typeof operator which returns a string indicating the type of the unevaluated operand. typeof operator returns undefined for variables that have not been assigned a value, and also for variables that have not been declared at all. However, trying to directly access an undeclared variable will still throw a ReferenceError.

To verify if a value is null, use the strict equality operator (===). Avoid using the abstract equality operator (==) for this purpose, as it will return true for both null and undefined values.

let myVar;
console.log(typeof myVar); // "undefined"
console.log(typeof undeclaredVar); // "undefined"
console.log(undeclaredVar); // ReferenceError: undeclaredVar is not defined

let myVar1 = null;
let myVar2 = undefined;
// Using strict equality
console.log(myVar1 === null); // true
console.log(myVar2 === null); // false
// Using abstract equality
console.log(myVar1 == null); // true
console.log(myVar2 == null); // true

For best practices, always ensure your variables are neither undeclared nor unassigned. If you don't plan to use them immediately after declaration, explicitly initialize them with null.