What's the difference between a variable that is: null , undefined ...

In JavaScript, the terms null, undefined, and undeclared refer to different states of variables, each representing a unique situation in the code. Understanding these differences is crucial for effective debugging and code management.

Null

A variable is null when it has been explicitly assigned the null value. It represents a deliberate assignment of "no value" or "empty" to a variable. The type of a null variable is an object, which intentionally signifies the absence of any object value. To check if a variable is null, you can use strict equality (===) since null is only equal to null or undefined under non-strict equality.

Example:

let variable = null;
if (variable === null) {
    console.log('variable is null');
}

Undefined

A variable is undefined if it has been declared but not assigned any value. It is a primitive type and is the default value of variables that have been declared but left uninitialized. To check if a variable is undefined, you can use the typeof operator or strict equality.

Example:

let variable;
if (typeof variable === 'undefined') {
    console.log('variable is undefined');
}

Undeclared

An undeclared variable is one that has not been declared at all (i.e., no var, let, or const has been used to declare it). Trying to access such a variable results in a ReferenceError. However, if you check the type of an undeclared variable using typeof, it returns 'undefined', which can be misleading because it does not throw an error like direct access does.

Example:

try {
    console.log(undeclaredVariable);
} catch (error) {
    console.log('variable is undeclared'); // This will catch the ReferenceError
}

Checking for these states:
To determine if a variable is null, undefined, or undeclared, you can use a combination of typeof checks and strict equality. Here’s a function that encapsulates these checks...