In this post, we are going to discuss what is undefined, not-defined, null and their behaviours.

undefined

javascript assigns undefined to any variable that has been declared but not given a value, in other words, undefined acts as a placeholder when you don't explicitly initialize it during declaration. here are some examples.

let x;
console.log(x); // undefined
console.log(x === undefined); // true

not defined

when you get a ReferenceError: variable is not defined it means you didn't declare that variable at that point in time. its an error in javascript.

let a = 10
console.log(a + x) // ReferenceError: a is not defined

as you can see in the above code when we tried to access x which is not declared anywhere, we get an error of not defined simply means javascript can't find any variable named x in the code.

Null

the value null represents the intentional absence of a value for a variable. it is treated as falsy for boolean values. you can assign null to a variable to denote that currently, that variable doesn't have any value but it will have later on

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

console.log(!null) // true

in this code you can see !null prints true since null is treated as a falsy value by default. in short if null is assigned that means the value is absent.

I'm glad you took the time to read this post 🌟