What do these words do in your code? const, let, var in Javascript

Behavior of "var", "let", and "const" used in variable declarations in Javascript

While writing my own code, I messed up some of the declarations.
Still, sometimes I could reference variables correctly and sometimes I couldn't, so I looked into what was going on.

First, here is the basic behavior of each.

const

Constants are block-scoped, much like variables declared using the let keyword. The value of a constant can't be changed through reassignment (i.e. by using the assignment operator), and it can't be redeclared (i.e. through a variable declaration). However, if a constant is an object or array its properties or items can be updated or removed. (MDN)

let

The let declaration declares a block-scoped local variable, optionally initializing it to a value. (MDN)

var

The var statement declares a function-scoped or globally-scoped variable, optionally initializing it to a value.(MDN)

now, basically we use the "const" and "let".
but if you need to understand older code, you have opportunity to read code by using "var".
What is the different on these words?

The strangest difference I noticed while writing the code is as follows

const.js

console.log(a);

const a = 1; //ReferenceError

let.js

console.log(a);

let a = 1; //ReferenceError

var.js

console.log(a);

var a = 1; //undefined

Why is it only "undefined" when using "var"?
Because, var declaration are processed before some code is executed.

var declaration has two parts.
declaration and assignment.
declaration part is rolled up to the nearest function or global scope.

sample.js

var a;
console.log(a); //undefined

var a = "value";
console.log(a); //"value"

Conclusion

Even if you only used const and let, you need to understand how var works.

Thank you.